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1

Asymmetric synthesis of fortucine and reassignment of its absolute configuration.  

PubMed

A convergent and enantioselective synthesis of fortucine was achieved from the starting materials tyrosine methyl ester and 3-hydroxy-4-methoxybenzaldehyde. The synthesis is based on two key steps mediated by a hypervalent iodine reagent. This work has enabled us to reassign the absolute configuration of the natural product reported in the literature. PMID:24782396

Beaulieu, Marc-André; Ottenwaelder, Xavier; Canesi, Sylvain

2014-06-16

2

Asymmetric synthesis of fortucine and reassignment of its absolute configuration.  

PubMed

Invited for the cover of this issue is the group of Sylvain Canesi at the Université du Québec à Montréal and Xavier Ottenwaelder at Concordia University. The image depicts the asymmetric total synthesis of the alkaloid fortucine isolated from the species of plant Fortune Narcissus of- Amarillydaceaes. Read the full text of the article at 10.1002/chem.201402323. PMID:24827927

Beaulieu, Marc-André; Ottenwaelder, Xavier; Canesi, Sylvain

2014-06-16

3

Synthesis and determination of the absolute configuration of cavicularin by a symmetrization/asymmetrization approach.  

PubMed

Taking the strain: The asymmetric total synthesis and stereochemical assignment of (-)-cavicularin, which features a highly strained polycyclophane ring system, has been achieved. The key features of this synthesis are 1) macrocyclization by an SN Ar reaction, 2) group-selective reaction to induce planar chirality in a highly stereoselective manner, and 3) radical transannulation to construct the highly strained ring system. PMID:23956143

Takiguchi, Hiromu; Ohmori, Ken; Suzuki, Keisuke

2013-09-27

4

Asymmetric synthesis of epicylindrospermopsin via intramolecular nitrone cycloaddition. Assignment of absolute configuration.  

PubMed

A synthesis of (-)-epicylindrospermopsin (2) was completed that establishes its absolute configuration and corroborates the corrected structural assignment previously made to this toxin by Weinreb et al. The hydroxylamine 3, prepared from 4-bromobenzyloxyacetaldehyde, was condensed with aldehyde 4, obtained in nine steps from (R)-methionine, to give nitrone 16. Intramolecular cycloaddition of 16 proceeded stereoselectively to yield the oxazabicyclo[2.2.1]heptane 17, which after reduction and deprotection afforded piperidine 18. The latter was transformed via cyclic urea 19 to the inverted C12 alcohol 20, and the derived azide 22 was cyclized to produce the guanidine moiety of 25. Final sulfation of the C12 hydroxyl group furnished (-)-2. PMID:11982346

White, James D; Hansen, Joshua D

2002-05-01

5

Asymmetric synthesis of hydroxyphosphonates  

Microsoft Academic Search

Hydroxyphosphonates have attracted considerable attention as biologically active compounds, enzyme inhibitors, and drugs. Over the last few years significant interest in the asymmetric synthesis and practical application of chiral hydroxyphosphonates has been reported, which shows the theoretical interest and the practical importance of hydroxyphosphonates. An overview of recent synthetic approaches to chiral hydroxyphosphonates and determinations of their absolute configuration is

Oleg I. Kolodiazhnyi

2005-01-01

6

The absolute configuration of (+)-oxopropaline D by theoretical calculation of specific rotation and asymmetric synthesis.  

PubMed

The specific optical rotations of (R)-oxopropaline D calculated by two ab initio MO methods were +52+/-31 degrees and +61+/-29 degrees, respectively, and (+)-oxopropaline D (3) was presumed to have an R-configuration. On the basis of this theoretical result, the reaction of 1-litio-beta-carboline with (R)-glyceraldehyde acetonide followed by oxidation with MnO(2) gave (R)-oxopropaline D acetonide (4a), which was consistent with the previously synthesized (+)-oxopropaline D acetonide (4) in all respects. From the results of theoretical calculations and the experimental synthesis, we determined that natural (+)-oxopropaline D (3) has an R-configuration. PMID:12520122

Kuwada, Takeshi; Fukui, Miyako; Hata, Toshiyuki; Choshi, Tominari; Nobuhiro, Junko; Ono, Yukio; Hibino, Satoshi

2003-01-01

7

Asymmetric synthesis of umuravumbolide.  

PubMed

[figure: see text] This first asymmetric synthesis of enantiopure desacetylumuravumbolide and umuravumbolide via asymmetric reduction, allylboration, and ring-closing metathesis confirms their revised structures and configurations. A convenient procedure to upgrade the enantiopurity of alpha,beta-acetylenic alcohols is also described. PMID:11429860

Reddy, M V; Rearick, J P; Hoch, N; Ramachandran, P V

2001-01-11

8

Chemical aspects of metoprolol metabolism. Asymmetric synthesis and absolute configuration of the 3-[4-(1-hydroxy-2-methoxyethyl)phenoxy]-1-(isopropylamino)-2-propanols , the diastereomeric benzylic hydroxylation metabolites.  

PubMed

Asymmetric synthesis of 3-[4-(1-hydroxy-2-methoxyethyl)phenoxy]-1-(isopropylamino)-2-propanol (2), the benzylic hydroxylation metabolite of metoprolol (1), is described, and the absolute configurations of the diastereoisomers were assigned. Ketone 3, prepared in a multistep synthesis, was reduced with a complex of (2S)-(-)-2-amino-3-methyl-1,1-diphenylbutan-1-ol (9) and borane, yielding 2, with a ratio of 82:18 for the diastereomers. The absolute configurations 1'S,2S and 1'S,2R were assigned for the diastereomers formed in excess on the basis of reductions on closely related alkyl phenyl ketones and the circular dichroism spectrum. Derivatization of the 1'-hydroxyl group of oxazolidinone 10 with a chiral Mosher acid chloride and the use of an HPLC procedure to resolve the resulting esters enabled us to determine the metabolic product stereoselectivity for 2. In the presence of the rat liver microsomal fraction, the benzylic hydroxylation of 1 was highly product stereoselective favoring 1'R stereochemistry at the new asymmetric center in racemic 1 and in both enantiomers of 1. Determination of the stereochemistry of 2 will facilitate study of this polymorphically controlled metabolic process. PMID:3336032

Shetty, H U; Nelson, W L

1988-01-01

9

Concise asymmetric synthesis of (-)-sparteine.  

PubMed

A six-step asymmetric synthesis of natural (-)-sparteine from ethyl 7-iodohept-2-enoate is reported, involving a connective Michael addition of an amino ester-derived enolate to an alpha,beta-unsaturated amino ester. PMID:15306905

Hermet, Jean-Paul R; McGrath, Matthew J; O'Brien, Peter; Porter, David W; Gilday, John

2004-08-21

10

Hydroxamic acids in asymmetric synthesis.  

PubMed

Metal-catalyzed stereoselective reactions are a central theme in organic chemistry research. In these reactions, the stereoselection is achieved predominantly by introducing chiral ligands at the metal catalyst's center. For decades, researchers have sought better chiral ligands for asymmetric catalysis and have made great progress. Nevertheless, to achieve optimal stereoselectivity and to catalyze new reactions, new chiral ligands are needed. Because of their high metal affinity, hydroxamic acids play major roles across a broad spectrum of fields from biochemistry to metal extraction. Dr. K. Barry Sharpless first revealed their potential as chiral ligands for asymmetric synthesis in 1977: He published the chiral vanadium-hydroxamic-acid-catalyzed, enantioselective epoxidation of allylic alcohols before his discovery of Sharpless asymmetric epoxidation, which uses the titanium-tartrate complex as the chiral reagent. However, researchers have reported few highly enantioselective reactions using metal-hydroxamic acid as catalysts since then. This Account summarizes our research on metal-catalyzed asymmetric epoxidation using hydroxamic acids as chiral ligands. We designed and synthesized a series of new hydroxamic acids, most notably the C2-symmetric bis-hydroxamic acid (BHA) family. V-BHA-catalyzed epoxidation of allylic and homoallylic alcohols achieved higher activity and stereoselectivity than Sharpless asymmetric epoxidation in many cases. Changing the metal species led to a series of unprecedented asymmetric epoxidation reactions, such as (i) single olefins and sulfides with Mo-BHA, (ii) homoallylic and bishomoallylic alcohols with Zr- and Hf-BHA, and (iii) N-alkenyl sulfonamides and N-sulfonyl imines with Hf-BHA. These reactions produce uniquely functionalized chiral epoxides with good yields and enantioselectivities. PMID:23157425

Li, Zhi; Yamamoto, Hisashi

2013-02-19

11

Hydroxamic Acids in Asymmetric Synthesis  

PubMed Central

Metal-catalyzed stereoselective reactions are a central theme in organic chemistry research. In these reactions, the stereoselection is achieved predominantly by introducing chiral ligands at the metal catalyst’s center. For decades, researchers have sought better chiral ligands for asymmetric catalysis and have made great progress. Nevertheless, to achieve optimal stereoselectivity and to catalyze new reactions, new chiral ligands are needed. Due to their high metal affinity, hydroxamic acids play major roles across a broad spectrum of fields from biochemistry to metal extraction. Dr. K. Barry Sharpless first revealed their potential as chiral ligands for asymmetric synthesis in 1977: He published the chiral vanadium-hydroxamic-acid-catalyzed, enantioselective epoxidation of allylic alcohols before his discovery of Sharpless Asymmetric Epoxidation, which uses titanium-tartrate complex as the chiral reagent. However, researchers have reported few highly enantioselective reactions using metal-hydroxamic acid as catalysts since then. This Account summarizes our research on metal-catalyzed asymmetric epoxidation using hydroxamic acids as chiral ligands. We designed and synthesized a series of new hydroxamic acids, most notably the C2-symmetric bis-hydroxamic acid (BHA) family. V-BHA-catalyzed epoxidation of allylic and homoallylic alcohols achieved higher activity and stereoselectivity than Sharpless Asymmetric Epoxidation in many cases. Changing the metal species led to a series of unprecedented asymmetric epoxidation reactions, such as (i) single olefins and sulfides with Mo-BHA, (ii) homoallylic and bishomoallylic alcohols with Zr- and Hf-BHA, and (iii) N-alkenyl sulfonamides and N-sulfonyl imines with Hf-BHA. These reactions produce uniquely functionalized chiral epoxides with good yields and enantioselectivities.

Li, Zhi; Yamamoto, Hisashi

2012-01-01

12

Photochirogenesis: Photochemical models on the absolute asymmetric formation of amino acids in interstellar space  

NASA Astrophysics Data System (ADS)

Proteins of all living organisms including plants, animals, and humans are made up of amino acid monomers that show identical stereochemical L-configuration. Hypotheses for the origin of this symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which interstellar ultraviolet (UV) circularly polarized light (CPL) induces an enantiomeric excess in chiral organic molecules in the interstellar/circumstellar media. This scenario is supported by a) the detection of amino acids in the organic residues of UV-photo-processed interstellar ice analogues, b) the occurrence of L-enantiomer-enriched amino acids in carbonaceous meteorites, and c) the observation of CPL of the same helicity over large distance scales in the massive star-forming region of Orion. These topics are of high importance in topical biophysical research and will be discussed in this review. Further evidence that amino acids and other molecules of prebiotic interest are asymmetrically formed in space comes from studies on the enantioselective photolysis of amino acids by UV-CPL. Also, experiments have been performed on the absolute asymmetric photochemical synthesis of enantiomer-enriched amino acids from mixtures of astrophysically relevant achiral precursor molecules using UV-circularly polarized photons. Both approaches are based on circular dichroic transitions of amino acids that will be highlighted here as well. These results have strong implications on our current understanding of how life's precursor molecules were possibly built and how life selected the left-handed form of proteinogenic amino acids.

Meinert, Cornelia; de Marcellus, Pierre; Le Sergeant D'Hendecourt, Louis; Nahon, Laurent; Jones, Nykola C.; Hoffmann, Søren V.; Bredehöft, Jan Hendrik; Meierhenrich, Uwe J.

2011-10-01

13

Asymmetric Strecker synthesis of ?-arylglycines.  

PubMed

A practically simple three-component Strecker reaction for the asymmetric synthesis of enantiopure ?-arylglycines has been developed. Addition of a range of aryl-aldehydes to a solution of sodium cyanide and (S)-1-(4-methoxyphenyl)ethylamine affords highly crystalline (S,S)-?-aminonitriles that are easily obtained in diastereomerically pure form. Heating the resultant (S,S)-?-aminonitriles in 6 M aqueous HCl at reflux resulted in cleavage of their chiral auxiliary fragments and concomitant hydrolysis of their nitrile groups to afford enantiopure (S)-?-arylglycines. The enantiopurities of these (S)-?-arylglycines were determined via derivatization of their corresponding methyl esters with 2-formylphenylboronic acid and (S)-BINOL, followed by (1)H NMR spectroscopic analysis of the resultant mixtures of diastereomeric iminoboronate esters. PMID:21627154

Pérez-Fuertes, Yolanda; Taylor, James E; Tickell, David A; Mahon, Mary F; Bull, Steven D; James, Tony D

2011-08-01

14

Asymmetric total synthesis of (-)-azaspirene, a novel angiogenesis inhibitor.  

PubMed

The asymmetric total synthesis of (-)-azaspirene, an angiogenesis inhibitor, has been accomplished, establishing its absolute stereochemistry. The key steps are a MgBr2.OEt2-mediated, diastereoselective Mukaiyama aldol reaction, a NaH-promoted, intramolecular cyclization of an alkynylamide, and the aldol reaction of a ketone containing functionalized gamma-lactam moiety without protection of tert-alcohol and amide functionalities. PMID:12371831

Hayashi, Yujiro; Shoji, Mitsuru; Yamaguchi, Junichiro; Sato, Kenji; Yamaguchi, Shinpei; Mukaiyama, Takasuke; Sakai, Ken; Asami, Yukihiro; Kakeya, Hideaki; Osada, Hiroyuki

2002-10-16

15

Asymmetric total synthesis of (+)-crassalactone D.  

PubMed

The asymmetric total synthesis of (+)-crassalactone D (4), a naturally occurring antitumor agent, has been achieved by employing an oxidative spirocyclization of furan 11 as the key step. Two close analogues, 7-epi-crassalactone D (14) and 5-epi-7-epi-crassalactone D (15), also have been prepared in the course of the synthesis of (+)-crassalactone D. PMID:19924876

Yang, Zhicai; Tang, Phung; Gauuan, Jolicia F; Molino, Bruce F

2009-12-18

16

Total Synthesis and Absolute Configuration of Laurenditerpenol: A Hypoxia Inducible Factor-1 Activation Inhibitor  

PubMed Central

The absolute stereo structure of the natural product laurenditerpenol (1S, 6R, 7S, 10R, 11R, 14S, 15R) has been accomplished from eight plausible stereoisomers by its first asymmetric total synthesis in a highly convergent and flexible synthetic pathway. Six stereoisomers of laurenditerpenol were synthesized and evaluated for their biological activity.

Chittiboyina, Amar G.; Kumar, Gundluru Mahesh; Carvalho, Paulo B.; Liu, Yang; Zhou, Yu-Dong; Nagle, Dale G.

2010-01-01

17

Lycopodium alkaloids: isolation and asymmetric synthesis.  

PubMed

Lycopodium alkaloids have attracted the attention of many natural product chemists and synthetic organic chemists due to their important biological activities and unique skeletal characteristics. In this review we describe isolation and asymmetric syntheses of several new alkaloids such as lycoposerramines-C, -V, -W, and cernuine, and show that asymmetric total synthesis played a key role in elucidating the structures of these complex natural products. PMID:21452079

Kitajima, Mariko; Takayama, Hiromitsu

2012-01-01

18

Asymmetric synthesis of tertiary thiols and thioethers  

PubMed Central

Summary Enantiomerically pure tertiary thiols provide a major synthetic challenge, and despite the importance of chiral sulfur-containing compounds in biological and medicinal chemistry, surprisingly few effective methods are suitable for the asymmetric synthesis of tertiary thiols. This review details the most practical of the methods available.

MacLellan, Paul

2011-01-01

19

Absolute Phase Measurement of Ultrashort Pulses by Asymmetric Ionization  

Microsoft Academic Search

Numerical solutions of the time-dependent Schroedinger equation,TDSE for a 3-D H atom in an intense (10**13-10**14 W\\/cm2) ultrashort (3-5 fs) 800nm laser pulse with linear polarization are used to propose a new experimental method to measure the absolute phase of such pulses based on the asymmetry of the ionization produced by such pulses. It will be shown that the asymmetry

Andre D. Bandrauk; Stephane Chelkowski

2004-01-01

20

Synthesis and absolute stereochemistry of the bicyclic goniofupyranone core  

NASA Astrophysics Data System (ADS)

A bicyclic goniofupyranone core was synthesized from argentilactone using epoxide as an intermediate. The absolute stereochemistry of all asymmetric centers of a compound possessing the 2,6-dioxabicyclo [3.2.1]-octan-3-one moiety were unambiguously determined directly from X-ray data for the first time. Additionally, the absolute stereochemistry of the major epoxide was reported.

Alves, Ricardo Justino; Barros, Olga Soares do Rego; Kato, Lucilia; Martins, Felipe Terra; de Oliveira, Cecilia Maria Alves

2013-03-01

21

Asymmetric synthesis of 3,3,5,5-tetrasubstituted 1,2-dioxolanes: total synthesis of epiplakinic acid F.  

PubMed

The first enantioselective total synthesis of epiplakinic acid F (1) was achieved through a pivotal step involving a radical-mediated asymmetric peroxidation of vinylcyclopropanes with molecular oxygen to construct highly substituted 1,2-dioxolanes. Subsequent conversions of the chiral 1,2-dioxolanes led to total synthesis of epiplakinic acid F (1) and the confirmation of its absolute configuration. The enantiomer of epiplakinic acid F methyl ester (2) was also prepared. PMID:24769843

Tian, Xiang-Yin; Han, Jian-Wei; Zhao, Qiong; Wong, Henry N C

2014-06-14

22

Catalytic asymmetric total synthesis of (+)-caprazol.  

PubMed

Catalytic asymmetric total synthesis of caprazol, a lipo-nucleoside antibiotic, has been accomplished employing two of the stereoselective C-C bond forming reactions as key transformations. The stereochemistries of the ?-hydroxy-?-aminoester moiety at the juncture of the uridine part and diazepanone part, and of the ?-hydroxy-?-amino acid moiety embedded in the diazepanone system, were constructed using a diastereoselective isocyanoacetate aldol reaction (dr = 88:12) and an enantioselective anti-nitroaldol reaction catalyzed by a Nd/Na-chiral amide ligand (dr = 12:1, 95% ee), respectively. PMID:24897294

Gopinath, Purushothaman; Wang, Lu; Abe, Hikaru; Ravi, Gandamala; Masuda, Takashi; Watanabe, Takumi; Shibasaki, Masakatsu

2014-06-20

23

Microscale Synthesis of Chiral Alcohols via Asymmetric Catalytic Transfer Hydrogenation  

ERIC Educational Resources Information Center

Synthesis of pure enantiomers is a key issue in industry, especially in areas connected to life sciences. Catalytic asymmetric synthesis has emerged as a powerful and practical tool. Here we describe an experiment on racemic reduction and asymmetric reduction via a catalytic hydrogen transfer process. Acetophenone and substituted acetophenones are…

Peeters, Christine M.; Deliever, Rik; De Vos, Dirk

2009-01-01

24

Catalytic asymmetric synthesis of R207910.  

PubMed

The first asymmetric synthesis of a very promising antituberculosis drug candidate, R207910, was achieved by developing two novel catalytic transformations; a catalytic enantioselective proton migration and a catalytic diastereoselective allylation of an intermediate alpha-chiral ketone. Using 2.5 mol % of a Y-catalyst derived from Y(HMDS)(3) and the new chiral ligand 9, 1.25 mol % of p-methoxypyridine N-oxide (MEPO), and 0.5 mol % of Bu(4)NCl, alpha-chiral ketone 3 was produced from enone 4 with 88% ee. This reaction proceeded through a catalytic chiral Y-dienolate generation via deprotonation at the gamma-position of 4, followed by regio- and enantioselective protonation at the alpha-position of the resulting dienolate. Preliminary mechanistic studies suggested that a Y: 9: MEPO = 2: 3: 1 ternary complex was the active catalyst. Bu(4)NCl markedly accelerated the reaction without affecting enantioselectivity. Enantiomerically pure 3 was obtained through a single recrystallization. The second key catalytic allylation of ketone 3 was promoted by CuF.3PPh(3).2EtOH (10 mol %) in the presence of KO(t)Bu (15 mol %), ZnCl(2) (1 equiv), and Bu(4)PBF(4) (1 equiv), giving the desired diastereomer 2 in quantitative yield with a 14: 1 ratio without any epimerization at the alpha-stereocenter. It is noteworthy that conventional organometallic addition reactions did not produce the desired products due to the high steric demand and a fairly acidic alpha-proton in substrate ketone 3. This first catalytic asymmetric synthesis of R207910 includes 12 longest linear steps from commercially available compounds with an overall yield of 5%. PMID:20481617

Saga, Yutaka; Motoki, Rie; Makino, Sae; Shimizu, Yohei; Kanai, Motomu; Shibasaki, Masakatsu

2010-06-16

25

Asymmetric catalytic synthesis of the proposed structure of trocheliophorolide B  

PubMed Central

A concise catalytic asymmetric synthesis of the proposed structure of Trocheliophorolide B is reported. The synthetic sequence notably features an asymmetric acetaldehyde alkynylation, Ru-catalyzed alder-ene reaction and Zn-ProPhenol ynone aldol condensation. Comparison with the reported data suggests a miss-assignment of the natural product structure.

Quintard, Adrien

2012-01-01

26

Asymmetric Total Synthesis of the Epoxykinamycin FL-120B'**  

PubMed Central

Turn up the heat: An asymmetric total synthesis of the epoxykinamycin FL-120B’ is reported. The synthesis establishes a route to epoxide-containing diazobenzofluorenes which could potentially serve as monomers to the dimeric lomaiviticins. Key steps include Sharpless asymmetric epoxidation, Stille coupling, and intramolecular Friedel-Crafts acylation of atropisomeric carboxylic acids at elevated temperatures to construct the FL-120B’ core structure.

Scully, Stephen S.; Porco, John A.

2012-01-01

27

Total Synthesis, Assignment of the Relative and Absolute Stereochemistry, and Structural Reassignment of Phostriecin (aka Sultriecin)  

PubMed Central

A total synthesis of phostriecin (2) previously known as sultriecin (1), its structural reassignment as a phosphate versus sulfate monoester, and the assignment of its relative and absolute stereochemistry are disclosed herein. Key elements of the work, which provided first the originally assigned sulfate monoester 1 and then the reassigned and renamed phosphate monoester 2, relied on diagnostic 1H NMR spectroscopic properties of the natural product for the assignment of relative and absolute stereochemistry as well as the subsequent structural reassignment, and a convergent asymmetric total synthesis to provide the unequivocal authentic materials. Key steps of the synthetic approach include a Brown allylation for diastereoselective introduction of the C9 stereochemistry, an asymmetric CBS reduction to establish the lactone C5-stereochemistry, diastereoselective oxidative ring expansion of an ?-hydroxyfuran to access the pyran lactone precursor, and single-step installation of the sensitive Z,Z,E-triene unit through a chelation-controlled cuprate addition with installation of the C11 stereochemistry. The approach allows ready access to analogues that can now be used to probe important structural features required for PP2A inhibition, the mechanism of action defined herein.

Burke, Christopher P.; Haq, Nadia; Boger, Dale L.

2010-01-01

28

A Catalytic, Asymmetric Formal Synthesis of (+)-Hamigeran B  

PubMed Central

A concise asymmetric, formal synthesis of (+)-hamigeran B is reported. A Pd-catalyzed, decarboxylative allylic alkylation, employing a trifluoromethylated derivative of t-BuPHOX, is utilized as the enantioselective step to form the critical quaternary carbon center in excellent yield and enantioselectivity. The product is converted in three steps to a late-stage intermediate previously used in the synthesis of hamigeran B.

Mukherjee, Herschel; McDougal, Nolan T.; Virgil, Scott C.

2011-01-01

29

Catalytic Asymmetric Synthesis of Highly Substituted Pyrrolizidines.  

PubMed

A catalytic asymmetric double (1,3)-dipolar cycloaddition reaction has been developed. Using a chiral silver catalyst, enantioenriched pyrrolizidines can be prepared in one flask from inexpensive, commercially available starting materials. The pyrrolizidine products contain a variety of substitution patterns and as many as six stereogenic centers. PMID:23565328

Lim, Andrew D; Codelli, Julian A; Reisman, Sarah E

2012-11-13

30

Catalytic Asymmetric Synthesis of Highly Substituted Pyrrolizidines  

PubMed Central

A catalytic asymmetric double (1,3)-dipolar cycloaddition reaction has been developed. Using a chiral silver catalyst, enantioenriched pyrrolizidines can be prepared in one flask from inexpensive, commercially available starting materials. The pyrrolizidine products contain a variety of substitution patterns and as many as six stereogenic centers.

Lim, Andrew D.; Codelli, Julian A.

2012-01-01

31

Characterization of Lassa Virus Cell Entry Inhibitors: Determination of the Active Enantiomer by Asymmetric Synthesis  

PubMed Central

The comparative characterization of a series of 4-acyl-1,6-dialkylpiperazin-2-ones as potent cell entry inhibitors of the hemorrhagic fever arenavirus Lassa (LASV) is disclosed. The resolution and examination of the individual enantiomers of the prototypical LASV cell entry inhibitor 3 (16G8) is reported and the more potent (–)-enantiomer was found to be 15-fold more active than the corresponding (+)-enantiomer. The absolute configuration of (–)-3 was established by asymmetric synthesis of the active inhibitor (–)-(S)-3 (lassamycin-1). A limited deletion scan of lassamycin-1 defined key structural features required of the prototypical inhibitors.

Whitby, Landon R.; Lee, Andrew M.; Kunz, Stefan; Oldstone, Michael B. A.; Boger, Dale L.

2009-01-01

32

Organocatalyzed asymmetric synthesis of dihydrodibenzofurans based on a dienamine process.  

PubMed

The first organocatalyzed asymmetric method for the synthesis of dihydrodibenzofurans based on a dienamine process has been developed. This two-step protocol works with a broad range of substrates and delivers only the cis-diastereomer in good yield with up to 91% ee. The enantioenriched products have been transformed to highly functionalized and partially hydrogenated dibenzofurans in excellent diastereoselectivities. PMID:24040890

Wang, Zi-Yu; Wong, Wing-Tak; Yang, Dan

2013-10-01

33

Pseudoephenamine: A Practical Chiral Auxiliary for Asymmetric Synthesis  

PubMed Central

Pseudoephenamine is shown to be a versatile chiral auxiliary for asymmetric synthesis. It is free from regulatory restrictions and exhibits remarkable stereocontrol in alkylation reactions, especially those that form quaternary carbon centers. Amides derived from pseudoephenamine exhibit a high propensity to be crystalline substances and provide sharp, well-defined peaks in NMR spectra.

Morales, Marvin R.; Mellem, Kevin T.; Myers, Andrew G.

2013-01-01

34

Asymmetric, stereodivergent synthesis of (-)-clusianone utilizing a biomimetic cationic cyclization.  

PubMed

We report a stereodivergent, asymmetric total synthesis of (-)-clusianone in six steps from commercial materials. We implement a challenging cationic cyclization forging a bond between two sterically encumbered quaternary carbon atoms. Mechanistic studies point to the unique ability of formic acid to mediate the cyclization forming the clusianone framework. PMID:24916169

Boyce, Jonathan H; Porco, John A

2014-07-21

35

Asymmetric synthesis of the aminocyclitol pactamycin, a universal translocation inhibitor.  

PubMed

An asymmetric total synthesis of the aminocyclopentitol pactamycin is described. The title compound is delivered in 15 steps from 2,4-pentanedione. Critical to this approach was the exploitation of a complex symmetry-breaking reduction strategy to assemble the C1, C2, and C7 relative stereochemistry within the first four steps of the synthesis. Multiple iterations of this reduction strategy are described, and a thorough analysis of stereochemical outcomes is detailed. In the final case, an asymmetric Mannich reaction was developed to install a protected amine directly at the C2 position. Symmetry-breaking reduction of this material gave way to a remarkable series of stereochemical outcomes leading to the title compound without recourse to nonstrategic downstream manipulations. This synthesis is immediately accommodating to the preparation of structural analogs. PMID:24245656

Sharpe, Robert J; Malinowski, Justin T; Johnson, Jeffrey S

2013-11-27

36

Catalytic asymmetric reactions using environmentally friendly reagents and catalyst: Asymmetric synthesis versus kinetic resolution  

Microsoft Academic Search

Kinetic resolution of racemic compounds is a familiar method for the preparation of optically active compounds. However, an inevitable and critical drawback from the point of view of green chemistry is the consequent wastage of half of the starting compound. A cat- alytic asymmetric synthesis of 5-arylcyclohex-2-enones was developed starting from racemic 5-(trimethylsilyl)cyclohex-2-enone, which overcomes the drawback of kinetic resolution.

Kiyoshi Tomioka

2006-01-01

37

Phase-transfer-catalysed asymmetric synthesis of tetrasubstituted allenes  

NASA Astrophysics Data System (ADS)

Allenes are molecules based on three carbons connected by two cumulated carbon-carbon double bonds. Given their axially chiral nature and unique reactivity, substituted allenes have a variety of applications in organic chemistry as key synthetic intermediates and directly as part of biologically active compounds. Although the demands for these motivated many endeavours to make axially chiral, substituted allenes by exercising asymmetric catalysis, the catalytic asymmetric synthesis of fully substituted ones (tetrasubstituted allenes) remained largely an unsolved issue. The fundamental obstacle to solving this conundrum is the lack of a simple synthetic transformation that provides tetrasubstituted allenes in the action of catalysis. We report herein a strategy to overcome this issue by the use of a phase-transfer-catalysed asymmetric functionalization of 1-alkylallene-1,3-dicarboxylates with N-arylsulfonyl imines and benzylic and allylic bromides.

Hashimoto, Takuya; Sakata, Kazuki; Tamakuni, Fumiko; Dutton, Mark J.; Maruoka, Keiji

2013-03-01

38

Catalytic asymmetric synthesis of ?-butenolides by direct vinylogous reactions.  

PubMed

The ?-butenolide structural motif is a prominent feature in many bioactive natural products and drugs. This short review summarizes catalytic asymmetric synthesis of ?-butenolides through direct vinylogous reactions by metal complexes and organocatalysts. In light of building chiral ?-tertiary and quaternary carbon centers of butenolides, three synthetic strategies are included: 1) the reactions with furanone derivatives as nucleophiles, 2) olefination of ?,?- disubstituted butenolides and 3) the reactions by using ?,?-disubstituted butenolides as nucleophiles. PMID:23544459

Yan, Lin; Wu, Xiaohong; Liu, Hongjun; Xie, Liangying; Jiang, Zhiyong

2013-05-01

39

The absolute asymmetric photoisomerization of a photochromic dithionite complex in chiral crystals.  

PubMed

An achiral photochromic dithionite (micro-O(2)SSO(2)) complex with n-propyl moieties forms chiral crystals and undergoes an enantioselective photoisomerization (ee > 90%) to an oxysulfur (micro-O(2)SOS*O) complex, containing an asymmetric S atom, with essentially 100% interconversion of the chiral crystals. PMID:19532921

Nakai, Hidetaka; Hatake, Mayu; Miyano, Yousuke; Isobe, Kiyoshi

2009-05-21

40

A Facile Synthesis of Asymmetric Hybrid Colloidal Particles  

PubMed Central

This paper describes a simple approach to the synthesis of asymmetric, hybrid colloidal particles by precipitation polymerization. The key is to introduce Au or Ag colloids 2 min after (rather than before) starting the polymerization. The hybrid particles were uniform in size, and each one of them only contained one Au (or Ag) nanoparticle in its surface. Due to the simplicity of this procedure, it should be possible to use it for large-scale production. This method can be extended to metal nanoparticles other than Au and Ag and with a range of sizes, as long as they have appropriate charges on the surface.

Ohnuma, Akira; Cho, Eun Chul; Camargo, Pedro H. C.; Au, Leslie; Ohtani, Bunsho; Xia, Younan

2009-01-01

41

Component mode synthesis for multibody systems with absolute coordinates  

Microsoft Academic Search

Abstract—In the present paper, the concept of the float- ing frame of reference is transformed to a description based on absolute coordinates. The equations are linearized for a certain configuration of the body based on a corotational strain tensor. Because of the transformation,to absolute coordinates, the mass matrix is constant and the stiffness matrix can be inverted efficiently. Pairs of

J. Gerstmayr

2007-01-01

42

Asymmetric organocatalytic synthesis of 4,6-bis(1H-indole-3-yl)-piperidine-2 carboxylates.  

PubMed

We developed an asymmetric organocatalytic synthesis of 4,6-bis(1H-indole-3-yl)-piperidine-2-carboxylates using 10 mol% of a chiral phosphoric acid. The products, which are novel bisindole-piperidine-amino acid hybrids, can be obtained in one step from 3-vinyl indoles with imino esters in dichloromethane at room temperature after 1 h of reaction time. A variety of these compounds could be synthesized in up to 70% yield and 99% ee, and they were experimentally and computationally analyzed regarding their relative and absolute stereochemistry. PMID:24733144

Zhong, Sabilla; Nieger, Martin; Bihlmeier, Angela; Shi, Min; Bräse, Stefan

2014-05-28

43

High-turnover hypoiodite catalysis for asymmetric synthesis of tocopherols.  

PubMed

The diverse biological activities of tocopherols and their analogs have inspired considerable interest in the development of routes for their efficient asymmetric synthesis. Here, we report that chiral ammonium hypoiodite salts catalyze highly chemo- and enantioselective oxidative cyclization of ?-(2-hydroxyphenyl)ketones to 2-acyl chromans bearing a quaternary stereocenter, which serve as productive synthetic intermediates for tocopherols. Raman spectroscopic analysis of a solution of tetrabutylammonium iodide and tert-butyl hydroperoxide revealed the in situ generation of the hypoiodite salt as an unstable catalytic active species and triiodide salt as a stable inert species. A high-performance catalytic oxidation system (turnover number of ~200) has been achieved through reversible equilibration between hypoiodite and triiodide in the presence of potassium carbonate base. We anticipate that these findings will open further prospects for the development of high-turnover redox organocatalysis. PMID:25035486

Uyanik, Muhammet; Hayashi, Hiroki; Ishihara, Kazuaki

2014-07-18

44

Formal synthesis of the anti-angiogenic polyketide (-)-borrelidin under asymmetric catalytic control.  

PubMed

Borrelidin (1) is a polyketide that possesses extremely potent anti-angiogenesis activity. This paper describes its formal total synthesis by the most efficient route to date. This modular approach takes optimal benefit of asymmetric catalysis and permits the synthesis of analogues; in addition, the high yields and selectivities obtained eliminate the need for separation of stereoisomers. The upper half of borrelidin has been accessed by iterative copper-catalysed asymmetric conjugate addition of methylmagnesium bromide, whereas synthesis of the lower half of the molecule was achieved by relying on asymmetric hydrogenation and cross-methathesis as key steps. PMID:20734306

Madduri, Ashoka V R; Minnaard, Adriaan J

2010-10-11

45

Lipase-catalyzed asymmetric synthesis of desprenyl-carquinostatin A and descycloavandulyl-lavanduquinocin.  

PubMed

An asymmetric synthesis of the core carbazole structure, 6-desprenyl-carquinostatin 3 and 6-descycloavandulyl-lavanduquinocin 3, toward a total synthesis of carquinostatin A (1) and lavanduquinocin (2), has been established. Lipase QLM (Meito) catalyzed enantioselective acetylation of the racemic alcohol 6 gave the (-)-acetate 7 and the (+)-alcohol 6 with high enantioselectivity. The absolute stereochemistry of the (-)- and (+)-alcohol 6 have been determined to be R- and S-configurations, respectively, by the advanced Mosher method. In the same manner, the (-)-acetate 13 and the (+)-alcohol 12 have been obtained from the racemic alcohol 12. The (R)-(-)-acetate 13, derived from the (R)-(-)-acetate 7, was the same as the (-)-acetate 13, which has been determined to be (R)-configuration. Oxidation of the (R)-(-)-acetate 13 followed by hydrolysis afforded (R)-(-)-6-desprenyl-carquinostatin [and (R)-(-)-6-descycloavandulyl-lavanduquinocin] 3. In addition, oxidation of the (S)-(+)-alcohol 12 provided (S)-(+)-3, which is the enantiomer of 6-desprenyl-carquinostatin A (R)-(-)-3. PMID:17603201

Choshi, Tominari; Uchida, Yoshinari; Kubota, Yukiko; Nobuhiro, Junko; Takeshita, Mitsuhiro; Hatano, Takushi; Hibino, Satoshi

2007-07-01

46

Total synthesis of (-)-galanthamine and (-)-lycoramine via catalytic asymmetric hydrogenation and intramolecular reductive Heck cyclization.  

PubMed

A synthetic strategy featuring efficient ruthenium-catalyzed asymmetric hydrogenation of racemic ?-aryloxy cyclic ketone via dynamic kinetic resolution and palladium-catalyzed intramolecular reductive Heck cyclization has been developed for the asymmetric total synthesis of (-)-galanthamine (20.1%, 12 steps) and (-)-lycoramine (40.2%, 10 steps). PMID:22612349

Chen, Ji-Qiang; Xie, Jian-Hua; Bao, Deng-Hui; Liu, Sheng; Zhou, Qi-Lin

2012-06-01

47

First enantioselective total synthesis of (+)-(R)-Pinnatolide using an asymmetric domino allylation reaction.  

PubMed

An efficient total synthesis of (+)-(R)-Pinnatolide is described. As a key step an asymmetric multicomponent domino allylation reaction of methyl levulinate is used to form the quaternary stereogenic center in a highly selective way. PMID:22853329

Tietze, Lutz F; Wolfram, Thomas; Holstein, Julian J; Dittrich, Birger

2012-08-17

48

Total synthesis, proof of absolute configuration, and biosynthetic origin of stylopsal, the first isolated sex pheromone of strepsiptera.  

PubMed

The asymmetric total synthesis of the diastereomers of stylopsal establishes the absolute configuration of the first reported sex pheromone of the twisted-wing parasite Stylops muelleri as (3R,5R,9R)-trimethyldodecanal. The key steps for the diastereo- and enantiodivergent introduction of the methyl groups are two different types of asymmetric conjugate addition reactions of organocopper reagents to ?,?-unsaturated esters, whereas the dodecanal skeleton is assembled by Wittig reactions. The structure of the natural product was confirmed by chiral gas chromatography (GC) techniques, GC/MS and GC/electroantennography (EAD) as well as field tests. An investigation into the biosynthesis of the pheromone revealed that it is likely to be produced by decarboxylation of a 4,6,10-trimethyltridecanoic acid derivative, which was found in substantial amounts in the fat body of the female, but not in the host bee Andrena vaga. This triple-branched fatty acid precursor thus seems to be biosynthesized de novo through a polyketide pathway with two consecutive propionate-propionate-acetate assemblies to form the complete skeleton. The simplified, motionless and fully host-dependent female exploits a remarkable strategy to maximize its reproductive success by employing a relatively complex and potent sex pheromone. PMID:23630024

Lagoutte, Roman; Šebesta, Petr; Jiroš, Pavel; Kalinová, Blanka; Jirošová, Anna; Straka, Jakub; ?erná, Kate?ina; Šobotník, Jan; Cva?ka, Josef; Jahn, Ullrich

2013-06-24

49

Enzyme-assisted asymmetric total synthesis of (-)-podophyllotoxin and (-)-picropodophyllin.  

PubMed

Described is the first catalytic, asymmetric synthesis of (-)-podophyllotoxin and its C(2)-epimer, (-)-picropodophyllin. Asymmetry is achieved via the enzymatic desymmetrization of advanced meso diacetate 20, through PPL-mediated ester hydrolysis. A second key feature of the synthesis is the strategically late introduction of the highly oxygenated natural ring E through an arylcopper species. The successful implementation of this approach augers well for the introduction of other functionalized rings E for future SAR work. The synthesis begins from piperonal, which is fashioned into isobenzofuran (IBF) precursor 14 in three steps (bromination, acetalization, and halogen-metal exchange/hydroxymethylation). Interestingly, treatment of 14 with HOAc in commerical dimethyl maleate (contains 5% dimethyl fumarate) leads to a nearly equimolar mixture of fumarate- (15) and maleate-IBF Diels-Alder adducts (16 and 17), indicating that IBF 11 reacts about 15 times faster with dimethyl fumarate than with dimethyl maleate. With scrupulously pure dimethyl maleate a 2.8:1 endo:exo mixture of maleate DA adducts is still obtained. On the other hand, the desired meso diester 16 is obtained pure and in nearly quantitative yield by employing neat dimethyl acetylene dicarboxylate as the dienophile, followed by catalytic hydrogenation. Reduction (LiAlH(4)) of 16 provides meso diol 19, which is then treated with Ac(2)O, BzCl, and PhCH(2)COCl to provide the corresponding meso diesters, 20-22. Screening of these meso benzoxabicyclo[2.2.1]heptyl substrate candidates across a battery of acyl transfer enzymes leads to an optimized match of diacetate 20 with PPL. Even on 10-20 g scales, asymmetry is efficiently introduced here, yielding the key chiral intermediate, monoacetate 25 (66% isolated yield, 83% corrected yield, 95% ee). Protecting group manipulation and oxidation (Swern) provide aldehyde 27b, which undergoes efficient retro-Michael ring opening to produce dihydronaphthalene 30, in which the C(3) and C(4) stereocenters are properly set. Following several unsuccessful approaches to the intramolecular delivery of ring E (via Claisen rearrangement, Heck-type cyclization, or radical cyclization), a highly diastereoselective, intermolecular conjugate addition of the arylcopper reagent derived from (3,4,5-trimethoxy)phenylmagnesium bromide and CuCN to acyl oxazolidinone 50 was developed (85% yield, only the required alpha-stereochemistry at C(1) is observed). The conjugate addition product is converted to (-)-picropodophyllin in two steps (lactonization, SEM deprotection) or to (-)-podophyllotoxin, in three steps, through the introduction of a C(2)-epimerization step, under Kende conditions, prior to the final conjugate addition. PMID:10814019

Berkowitz, D B; Choi, S; Maeng, J H

2000-02-11

50

Synthesis of asymmetric tetracarboxylic acids and corresponding dianhydrides  

NASA Technical Reports Server (NTRS)

This invention relates to processes for preparing asymmetrical biphenyl tetracarboxylic acids and the corresponding asymmetrical dianhydrides, namely 2,3,3',4'-biphenyl dianhydride (a-BPDA), 2,3,3',4'-benzophenone dianhydride (a-BTDA) and 3,4'-methylenediphthalic anhydride (-MDPA). By cross-coupling reactions of reactive metal substituted o-xylenes or by cross-coupling o-xylene derivatives in the presence of catalysts, this invention specifically produces asymmetrical biphenyl intermediates that are subsequently oxidized or hydrolyzed and oxidized to provide asymmetric biphenyl tetracarboxylic acids in comparatively high yields. These asymmetrical biphenyl tetracarboxylic acids are subsequently converted to the corresponding asymmetrical dianhydrides without contamination by symmetrical biphenyl dianhydrides.

Chuang, Chun-Hua (Inventor)

2008-01-01

51

Studies on the Asymmetric Synthesis of alpha-Amino Acids. Ii. New Systems for Highly Specific Asymmetric Synthesis with Conservation of the Chiral Reagent.  

National Technical Information Service (NTIS)

A new approach for the asymmetric synthesis of alpha-amino acids from alpha-keto acids has been extended with the development of a highly selective stereochemical system. The levorotatory chiral reagent 1-amino-1'-methyl indoline-2-methanol(I), has been p...

E. J. Corey H. S. Sachdev J. Z. Gougoutas W. Saenger

1969-01-01

52

Synthesis of the tetracyclic core of Illicium sesquiterpenes using an organocatalyzed asymmetric Robinson annulation  

PubMed Central

Summary An enantioselective synthesis of the core framework of neurotrophic Illicium majucin-type sesquiterpenes is described here. This strategy is based on an organocatalyzed asymmetric Robinson annulation and provides an efficient approach for a diversity-oriented synthesis of Illicium natural products that holds remarkable therapeutic potential for neurodegenerative diseases.

Trzoss, Lynnie; Xu, Jing; Lacoske, Michelle H

2013-01-01

53

Asymmetric total synthesis of a pentacyclic Lycopodium alkaloid: huperzine-Q.  

PubMed

Right on Q: the first asymmetric total synthesis of (-)-huperzine-Q, which possesses six stereogenic centers and a spiroaminal moiety, has been achieved in 19 steps and 16.4 % overall yield. This synthesis involved a novel stereoselective Pauson-Khand reaction, a vinyl Claisen rearrangement, and a biomimetic spiroaminal formation. TBDPS=tert-butyldiphenylsilyl. PMID:21751323

Nakayama, Atsushi; Kogure, Noriyuki; Kitajima, Mariko; Takayama, Hiromitsu

2011-08-22

54

Asymmetric total synthesis of smyrindiol employing an organocatalytic aldol key step  

PubMed Central

Summary The first organocatalytic asymmetric synthesis of smyrindiol, by using an (S)-proline catalyzed enantioselective intramolecular aldol reaction as the key step, is described. Smyrindiol was synthesized from commercially available 2,4-dihydroxybenzaldehyde in 15 steps, with excellent stereoselectivity (de = 99%, ee = 99%). In the course of this total synthesis a new and mild coumarin assembly was developed.

Fronert, Jeanne; Bisschops, Tom; Boeck, Florian

2012-01-01

55

Synthesis of optically active strigolactones: enzymatic resolution and asymmetric hydroxylation  

Microsoft Academic Search

A synthetic intermediate of an optically active strigolactone analogue was prepared in two ways: enzymatic resolution and asymmetric hydroxylation. The 4-hydroxy tricyclic lactone 4 was enzymatically resolved to give the corresponding enantiomers in an enantiomerically pure state, while the tricyclic lactone 5 was hydroxylated asymmetrically at the 4-position by the action of cytochrome P450 monooxygenase.

Hirosato Takikawa; Hiromasa Imaishi; Aya Tanaka; Satoshi Jikumaru; Mami Fujiwara; Mitsuru Sasaki

2010-01-01

56

Examples of catalytic asymmetric amine synthesis using organic catalysts  

Microsoft Academic Search

The asymmetric construction of optically active amine compounds is of paramount interest for the chemical industry and academic research. This review describes the use of catalytic asymmetric transformations, such as Mannich-type, Michael, cycloaddition and transfer-hydrogenation reactions using small organic molecules (eg, amino acids, secondary amines, peptides, thioureas and phosphoric acids) as catalysts, for the construction of important compounds for medicinal

R. Rios; A. Cordova

2009-01-01

57

Quantifying absolute protein synthesis rates reveals principles underlying allocation of cellular resources.  

PubMed

Quantitative views of cellular functions require precise measures of rates of biomolecule production, especially proteins-the direct effectors of biological processes. Here, we present a genome-wide approach, based on ribosome profiling, for measuring absolute protein synthesis rates. The resultant E. coli data set transforms our understanding of the extent to which protein synthesis is precisely controlled to optimize function and efficiency. Members of multiprotein complexes are made in precise proportion to their stoichiometry, whereas components of functional modules are produced differentially according to their hierarchical role. Estimates of absolute protein abundance also reveal principles for optimizing design. These include how the level of different types of transcription factors is optimized for rapid response and how a metabolic pathway (methionine biosynthesis) balances production cost with activity requirements. Our studies reveal how general principles, important both for understanding natural systems and for synthesizing new ones, emerge from quantitative analyses of protein synthesis. PMID:24766808

Li, Gene-Wei; Burkhardt, David; Gross, Carol; Weissman, Jonathan S

2014-04-24

58

Asymmetric Synthesis of 2,4,5-Trisubstituted ?2-Thiazolines  

PubMed Central

?2-Thiazolines are interesting heterocycles that display a wide variety of biological characteristics. They are also common in chiral ligands used for asymmetric syntheses and as synthetic intermediates. Herein, we present asymmetric routes to 2,4,5-trisubstituted ?2-thiazolines. These ?2-thiazolines were synthesized from readily accessible/commercially available ?,?-unsaturated methyl esters through a Sharpless asymmetric dihydroxylation and an O?N acyl migration reaction as key steps. The final products were obtained in good yields with up to 97% enantiomeric excess.

Bengtsson, Christoffer; Nelander, Hanna; Almqvist, Fredrik

2013-01-01

59

Phosphonic acid functionalized asymmetric phthalocyanines: synthesis, modification of indium tin oxide, and charge transfer.  

PubMed

Metalated and free-base A(3)B-type asymmetric phthalocyanines (Pcs) bearing, in the asymmetric quadrant, a flexible alkyl linker of varying chain lengths terminating in a phosphonic acid (PA) group have been synthesized. Two parallel series of asymmetric Pc derivatives bearing aryloxy and arylthio substituents are reported, and their synthesis and characterization through NMR, combustion analysis, and MALDI-MS are described. We also demonstrate the modification of indium tin oxide (ITO) substrates using the PA functionalized asymmetric Pc derivatives and monitoring their electrochemistry. The PA functionalized asymmetric Pcs were anchored to the ITO surface through chemisorption and their electrochemical properties characterized using cyclic voltammetry to investigate the effects of PA structure on the thermodynamics and kinetics of charge transfer. Ionization energies of the modified ITO surfaces were measured using ultraviolet photoemission spectroscopy. PMID:22047210

Polaske, Nathan W; Lin, Hsiao-Chu; Tang, Anna; Mayukh, Mayunk; Oquendo, Luis E; Green, John T; Ratcliff, Erin L; Armstrong, Neal R; Saavedra, S Scott; McGrath, Dominic V

2011-12-20

60

Asymmetric synthesis of ?-substituted Baylis–Hillman products via lithium amide conjugate addition  

Microsoft Academic Search

A three-step protocol for the asymmetric synthesis of a range of ?-substituted Baylis–Hillman products has been developed. This procedure involves the diastereoselective conjugate addition of lithium (R)-N-methyl-N-(?-methylbenzyl)amide to an ?,?-unsaturated ester to generate an N-protected ?-amino ester in high de. Subsequent asymmetric aldol reaction via deprotonation with LDA, transmetallation with B(OMe)3 and addition of an aldehyde gives a range of

Alexander Chernega; Stephen G. Davies; Dirk. L. Elend; Christian A. P. Smethurst; Paul M. Roberts; Andrew D. Smith; G. Darren Smyth

2007-01-01

61

Total Synthesis of Chiral Biaryl Natural Products by Asymmetric Biaryl Coupling ‡  

PubMed Central

This tutorial review highlights the use of catalytic asymmetric 2-naphthol couplings in total synthesis. The types of chirality, chiral biaryl natural products, prior approaches to chiral biaryl natural products, and other catalytic asymmetric biaryl couplings are outlined. The three main categories of chiral catalysts for 2-naphthol coupling (Cu, V, Fe) are described with discussion of their limitations and advantages. Applications of the copper catalyzed couplings in biomimetic syntheses are discussed including nigerone, hypocrellin, calphostin D, phleichrome, and cercosporin.

Kozlowski, Marisa C.; Morgan, Barbara J.; Linton, Elizabeth C.

2010-01-01

62

Synthesis of a selective estrogen receptor ?-modulator via asymmetric phase-transfer catalysis  

Microsoft Academic Search

An efficient asymmetric synthesis of selective estrogen receptor ?-modulator (S)-4-bromo-9a-butyl-8-chloro-6-fluoro-7-hydroxy-1,2,9,9a-tetrahydro-fluoren-3-one was developed. The route features a chemoselective aromatic chlorination reaction, an asymmetric phase-transfer-catalyzed alkylation of an indanone with efficient ee upgrade by racemate crystallization, and a robust bromination reaction using imidazole as an in situ bromine trap to avoid overreaction. The synthesis proceeds in 34% yield over 8 steps from

Mark A. Huffman; Jonathan D. Rosen; Roger N. Farr; Joseph E. Lynch

2007-01-01

63

A metallocene-pyrrolidinopyridine nucleophilic catalyst for asymmetric synthesis.  

PubMed

[reaction: see text] A highly active chiral 4-aminopyridine nucleophilic catalyst, available in three steps from (S,S)-hexane-2,5-diol, was applied to the asymmetric Steglich rearrangement of O-aceylated azlactones (1 mol % loading, up to 76% ee). PMID:16468763

Nguyen, Huy V; Butler, David C D; Richards, Christopher J

2006-02-16

64

Enantioselective Synthesis of the ABC Ring Motif of Norzoanthamine Based on Asymmetric Robinson Annulation Reactions  

PubMed Central

An enantioselective strategy for the synthesis of tetracyclic motif 5, representing the northern fragment of norzoanthamine, is presented. Key to the strategy is the use of two asymmetric Robinson annulation reactions that produce the tricyclic ABC ring system with excellent stereoselectivity. Further functionalization at the periphery of the C ring produces compound 5 containing six contiguous stereocenters of the natural product.

Nguyen, Thong X.; Dakanali, Marianna; Trzoss, Lynnie; Theodorakis, Emmanuel A.

2011-01-01

65

One-pot asymmetric synthesis of 2- and 2,3-disubstituted tetrahydrofuran derivatives.  

PubMed

A novel and convenient one-pot asymmetric synthesis of 2- and 2,3-disubstituted tetrahydrofurans has been achieved in 56-81% yields and 86-99% ee from aliphatic and aromatic aldehydes via an allyl/crotyl/alkoxyallylboration-hydroboration-iodination-cyclization reaction sequence. PMID:22607087

Ramachandran, P Veeraraghavan; Nair, Hari N G; Gagare, Pravin D

2012-06-15

66

?-Transaminase-catalyzed asymmetric synthesis of unnatural amino acids using isopropylamine as an amino donor.  

PubMed

Isopropylamine is an ideal amino donor for reductive amination of carbonyl compounds by ?-transaminase (?-TA) owing to its cheapness and high volatility of a ketone product. Here we developed asymmetric synthesis of unnatural amino acids via ?-TA-catalyzed amino group transfer between ?-keto acids and isopropylamine. PMID:23897436

Park, Eul-Soo; Dong, Joo-Young; Shin, Jong-Shik

2013-09-25

67

Asymmetric synthesis of ( S)-(?)- N-acetylcolchinol via Ullmann biaryl coupling  

Microsoft Academic Search

A modified Ziegler Ullmann coupling process has been developed as the key step in an effective synthesis of (S)-(?)-N-acetylcolchinol, analogues of which are selective vascular targeting agents with potential importance in cancer chemotherapy. Asymmetric induction is achieved by enamide hydrogenation using FerroTANE catalysts.

Simon D. Broady; Michael D. Golden; John Leonard; James C. Muir; Mickael Maudet

2007-01-01

68

Asymmetric synthesis of trifluoromethylated amines via catalytic enantioselective isomerization of imines.  

PubMed

A new approach toward the asymmetric synthesis of optically active trifluoromethylated amines was enabled by an unprecedented, highly enantioselective catalytic isomerization of trifluoromethyl imines with a new chiral organic catalyst. Not only aryl but also alkyl trifluoromethylated amines could be obtained in high enantioselectivities. PMID:22906148

Wu, Yongwei; Deng, Li

2012-09-01

69

New ureas containing glycosyl and diphenylphosphinyl scaffolds: synthesis and the first attempts to use them in asymmetric synthesis.  

PubMed

Chiral ureas containing glycosyl and diphenylphosphinyl scaffolds were found to be an effective organocatalyst. They were synthesised in high yields by a one-pot tandem Staudinger/aza-Wittig coupling reaction. The first attempts of using them in asymmetric synthesis are presented. Yields of the Morita-Baylis-Hillman reaction were moderate with an enantiomeric excess of up to 80%. PMID:24905550

Porwa?ski, Stanis?aw

2014-07-23

70

Asymmetric total synthesis of onoseriolide, bolivianine, and isobolivianine.  

PubMed

In this article, we describe our efforts on the total synthesis of bolivianine (1) and isobolivianine (2), involving the synthesis of onoseriolide (3). The first generation synthesis of bolivianine was completed in 21 steps by following a chiral resolution strategy. Based on the potential biogenetic relationship between bolivianine (1), onoseriolide (3), and ?-(E)-ocimene (8), the second generation synthesis of bolivianine was biomimetically achieved from commercially available (+)-verbenone in 14 steps. The improved total synthesis features an unprecedented palladium-catalyzed intramolecular cyclopropanation through an allylic metal carbene, for the construction of the ABC tricyclic system, and a Diels-Alder/intramolecular hetero-Diels-Alder (DA/IMHDA) cascade for installation of the EFG tricyclic skeleton with the correct stereochemistry. Transformation from bolivianine to isobolivianine was facilitated in the presence of acid. The biosynthetic mechanism and the excellent regio- and endo selectivities in the cascade are well supported by theoretical chemistry based on the DFT calculations. PMID:24488706

Du, Biao; Yuan, Changchun; Yu, Tianzi; Yang, Li; Yang, Yang; Liu, Bo; Qin, Song

2014-02-24

71

Asymmetric synthesis catalyzed by chiral ferrocenylphosphine-metal complexes  

Microsoft Academic Search

Optically active ferrocenylphosphines containing, on the side chain, hydroxy group appropriately distant from the ferrocene nucleus have been found to be effective ligands for the palladium-catalyzed asymmetric allylic alkylation of 1,3-disubstituted 2-propenyl acetates with sodium acetylacetonate and related soft carbon nucleophiles to give the alkylation products of up to 96% ee. chiral ferrocenylphosphines which have tertiary amines on the side

Tamio Hayashi

1988-01-01

72

Inherently Chiral Calixarenes: Synthesis, Optical Resolution, Chiral Recognition and Asymmetric Catalysis  

PubMed Central

Inherently chiral calixarenes, whose chirality is based on the absence of a planar symmetry or an inversion center in the molecules as a whole through the asymmetric array of several achiral groups upon the three-dimensional calix-skeletons, are challenging and attractive chiral molecules, because of their potential in supramolecular chemistry. The synthesis and optical resolution of all varieties of inherently chiral calixarenes are systematically discussed and classified, and their applications in chiral recognition and asymmetric catalysis are thoroughly illustrated in this review.

Li, Shao-Yong; Xu, Yao-Wei; Liu, Jun-Min; Su, Cheng-Yong

2011-01-01

73

Unusual amino acids. I: Asymmetric synthesis of furylalanine derivatives.  

PubMed

Nonproteinogenic amino acids are valuable active compounds from their pharmacological and biochemical effects and also as novel building blocks for peptides. The preparation of furylalanine derivatives by asymmetric hydrogenation is described. Amino-phosphine-phosphinite-rhodium complexes catalyzed the hydrogenation of the prochiral dehydroamino acid precursors in high rate and with enantioselectivities of 70-90% ee. Substrate-catalyst ratios up to 2,000 can be used depending on the catalyst applied. The procedure turns out to be suitable for larger scale preparations. PMID:1616825

Krause, H W; Wilcke, F W; Kreuzfeld, H J; Döbler, C

1992-01-01

74

Asymmetric Total Synthesis of (+)- and ent-(-)-Yatakemycin and Duocarmycin SA: Evaluation of Yatakemycin Key Partial Structures and its Unnatural Enantiomer  

PubMed Central

Complementary to studies that provided the first yatakemycin total synthesis resulting in its structure revision and absolute stereochemistry assignment, a second generation asymmetric total synthesis is disclosed herein. Since the individual yatakemycin subunits are identical to those of duocarmycin SA (alkylation subunit) or CC-1065 (central and right-hand subunits), the studies also provide an improvement in our earlier total synthesis of CC-1065 and, as detailed herein, have been extended to an asymmetric total synthesis of (+)-duocarmycin SA. Further extensions of the studies provided key yatakemycin partial structures and analogues for comparative assessments. This included the definition of the DNA selectivity (adenine central to a five base-pair AT sequence, eg. 5?-AAAAA), efficiency, relative rate, and reversibility of ent-(?)-yatakemycin and its comparison with the natural enantiomer (identical selectivity and efficiency), structural characterization of the adenine N3 adduct confirming the nature of the DNA reaction, and comparisons of the cytotoxic activity of the natural product (L1210 IC50 = 5 pM) with its unnatural enantiomer (IC50 = 5 pM) and a series of key partial structures including those that probe the role of the C-terminus thiomethyl ester. The only distinguishing features between the enantiomers is that ent-(?)-yatakemycin alkylates DNA at a slower rate (krel = 0.13) and is reversible, whereas (+)-yatakemycin is not. Nonetheless, even ent-(?)-yatakemycin alkylates DNA at a faster rate and with a greater thermodynamic stability than (+)-duocarmycin SA illustrating the unique characteristics of such “sandwiched” agents.

Tichenor, Mark S.; Trzupek, John D.; Kastrinsky, David B.; Shiga, Futoshi; Hwang, Inkyu; Boger, Dale L.

2008-01-01

75

The synthesis and metabolism of asymmetric dimethylarginine (ADMA)  

Microsoft Academic Search

Numerous reports have indicated that the plasma concentration of endogenously produced inhibitors of nitric oxide synthase\\u000a are elevated in human disease states. In this review we discuss recent advances in our understanding of the enzymes responsible\\u000a for the synthesis and metabolism of these inhibitors.

James M. Leiper; Patrick Vallance

2006-01-01

76

Large-scale asymmetric synthesis of a cathepsin S inhibitor.  

PubMed

A potent reversible inhibitor of the cysteine protease cathepsin-S was prepared on large scale using a convergent synthetic route, free of chromatography and cryogenics. Late-stage peptide coupling of a chiral urea acid fragment with a functionalized aminonitrile was employed to prepare the target, using 2-hydroxypyridine as a robust, nonexplosive replacement for HOBT. The two key intermediates were prepared using a modified Strecker reaction for the aminonitrile and a phosphonation-olefination-rhodium-catalyzed asymmetric hydrogenation sequence for the urea. A palladium-catalyzed vinyl transfer coupled with a Claisen reaction was used to produce the aldehyde required for the side chain. Key scale up issues, safety calorimetry, and optimization of all steps for multikilogram production are discussed. PMID:20102230

Lorenz, Jon C; Busacca, Carl A; Feng, XuWu; Grinberg, Nelu; Haddad, Nizar; Johnson, Joe; Kapadia, Suresh; Lee, Heewon; Saha, Anjan; Sarvestani, Max; Spinelli, Earl M; Varsolona, Rich; Wei, Xudong; Zeng, Xingzhong; Senanayake, Chris H

2010-02-19

77

Enantioselective synthesis of (s)- and (R)-tolterodine by asymmetric hydrogenation of a coumarin derivative obtained by a Heck reaction.  

PubMed

An efficient and short enantioselective synthesis of (S)- and (R)-tolterodine was performed by asymmetric hydrogenation of a coumarin intermediate, easily obtained by a Heck reaction from inexpensive and commercially available starting materials. PMID:17625882

Ulgheri, Fausta; Marchetti, Mauro; Piccolo, Oreste

2007-08-01

78

Supported liquid membrane as a novel tool for driving the equilibrium of ?-transaminase catalyzed asymmetric synthesis.  

PubMed

An attractive option to produce chiral amines of industrial importance is through asymmetric synthesis using ?-transaminase. However, reaching high yields often requires a strategy for shifting the equilibrium position. This paper describes a novel strategy for handling this problem. It involves the use of a supported liquid membrane (SLM) together with a packed bed reactor. The reactor contains Escherichia coli cells with ?-transaminase from Arthrobacter citreus, immobilized by flocculation with chitosan. The SLM consists of a hollow fibre membrane contactor in which the pores contain undecane. The system enables continuous extraction of the amine product and was used to successfully shift the equilibrium in asymmetric synthesis of (S)-?-methylbenzylamine (MBA). A conversion of 98% was reached, compared to 50% without product extraction. Moreover, a selective extraction of the produced MBA was realized. A high product concentration of 55g/l was reached after 80h, and the system showed promising potential for continuous operation. PMID:24675224

Rehn, Gustav; Adlercreutz, Patrick; Grey, Carl

2014-06-10

79

Asymmetric synthesis and effect of absolute stereochemistry of YCZ-2013, a brassinosteroid biosynthesis inhibitor.  

PubMed

The four stereoisomers of 2RS,4RS-1-[[2-(2,4-dichlorophenyl)-4-(2-(2-propenyloxy)phenoxymethyl)-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole (YCZ-2013), a novel brassinosteroid biosynthesis inhibitor, were prepared. The diastereomers of 2RS,4R-5 and 2RS,4S-5 were prepared by using the corresponding optically pure R and S toluene-4-sulfonic acid 2,3-dihydroxypropyl ester (R-4,S-4). The enatiomerically and diastereomerically pure acetonide (5) was obtained by a method involving diastereoselective crystallisation of the tosylate salt, followed by re-equilibration with the mother liquor and chromatography. The optical purity of four target compounds (YCZ-2013) was confirmed by chiral high-performance liquid chromatography (HPLC) and NMR. The effects of these stereoisomers on Arabidopsis stem elongation indicated that the cis isomers of 2S,4R-YCZ-2013 and 2R,4S-YCZ-2013 exhibited potent inhibitory activity with IC50 values of approximately 24 ± 3 and 24 ± 2 nM, respectively. The IC50 values of the trans isomers of 2S,4S-YCZ-2013 and 2R,4R-YCZ-2013 are approximately 1510 ± 50 and 3900 ± 332 nM, respectively. Co-application of brassinolide (10nM), the most potent BR, and GA3 (1 ?M) to Arabidopsis seedlings grown in the dark with 2R,4S-YCZ-2013 and 2S,4R-YCZ-2013 revealed that brassinolide recovered the induced dwarfism of Arabidopsis seedlings, whereas GA3 showed no effect. PMID:24269478

Oh, Keimei; Yamada, Kazuhiro; Yoshizawa, Yuko

2013-12-15

80

Asymmetric synthesis of ?-alkyl ?-selenocarbonyl compounds catalyzed by bifunctional organocatalysts.  

PubMed

A new organocatalytic approach for the synthesis of a variety of ?-alkyl, ?-phenylselenyl ketones as well as their corresponding esters and amides, by the addition of ?-selenocarbonyl derivatives to nitroalkenes catalyzed by thiourea or squaramide cinchona catalysts, is presented. This catalytic system allows the preparation in high yields of enantiomerically enriched selenocarbonyl derivatives bearing two chiral centers with excellent ee's and dr's by using catalytic loadings of 3 mol%. PMID:21591622

Marcos, Vanesa; Alemán, Jose; Ruano, Jose L Garcia; Marini, Francesca; Tiecco, Marcello

2011-06-17

81

Asymmetric Synthesis of Spiropyrazolones through Phosphine-Catalyzed [4+1] Annulation.  

PubMed

An enantioselective synthesis of spiropyrazolones from allenoate-derived MBH acetates and pyrazolones through a phosphine-mediated [4+1] annulation process has been developed. Spiropyrazolones were readily prepared in good chemical yields and good to high enantioselectivities. This is the first asymmetric example in which ?-substituted allenoates were utilized as a C4 synthon for phosphine-catalyzed [4+1] annulation. PMID:24737675

Han, Xiaoyu; Yao, Weijun; Wang, Tianli; Tan, Yong Ren; Yan, Ziyu; Kwiatkowski, Jacek; Lu, Yixin

2014-05-26

82

Asymmetric synthesis of ?,?-disubstituted amino acids by cycloaddition of (E)-ketonitrones with vinyl ethers.  

PubMed

Original acyclic (E)-?,?-dialkylketonitrones bearing a chiral auxiliary on their nitrogen atom were synthesized and successfully employed for the asymmetric synthesis of ?,?-disubstituted amino acids using regio- and stereocontrolled 1,3-dipolar cycloaddition reactions with vinyl ethers. N-Glycosyl chiral auxiliaries were found to provide excellent exo- and ?-facial stereocontrol. The obtained enantiopure cycloadducts were selectively transformed into functional ?,?-disubstituted amino acids and related ?-peptides through the highly regioselective opening of an intermediate quaternary anhydride. PMID:24650145

Zhang, Xiaofei; Cividino, Pascale; Poisson, Jean-François; Shpak-Kraievskyi, Pavlo; Laurent, Mathieu Y; Martel, Arnaud; Dujardin, Gilles; Py, Sandrine

2014-04-01

83

Asymmetric synthesis of a C1?C19 fragment of ulapualide A  

Microsoft Academic Search

The stereocontrolled synthesis of a C1–C19 fragment of ulapualide A has been accomplished. The C3 hydroxyl-bearing stereocenter was established by Jacobsen's hydrolytic kinetic resolution (HKR) of a terminal epoxide, while the C9 methyl stereocenter was introduced through an asymmetric crotylation using chiral organosilane (S)-7. Union of the C1?C6 and the C7?C19 fragments by a Kishi–Nozaki coupling, followed by oxidation and

Cassandra A. Celatka; James S. Panek

2002-01-01

84

Concise, Asymmetric, Stereocontrolled Total Synthesis of Stephacidins A, B and Notoamide B  

PubMed Central

Concise asymmetric total syntheses of the fungal metabolites (?)-stephacidin A, (+)-stephacidin B, and (+)-notoamide B are described. Key features of these total syntheses include (1) a facile synthesis of (R)-allyl proline methyl ester, (2) a revised route toward the pyranoindole ring system, (3) a novel cross-metathesis strategy for the introduction of important functional groups, and (4) an SN2? cyclization to form the [2.2.2] bridged bicyclic ring system. Furthermore, our synthesis has taken advantage of microwave heating to shorten reaction times as well as increase yields for the preparation of vital intermediates.

Artman, Gerald D.; Grubbs, Alan W.; Williams, Robert M.

2007-01-01

85

Synthesis of (+)-CP-99,994 via Pd(0)-catalyzed asymmetric allylic and homoallylic C-H Diamination of terminal olefin.  

PubMed

This paper describes an asymmetric synthesis of the potent substance P receptor antagonist (+)-CP-99,994 from 4-phenyl-1-butene via Pd(0)-catalyzed asymmetric allylic and homoallylic C-H diamination. PMID:19778085

Fu, Renzhong; Zhao, Baoguo; Shi, Yian

2009-10-01

86

Practical Catalytic Asymmetric Synthesis of Diaryl-, Aryl Heteroaryl- and Diheteroarylmethanols  

PubMed Central

Enantioenriched diaryl-, aryl heteroaryl- and diheteroarylmethanols exhibit important biological and medicinal properties. One-pot catalytic asymmetric syntheses of these compounds beginning from readily available aryl bromides are introduced. Thus, lithium-bromide exchange with commercially available aryl bromides and n-BuLi was followed by salt metathesis with ZnCl2 to generate ArZnCl. A second equivalent of n-BuLi was added to form the mixed organozinc, ArZnBu. In the presence of enantioenriched amino alcohol-based catalysts, ArZnBu adds to aldehydes to afford essentially racemic diarylmethanols. The low enantioselectivities were attributed to a LiCl-promoted background reaction. To inhibit this background reaction, the chelating diamine TEEDA (tetraethylethylene diamine) was introduced prior to aldehyde addition. Under these conditions, enantioenriched diarylmethanols were obtained with >90% ee. Arylations of enals generated allylic alcohols with 81–90% ee. This procedure was unsuccessful, however, when applied to heteraryl bromides, which was attributed to decomposition of the heteroaryl lithium under the salt metathesis conditions. To avoid this problem, the metathesis was conducted with EtZnCl, which enabled the salt metathesis to proceed at low temperatures. The resulting EtZn(ArHetero) intermediates (ArHetero=2- and 3-thiophenyl, 2-benzothiophenyl, 3-furyl, and 5-indolyl) were successfully added to aldehydes and heteroaryl aldehydes with enantioselectivities between 81–99%. These are the first examples of catalytic and highly enantioselective syntheses of diheteroarylmethanols. In a similar fashion, ferrocenyl bromide was used to generate FcZnEt and the ferrocenyl group added to benzaldehyde and heteroaromatic aldehydes to form ferrocene-based ligand precursors in 86–95% yield with 96–98% ee. It was also found that the arylation and heteroarylation of enals could be followed by diastereoselective epoxidations to provide epoxy alcohols with high enantio- and diastereoselectivities in a one-pot procedure.

Salvi, Luca; Kim, Jeung Gon; Walsh, Patrick J.

2009-01-01

87

The origins of homochirality examined by using asymmetric autocatalysis.  

PubMed

Pyrimidyl alkanol was found to act as an asymmetric autocatalyst in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde. Asymmetric autocatalysis of 2-alkynylpyrimidyl alkanol with an extremely low enantiomeric excess (ca. 0.00005% ee) exhibits enormous asymmetric amplification to afford the same compound with >99.5% ee. This asymmetric autocatalysis with amplification of ee has been employed to examine the validity of proposed theories of the origins of homochirality. Circularly polarized light, quartz, sodium chlorate, cinnabar, chiral organic crystals and spontaneous absolute asymmetric synthesis were considered as possible candidates for the origin of chirality; each could act as a chiral source in asymmetric autocatalysis. Asymmetric autocatalysis can discriminate the isotope chirality arising from the small difference between carbon (carbon-13/carbon-12) and hydrogen (D/H) isotopes. Cryptochiral compounds were also discriminated by asymmetric autocatalysis. PMID:24449510

Soai, Kenso; Kawasaki, Tsuneomi; Matsumoto, Arimasa

2014-02-01

88

Enantioselective Total Synthesis and Confirmation of the Absolute and Relative Stereochemistry of Streptorubin B  

PubMed Central

The enantioselective total synthesis of the pyrrolophane natural product streptorubin B is described. Key steps in the concise route include application of a one-pot enantioselective aldol cyclization/Wittig reaction and an anionic oxy-Cope rearrangement to forge the crucial 10-membered ring. Comparisons between CD-spectra of synthetic and natural samples of streptorubin B, coupled with X-ray crystallography, allowed for the determination of the absolute stereochemistry of this natural product for the first time. These studies also provided unambiguous proof of the relative configuration between the butyl sidechain and bis-pyrrole subunit. Additional studies revealed a novel atropstereoselective Paal–Knorr pyrrole condensation and provided fundamental experimental insight into the barrier for atropisomerization of the natural product.

Hu, Dennis X.; Clift, Michael D.; Lazarski, Kiel E.; Thomson, Regan J.

2010-01-01

89

Catalytic asymmetric synthesis of α-amino phosphonates using lanthanoid-potassium-BINOL complexes  

Microsoft Academic Search

α-Amino phosphonic acids 3 are interesting compounds in the design of enzyme inhibitors. The concept of mimicking tetrahedral transition states of enzyme-medicated peptide bond hydrolysis previously led to the successful design and synthesis of phosphonamide-containing peptides as a promising new class of proteinase inhibitors. It is not surprising that the absolute configuration of the α-carbon strongly influences the biological properties

Hiroaki Sasai; Shigeru Arai; Masakatsu Shibasaki; Masakatsu Shibasaki

1995-01-01

90

Asymmetric synthesis of alpha-branched primary amines on solid support via novel hydrazine resins.  

PubMed

[reaction: see text]. Two novel chiral hydrazine resins for asymmetric solid-phase synthesis have been developed. The enantiopure beta-methoxyamino auxiliaries, derived from trans-4-hydroxy-(S)-proline and (R)-leucine, were attached to Merrifield resin and transformed into their corresponding hydrazines. Immobilization of various aldehydes, followed by 1,2-addition of organolithium reagents to the resulting enantiopure hydrazones and reductive cleavage from the solid support, furnished alpha-branched amines, which were isolated as their corresponding amides in good overall yields and enantiomeric excesses of up to 86%. PMID:11348204

Enders, D; Kirchhoff, J H; Köbberling, J; Peiffer, T H

2001-04-19

91

Synthesis, physical properties, and self-assembly of a novel asymmetric aroyleneimidazophenazine.  

PubMed

The synthesis, physical properties, and self-assembly of a novel asymmetric aroyleneimidazophenazine (IZ1) is reported. The as-prepared IZ1 nanowires display an obvious red fluorescence. A heterojunction light-emitting diode (LED) device with the structure ITO/IZ1 nanowires/p-SiC/Al (10 nm)/Ti (80 nm)/Al (380 nm)/ITO was fabricated, and electroluminescence emission with two peaks at about 412 nm and 613 nm was detected with a forward bias ranging from 5 to 10 V. PMID:23362107

Zhao, Jianfeng; Wong, Jen It; Wang, Chengyuan; Gao, Junkuo; Ng, Verena Zi Yun; Yang, Hui Ying; Loo, Say Chye Joachim; Zhang, Qichun

2013-03-01

92

Highly enantioselective synthesis of ?-, ?-, and ?-chiral 1-alkanols via Zr-catalyzed asymmetric carboalumination of alkenes (ZACA)-Cu- or Pd-catalyzed cross-coupling.  

PubMed

Despite recent advances of asymmetric synthesis, the preparation of enantiomerically pure (?99% ee) compounds remains a challenge in modern organic chemistry. We report here a strategy for a highly enantioselective (?99% ee) and catalytic synthesis of various ?- and more-remotely chiral alcohols from terminal alkenes via Zr-catalyzed asymmetric carboalumination of alkenes (ZACA reaction)-Cu- or Pd-catalyzed cross-coupling. ZACA-in situ oxidation of tert-butyldimethylsilyl (TBS)-protected ?-alkene-1-ols produced both (R)- and (S)-?,?-dioxyfunctional intermediates (3) in 80-88% ee, which were readily purified to the ?99% ee level by lipase-catalyzed acetylation through exploitation of their high selectivity factors. These ?,?-dioxyfunctional intermediates serve as versatile synthons for the construction of various chiral compounds. Their subsequent Cu-catalyzed cross-coupling with various alkyl (primary, secondary, tertiary, cyclic) Grignard reagents and Pd-catalyzed cross-coupling with aryl and alkenyl halides proceeded smoothly with essentially complete retention of stereochemical configuration to produce a wide variety of ?-, ?-, and ?-chiral 1-alkanols of ?99% ee. The M?NP ester analysis has been applied to the determination of the enantiomeric purities of ?- and ?-chiral primary alkanols, which sheds light on the relatively undeveloped area of determination of enantiomeric purity and/or absolute configuration of remotely chiral primary alcohols. PMID:24912191

Xu, Shiqing; Oda, Akimichi; Kamada, Hirofumi; Negishi, Ei-Ichi

2014-06-10

93

Symmetry Breaking in Asymmetric Autocatalysis of Pyrimidyl Alkanol  

SciTech Connect

Asymmetric autocatalysis is a reaction in which chiral product acts as a chiral catalyst for its own production. Pyrimidyl alkanol was found to act as asymmetric autocatalysts with significant amplification of enantiopurity in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde. Symmetry breaking is observed in the reaction between the aldehyde and diisopropylzinc without adding chiral substance. Enantioenriched pyrimidyl alkanol was obtained with stochastic distribution of R and S-enantiomers. This is a unique example of spontaneous absolute asymmetric synthesis. Asymmetric autocatalysis is also described using chiral initiators such as circularly polarized light, quartz, chiral crystal of achiral cytosine.

Soai, Kenso; Kawasaki, Tsuneomi [Department of Applied Chemistry, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)

2008-03-05

94

An enantioselective synthesis of the key intermediate for triazole antifungal agents; application to the catalytic asymmetric synthesis of efinaconazole (Jublia).  

PubMed

A new synthetic route, the shortest reported to date, to access a key intermediate for the synthesis of various triazole antifungal agents was developed. The elusive tetrasubstituted stereogenic center that is essential in advanced triazole antifungal agents was constructed via the catalytic asymmetric cyanosilylation of a ketone. The subsequent transformations were performed in two one-pot operations, enhancing the overall synthetic efficiency toward the intermediate. This streamlined synthetic approach was successfully applied to efficient enantioselective syntheses of efinaconazole (Jublia) and ravuconazole. PMID:24635354

Tamura, Keiji; Kumagai, Naoya; Shibasaki, Masakatsu

2014-04-01

95

The Development of the Enantioselective Addition of Ethyl Diazoacetate to Aldehydes: Asymmetric Synthesis of 1,2-Diols  

PubMed Central

A novel synthetic strategy towards the asymmetric synthesis of vicinal diols bearing a tertiary center is presented. The method encompasses the dinuclear Mg-catalyzed asymmetric addition of ethyl diazoacetate into several aldehydes, oxidation of the diazo functionality, and diastereoselective alkyl transfer of various organometallics into the resulting chiral ?-hydroxy-?-ketoesters to afford a diverse range of 1,2-diols in high yield, diastereoselectivity, and chirality transfer.

Trost, Barry M.; Malhotra, Sushant; Koschker, Philipp; Ellerbrock, Pascal

2011-01-01

96

Engineering methylaspartate ammonia lyase for the asymmetric synthesis of unnatural amino acids  

NASA Astrophysics Data System (ADS)

The redesign of enzymes to produce catalysts for a predefined transformation remains a major challenge in protein engineering. Here, we describe the structure-based engineering of methylaspartate ammonia lyase (which in nature catalyses the conversion of 3-methylaspartate to ammonia and 2-methylfumarate) to accept a variety of substituted amines and fumarates and catalyse the asymmetric synthesis of aspartic acid derivatives. We obtained two single-active-site mutants, one exhibiting a wide nucleophile scope including structurally diverse linear and cyclic alkylamines and one with broad electrophile scope including fumarate derivatives with alkyl, aryl, alkoxy, aryloxy, alkylthio and arylthio substituents at the C2 position. Both mutants have an enlarged active site that accommodates the new substrates while retaining the high stereo- and regioselectivity of the wild-type enzyme. As an example, we demonstrate a highly enantio- and diastereoselective synthesis of threo-3-benzyloxyaspartate (an important inhibitor of neuronal excitatory glutamate transporters in the brain).

Raj, Hans; Szyma?ski, Wiktor; de Villiers, Jandré; Rozeboom, Henriëtte J.; Veetil, Vinod Puthan; Reis, Carlos R.; de Villiers, Marianne; Dekker, Frank J.; de Wildeman, Stefaan; Quax, Wim J.; Thunnissen, Andy-Mark W. H.; Feringa, Ben L.; Janssen, Dick B.; Poelarends, Gerrit J.

2012-06-01

97

Multi-component Cycloaddition Approaches in the Catalytic Asymmetric Synthesis of Alkaloid Targets†  

PubMed Central

Cycloaddition reactions are attractive strategies for rapid formation of molecular complexity in organic synthesis as multiple bonds are formed in a single process. To this end, several research groups have been actively involved in the development of catalytic methods to activate readily accessible ?-components to achieve cycloadditions. However, the use of C-N ?-components for the formation of heterocycles by these processes is less well developed. It has been previously demonstrated that the combination of different isocyanates with two alkynes yields pyridones of several types by metal-catalyzed [2+2+2] cycloadditions. The potential of this chemistry has been extended to alkenes as C-C ?-components, allowing the formation of sp3-stereocenters. In this tutorial review directed towards [n+2+2] cycloaddition of heterocumulenes, alkynes and alkenes, the recent advances in catalytic asymmetric synthesis of indolizidine, quinolizidine and azocine skeletons are discussed.

Perreault, Stephane; Rovis, Tomislav

2010-01-01

98

Investigation of Lewis acid versus Lewis base catalysis in asymmetric cyanohydrin synthesis.  

PubMed

The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}(2)] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO(3) or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}(2)O] and triphenylphosphine oxide, a non-linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron-deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case. PMID:20715204

North, Michael; Omedes-Pujol, Marta; Williamson, Courtney

2010-10-01

99

Adding a Functional Handle to Nature's Building Blocks: The Asymmetric Synthesis of ?-Hydroxy-?-Amino Acids.  

PubMed

?-Hydroxy-?-amino acids are not only used by synthetic chemists but are also found in natural products, many of which show anti-microbial or anti-cancer properties. Over the past 30?years, chemists have searched for many asymmetric routes to these useful building blocks. Initial attempts to synthesize these compounds utilized chiral auxiliaries and the reactions of glycine equivalents with aldehydes to form two stereocenters in one step. Other methods with the formation of specific intermediates or that were aimed at a specific amino acid have also been investigated. Asymmetric hydrogenation by dynamic kinetic resolution has emerged as a high-yielding method for the synthesis of an array of modified amino acids with good stereoselectivity. More recently, amino-acid functionalization and multicomponent reactions have increased the atom economy and simplified many long and difficult routes. In this Focus Review, many of the elegant syntheses of these compounds are explored. The applications of ?-hydroxy-?-amino acids in natural-product synthesis are also mentioned. PMID:24840231

Zhang, Yinfeng; Farrants, Helen; Li, Xuechen

2014-07-01

100

Synthesis, structure revision, and absolute configuration of (+)-didemniserinolipid B, a serinol marine natural product from a tunicate Didemnum sp.  

PubMed

[structure: see text] En route to proving the absolute and relative stereochemistry, through synthesis, of (+)-didemniserinolipid B (1), the first natural serinolipid isolated from a tunicate Didemnum sp., it was discovered that the isolated natural product was in fact the 31-sulfate configured 8R,9R,10R,13S,30S. This structural reassignment was only possible after the development of a microwave-assisted method for the sulfation of unreactive hydroxyl groups. PMID:12227754

Kiyota, Hiromasa; Dixon, Darren J; Luscombe, Christine K; Hettstedt, Stephan; Ley, Steven V

2002-09-19

101

Catalytic asymmetric amination of N-nonsubstituted ?-alkoxycarbonyl amides: concise enantioselective synthesis of mycestericin F and G.  

PubMed

In an attempt to explore the synthetic utility of a ternary asymmetric catalyst comprising La(NO(3))(3)·6H(2)O, amide-based ligand (R)-L1, and D-valine tert-butyl ester H-D-Val-OtBu, we investigated a catalytic, asymmetric amination of functionalized N-nonsubstituted ?-alkoxycarbonyl amides using di-tert-butyl azodicarboxylate as an electrophilic aminating reagent. A highly functionalized, cyclic N-nonsubstituted ?-alkoxycarbonyl amide delivered the desired amination product in up to 96% enantiometric excess, with the requisite functionalities of the polar heads of sphingosines with the appropriate stereochemical arrangement. The rapid asymmetric assembly of these functional groups allowed a concise enantioselective synthetic route to sphingosines to be established with a broad flexibility towards derivative synthesis. These studies have culminated in an efficient catalytic enantioselective total synthesis of immunosuppressive fungal metabolites mycestericin?F (3a) and G (3b). PMID:21274942

Berhal, Farouk; Takechi, Sho; Kumagai, Naoya; Shibasaki, Masakatsu

2011-02-01

102

Asymmetric synthesis of 3,5-dihydroxy-6( E)-heptenoate-containing HMG-CoA reductase inhibitors  

Microsoft Academic Search

A ‘one-pot’ conversion of aldehyde 6 to hydroxyketoester 10 with high enantioselection, culminating in a practical asymmetric synthesis of (3R,5S) isomer of the antihyperlipoproteinemic agent fluvastatin, 1, is described. All four 3,5-dihydroxy-6(E)-heptenoate stereoisomers were prepared in enantiopure form starting from 10, utilizing selective reduction and oxidation methods.

Orin Tempkin; Stephan Abel; Chung-Pin Chen; Russell Underwood; Kapa Prasad; Kau-Ming Chen; Oljan Repic; Thomas J Blacklock

1997-01-01

103

Development of an Asymmetric Trimethylenemethane Cycloaddition Reaction: Application in the Enantioselective Synthesis of Highly Substituted Carbocycles  

PubMed Central

A protocol for the enantioselective [3+2] cycloaddition of trimethylenemethane with electron-deficient olefins has been developed. The synthesis of novel phosphoramidite ligands was critical in this effort, and the preparation and reactivity of these ligands is detailed. The evolution of the ligand design, commencing with acyclic amine-derived phosphoramidites and leading to cyclic pyrrolidine and azetidine structures is discussed. The conditions developed to effect an asymmetric TMM reaction using 2-trimethylsilylmethyl allyl acetate were shown to be tolerant of a wide variety of alkene acceptors, providing the desired methylenecyclopentanes with high levels of enantioselectivity. The donor scope was also explored and substituted systems were tolerated, including one bearing a nitrile moiety. These donors were reactive with unsaturated acylpyrroles, giving the product cyclopentane rings bearing three stereocenters in high enantioselectivity and complete diastereoselectivity.

Trost, Barry M.; Silverman, Steven M.; Stambuli, James P.

2011-01-01

104

Catalytic asymmetric synthesis of tertiary alkyl fluorides: Negishi cross-couplings of racemic ?,?-dihaloketones.  

PubMed

The development of new approaches to the construction of fluorine-containing target molecules is important for a variety of scientific disciplines, including medicinal chemistry. In this Article, we describe a method for the catalytic enantioselective synthesis of tertiary alkyl fluorides through Negishi reactions of racemic ?-halo-?-fluoroketones, which represents the first catalytic asymmetric cross-coupling that employs geminal dihalides as electrophiles. Thus, selective reaction of a C-Br (or C-Cl) bond in the presence of a C-F bond can be achieved with the aid of a nickel/bis(oxazoline) catalyst. The products of the stereoconvergent cross-couplings, enantioenriched tertiary ?-fluoroketones, can be converted into an array of interesting organofluorine compounds. PMID:24678878

Liang, Yufan; Fu, Gregory C

2014-04-01

105

Asymmetric phosphorylation through catalytic P(III) phosphoramidite transfer: Enantioselective synthesis of d-myo-inositol-6-phosphate  

PubMed Central

Despite the ubiquitous use of phosphoramidite chemistry in the synthesis of biophosphates, catalytic asymmetric phosphoramidite transfer remains largely unexplored for phosphate ester synthesis. We have discovered that a tetrazole-functionalized peptide, in the presence of 10-? molecular sieves, functions as an enantioselective catalyst for phosphite transfer. This chemistry in turn has been used as the key step in a streamlined synthesis of myo-inositol-6-phosphate. Mechanistic insights implicate phosphate as a directing group for a highly selective kinetic resolution of a protected inositol monophosphate. This work represents a distinct and efficient method for the selective catalytic phosphorylation of natural products.

Jordan, Peter A.; Kayser-Bricker, Katherine J.; Miller, Scott J.

2010-01-01

106

Catalytic asymmetric synthesis of butane diacetal-protected (4S,5S)-dihydroxycyclohexen-1-one and use in natural product synthesis.  

PubMed

Due to the lack of availability of unnatural (+)-quinic acid as a starting material, a 6-step synthesis of butane diacetal-protected (4S,5S)-dihydroxycyclohexen-1-one (formally derived from (+)-quinic acid) has been devised. The key catalytic asymmetric step involves a chiral Co-salen-catalysed epoxide ring-opening reaction. (4S,5S)-Dihydroxycyclohexen-1-one was utilised in the synthesis of two cyclohexenone natural products isolated from the mycelia of Lasiodiplodia theobromae. PMID:22930235

Burns, David J; Hachisu, Shuji; O'Brien, Peter; Taylor, Richard J K

2012-10-14

107

Chiral ionic liquids, a renewal for the chemistry of chiral solvents? Design, synthesis and applications for chiral recognition and asymmetric synthesis  

Microsoft Academic Search

Chiral ionic solvents were almost unexplored before the last five years. This field which is of increasing importance could constitute a renewal for the chemistry of chiral solvents. So far reported examples are designed either from the chiral pool (aminoacids, hydroxyacids, amines, aminoalcohols, terpenes and alkaloids) or by asymmetric synthesis; they can bear central, axial or planar chirality. Modern applications

Christine Baudequin; Delphine Brégeon; Jocelyne Levillain; Frédéric Guillen; Jean-Christophe Plaquevent; Annie-Claude Gaumont

2005-01-01

108

Collective asymmetric synthesis of (-)-antofine, (-)-cryptopleurine, (-)-tylophorine, and (-)-tylocrebrine with tert-butanesulfinamide as a chiral auxiliary.  

PubMed

A collective asymmetric synthesis of phenanthroindolizidine and phenanthroquinolizidine alkaloids (-)-antofine, (-)-cryptopleurine, (-)-tylophorine, and (-)-tylocrebrine was achieved by means of a reaction sequence involving efficient generation of chiral homoallylic amine intermediates by asymmetric allylation of the corresponding tert-butanesulfinyl imine. From these intermediates, the pyrrolidine and piperidine rings were constructed by means of an intramolecular SN2 substitution reaction and a ring-closing metathesis reaction, respectively. The unusual C5-methoxy-substituted phenanthrene moiety of (-)-tylocrebrine was generated by means of an InCl3-catalyzed cycloisomerization reaction of an o-propargylbiaryl compound. PMID:24679059

Zheng, Yanlong; Liu, Yuxiu; Wang, Qingmin

2014-04-18

109

Enzymatic- and iridium-catalyzed asymmetric synthesis of a benzothiazepinylphosphonate bile acid transporter inhibitor.  

PubMed

A synthesis of the benzothiazepine phosphonic acid 3, employing both enzymatic and transition metal catalysis, is described. The quaternary chiral center of 3 was obtained by resolution of ethyl (2-ethyl)norleucinate (4) with porcine liver esterase (PLE) immobilized on Sepabeads. The resulting (R)-amino acid (5) was converted in two steps to aminosulfate 7, which was used for construction of the benzothiazepine ring. Benzophenone 15, prepared in four steps from trimethylhydroquinone 11, enabled sequential incorporation of phosphorus (Arbuzov chemistry) and sulfur (Pd(0)-catalyzed thiol coupling) leading to mercaptan intermediate 18. S-Alkylation of 18 with aminosulfate 7 followed by cyclodehydration afforded dihydrobenzothiazepine 20. Iridium-catalyzed asymmetric hydrogenation of 20 with the complex of [Ir(COD)2BArF] (26) and Taniaphos ligand P afforded the (3R,5R)-tetrahydrobenzothiazepine 30 following flash chromatography. Oxidation of 30 to sulfone 31 and phosphonate hydrolysis completed the synthesis of 3 in 12 steps and 13% overall yield. PMID:24256447

Cowan, David J; Collins, Jon L; Mitchell, Mark B; Ray, John A; Sutton, Peter W; Sarjeant, Amy A; Boros, Eric E

2013-12-20

110

Enantiopure N-Acyldihydropyridones as Synthetic Intermediates: Asymmetric Synthesis of (-)-Septicine and (-)-Tylophorine.  

PubMed

A concise asymmetric synthesis of (-)-septicine (1) and (-)-tylophorine (2) was accomplished with a high degree of stereocontrol in eight and nine steps, respectively. Addition of 4-(1-butenyl)magnesium bromide to 1-acylpyridinium salt 3, prepared in situ from 4-methoxy-3-(triisopropylsilyl)pyridine and the chloroformate of (-)-trans-2-(alpha-cumyl)cyclohexanol, gave a 91% yield of diastereomerically pure dihydropyridone 7. Oxidative cleavage of 7 and subsequent reduction provided alcohol 6 in 81% yield. Conversion of 6 to the chloride followed by treatment with sodium methoxide gave indolizidinone 9 in high yield. Bromination and conjugate reduction of 9 with L-Selectride, and trapping the intermediate enolate with N-(5-chloro-2-pyridyl)triflimide, provided bromovinyl triflate 11. Palladium-catalyzed cross-coupling of excess (3,4-dimethoxyphenyl)zinc bromide and 11 gave (-)-septicine (1). On the basis of this synthesis, (-)-1 was assigned the Rconfiguration. Reaction of 1 with vanadium(V) trifluoride oxide in TFA/CH(2)Cl(2) effected oxidative coupling to give a 68% yield of (-)-tylophorine (2). PMID:11671861

Comins, Daniel L.; Chen, Xinghai; Morgan, Lawrence A.

1997-10-17

111

An advance on exploring N-tert-butanesulfinyl imines in asymmetric synthesis of chiral amines.  

PubMed

Although catalytic asymmetric synthesis has undergone tremendous growth in the last 30 years, chiral auxiliary-aided asymmetric synthesis continues to attract considerable attention. Chiral N- tert-butanesulfinamide, as pioneered by Ellman and co-workers, is undoubtedly one of the most efficient auxiliaries developed to date; it allows the preparation, through simple conversion, of a diverse range of enantiopure amines, which are ubiquitous in natural products and biologically active molecules. Following on from our studies of the SmI(2)-mediated asymmetric syntheses of alpha,gamma-substituted gamma-butyrolactones, we found that simple homocoupling of chiral N- tert-butanesulfinyl imines in the presence of SmI(2) produced enantiopure vicinal C2-symmetric diamines in high yield. In addition, C2-unsymmetric chiral diamines are readily prepared through SmI(2)-mediated cross-couplings of N- tert-butanesulfinyl imines and nitrones; these transformations represented the first successful examples of asymmetric cross-coupling between two different imine species. Subsequently, we discovered another useful reaction induced by SmI(2), the efficient cross-coupling of N- tert-butanesulfinyl imines and aldehydes, which provides ready access to enantiopure anti-1,2-amino alcohols. The synthetic applicability of this reaction was demonstrated through its use in the facile total syntheses of (3R,4S)-statine, d- erythro-sphinganine, (+)-CP-99,994, and (+)-L-733,060. The Zn/In-mediated allylation of chiral N- tert-butanesulfinyl imines yields homoallylic amines. After pondering the reaction mechanism, we developed optimal reaction conditions for reversing the stereogenic outcome, thereby allowing the preparation of enantiopure homoallylic amines of either handedness from single enantiomers of the (R)- or (S)-sulfinyl imine. When a benzoyl-substituted allyl bromide is used for allylation, the reaction proceeds smoothly to give 2-vinyl-substituted anti-1,2-amino alcohols in high yields and diastereoselectivities, another simple method for preparing enantiopure amino alcohols. We employed these reactions in the syntheses of enantiopure allylglycine, 3-allyl-isoindolinones, and (-)-cytoxazone. Further studies led to the discovery that the allylations of N- tert-butanesulfinyl aldimines can be performed in water. The reactions described in this Account are among the simplest and most efficient synthetic methods available for preparing enantio-enriched diamines, amino alcohols, homoallylic amines, and other amine derivatives. These reactions are additionally attractive because of the ready availability of the starting materials, the simplicity of the reaction conditions, and the high degree of stereochemical control. Their applications in the total syntheses of several biologically interesting molecules illustrate the versatility of these transformations; we hope that they will stimulate the development of new synthetic methods. PMID:18533688

Lin, Guo-Qiang; Xu, Ming-Hua; Zhong, Yu-Wu; Sun, Xing-Wen

2008-07-01

112

Asymmetric Total Synthesis of (+)- and (-)-Clusianone and (+)- and (-)Clusianone Methyl Enol Ether via ACC Alkylation and Evaluation of their Anti-HIV Activity  

PubMed Central

The total asymmetric synthesis of (+)- and (?)-clusianone and (+)- and (?)-clusianone methyl enol ether is reported. Asymmetric induction is achieved through the use of ACC alkylation, providing the key intermediates with an er of 99:1. The four synthetic compounds were evaluated for their anti-HIV activity. Both (+)- and (?)-clusianone displayed significant anti-HIV activity.

Garnsey, Michelle R.; Matous, James A.; Kwiek, Jesse J.; Coltart, Don M.

2011-01-01

113

Asymmetric total synthesis of (+)-aphanamol I based on the transition metal catalyzed [5 + 2] cycloaddition of allenes and vinylcyclopropanes.  

PubMed

A concise asymmetric total synthesis of (+)-aphanamol I is described, based on the transition metal catalyzed [5 + 2] allenyl-vinylcyclopropane cycloaddition. The key cycloaddition precursor is convergently assembled from (R)-(+)-limonene and cyclopropane diester through a novel decarboxylative dehydration reaction. The metal-catalyzed [5 + 2] cycloaddition of this precursor proceeds with complete chemo, endo/exo, and diastereoselectivity in 93% yield, representing an effective general route to bicyclo[5.3.0]decane derivatives. PMID:10930274

Wender, P A; Zhang, L

2000-07-27

114

Facile and efficient Lewis acid catalyzed synthesis of an asymmetric tetrazine useful for bio-orthogonal click chemistry applications  

PubMed Central

Bio-orthogonal tetrazine click reactions have recently attracted significant interest for applications spanning biological imaging, cancer targeting, and biomaterials science. Here, we report a simple and efficient two-step scheme for the synthesis of an asymmetric tetrazine molecule containing a carboxylic acid handle for subsequent macromolecular conjugation. Yields as high as 75% were achieved using as little as 0.005 equivalents of nickel triflate catalyst, which is a significant improvement over previous methodologies.

Alge, Daniel L.; Donohue, Dillon F.; Anseth, Kristi S.

2013-01-01

115

De novo asymmetric synthesis of oligo-rhamno di- and tri-saccharides related to the anthrax tetrasaccharide  

PubMed Central

An asymmetric synthesis of the di- and trisaccharide portion of the naturally occurring anthrax tetrasaccharide from acetylfuran has been developed. The construction of the di- and trisaccharide subunits is based upon our previously disclosed route to anthrax tetrasaccharide. The approach uses iterative diastereoselective palladium-catalyzed glycosylations, Luche reductions, diastereoselective dihydroxylations, and regioselective protections for the assembly of the rhamno- di- and tri-saccharide. The route was also modified for the preparation of the mixed D-/L-disaccharide analogue.

Wang, Hua-Yu Leo; Guo, Haibing; O'Doherty, George A.

2013-01-01

116

Catalytic, enantioselective synthesis of 1,2-anti-diols by asymmetric ring-opening/cross-metathesis.  

PubMed

An enantioselective method for the synthesis of 1,2-anti-diols has been developed. A cyclometalated chiral-at-ruthenium complex catalyzes the asymmetric ring-opening/cross-metathesis of dioxygenated cyclobutenes, thus resulting in functionally rich synthetic building blocks. Syntheses of the insect pheromone (+)-endo-brevicomin and monosaccharide ribose demonstrate the synthetic utility of the 1,2-anti-diol fragments generated in the title reaction. PMID:24554613

Hartung, John; Grubbs, Robert H

2014-04-01

117

Asymmetric synthesis of chloroisothreonine derivatives via syn-stereoselective Mannich-type additions across N-sulfinyl-?-chloroimines.  

PubMed

Mannich-type reactions of O-Boc glycolic esters across chiral N-sulfinyl-?-chloroaldimines resulted in the efficient and syn-stereoselective synthesis of new ?-chloro-?-hydroxy-?-amino esters (dr > 99?:?1). The ?-coordinating ability of the chlorine atom was of great importance for the diastereoselectivity of the Mannich-type reaction and overruled the chelation of the sulfinyl oxygen with the lithium ion of the incoming E-enolate in the transition state model. These novel chloroisothreonine derivatives proved to be excellent building blocks in asymmetric synthesis of novel syn-?,?-aziridino-?-hydroxy esters and biologically relevant trans-oxazolidinone carboxylic esters. PMID:24740384

Callebaut, Gert; Colpaert, Filip; Nonn, Melinda; Kiss, Loránd; Sillanpää, Reijo; Törnroos, Karl W; Fülöp, Ferenc; De Kimpe, Norbert; Mangelinckx, Sven

2014-06-01

118

?-Allyl-?-aryl ?-Amino Esters in the Asymmetric Synthesis of Acyclic and Cyclic Amino Acid Derivatives by Alkene Metathesis.  

PubMed

Allylating agents were explored for the asymmetric synthesis of ?-allyl-?-aryl ?-amino acids by tandem N-alkylation/?-allylation. Cross-metathesis of the tandem product was developed to provide allylic diversity not afforded in the parent reaction; the synthesis of homotyrosine and homoglutamate analogues was completed. Cyclic ?-amino acid derivatives could be accessed by ring-closing metathesis presenting a viable strategy to higher ring homologue of enantioenriched ?-substituted proline. The eight-membered proline analogue was successfully converted to the pyrrolizidine natural product backbone. PMID:24828423

Curto, John M; Kozlowski, Marisa C

2014-06-01

119

Mechanism of the Cobalt Oxazoline Palladacycle (COP)-Catalyzed Asymmetric Synthesis of Allylic Esters  

PubMed Central

The catalytic enantioselective SN2? displacement of (Z)-allylic trichloroacetimidates catalyzed by the palladium(II) complex [COP-OAc]2 is a broadly useful method for the asymmetric synthesis of chiral branched allylic esters. A variety of experiments aimed at elucidating the nature of the catalytic mechanism and its rate- and enantiodetermining steps are reported. Key findings include: (a) the demonstration that a variety of bridged-dipalladium complexes are present and constitute resting states of the COP catalyst, however, monomeric palladium (II) complexes are likely involved in the catalytic cycle; (b) labeling experiments establishing that the reaction proceeds in an overall antarafacial fashion; (c) secondary deuterium kinetic isotope effects that suggest substantial rehybridization at both C1 and C3 in the rate-limiting step; and (d) DFT computational studies (B3-LYP/def2-TZVP) that provide evidence for bidentate substrate-bound intermediates and an anti-oxypalladation/syn-deoxypalladation pathway. These results are consistent with a novel mechanism in which chelation of the imidate nitrogen to form a cationic palladium (II) intermediate activates the alkene for attack by external carboxylate in the enantiodetermining step. Computational modeling of the transition-state structure for the acyloxy palladation step provides a model for enantioinduction.

Cannon, Jeffrey S.; Kirsch, Stefan F.; Overman, Larry E.; Sneddon, Helen F.

2010-01-01

120

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate  

PubMed Central

Summary Propylene carbonate can be used as a green solvent for the asymmetric synthesis of cyanohydrin trimethylsilyl ethers from aldehydes and trimethylsilyl cyanide catalysed by VO(salen)NCS, though reactions are slower in this solvent than the corresponding reactions carried out in dichloromethane. A mechanistic study has been undertaken, comparing the catalytic activity of VO(salen)NCS in propylene carbonate and dichloromethane. Reactions in both solvents obey overall second-order kinetics, the rate of reaction being dependent on the concentration of both the aldehyde and trimethylsilyl cyanide. The order with respect to VO(salen)NCS was determined and found to decrease from 1.2 in dichloromethane to 1.0 in propylene carbonate, indicating that in propylene carbonate, VO(salen)NCS is present only as a mononuclear species, whereas in dichloromethane dinuclear species are present which have previously been shown to be responsible for most of the catalytic activity. Evidence from 51V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane.

Omedes-Pujol, Marta

2010-01-01

121

Molecular Models of Reactants and Products from an Asymmetric Synthesis of a Chiral Carboxylic Acid  

NSDL National Science Digital Library

Our JCE Featured Molecules for this month come from the paper by Thomas E. Smith, David P. Richardson, George A. Truran, Katherine Belecki, and Megumi Onishi (1). The authors describe the use of a chiral auxiliary, 4-benzyl-2-oxazolidinone, in the synthesis of a chiral carboxylic acid. The majority of the molecules used in the experiment, together with several of the pharmaceuticals mentioned in the paper, have been added to our molecule collection. In many instances multiple enantiomeric and diastereomeric forms of the molecules have been included. This experiment could easily be extended to incorporate various aspects of computation for use in an advanced organic or integrated laboratory. Here are some possible exercises using the R and S forms of the 4-benzyl-2-oxazolidinone as the authors point out that both forms are available commercially. Calculation of the optimized structures and energies of the enantiomers at the HF/631-G(d) level using Gaussian03 (2) produces the results shown in Table 1. Evaluation of the vibrational frequencies results in no imaginary frequencies and the 66 real frequencies are identical for the two forms. Examination of the computed IR spectra also shows them to be identical. Additionally, the Raman and NMR spectra can be calculated for the enantiomers and compared to experimental values and spectral patterns. A tool that is becoming increasingly important for assigning absolute configuration is vibrational circular dichroism (VCD). Although the vibrational spectra of an enantiomeric pair are identical, the VCD spectra show opposite signs, as shown in Figure 1. One can imagine a synthesis, using an unknown enantiomer of the chiral auxiliary, followed by calculations of the electronic and vibrational properties of all of the intermediates and the product, and determination of absolute configuration of reactants and products by comparison of experimental and computed VCD spectra. Using a viewer capable of displaying two molecules that can be moved independently, students could more easily visualize the origin of the enantiomeric preference in the reaction between the chelated enolate and allyl iodide.

122

New enantiomeric fluorine-containing derivatives of sulforaphane: synthesis, absolute configurations and biological activity.  

PubMed

Three pairs of enantiomers of the unknown sulforaphane analogs bearing organofluorine substituents bonded to the sulfinyl sulfur atom and having different number of methylene groups in the central carbon chain were synthesized and fully characterized, including determination of their absolute configurations. All the new compounds were tested in vitro for their cytotoxicity against melanoma cells to show increased activity in comparison with the natural sulforaphane. The influence of the particular structural changes in the molecule on the cytotoxicity is discussed. PMID:24589488

Kie?basi?ski, Piotr; ?uczak, Jerzy; Cierpia?, Tomasz; B?aszczyk, Jaros?aw; Siero?, Les?aw; Wiktorska, Katarzyna; Lubelska, Katarzyna; Milczarek, Ma?gorzata; Chilmo?czyk, Zdzis?aw

2014-04-01

123

Inhibition of insect juvenile hormone epoxide hydrolase: asymmetric synthesis and assay of glycidol-ester and epoxy-ester inhibitors of trichoplusia ni epoxide hydrolase.  

PubMed

Juvenile hormone (JH) undergoes metabolic degradation by two major pathways involving JH esterase and JH epoxide hydrolase (EH). While considerable effort has been focussed on the study of JH esterase and the development of inhibitors for this enzyme, much less has been reported on the study of JH-EH. In this work, the asymmetric synthesis of two classes of inhibitors of recombinant JH-EH from Trichoplusia ni, a glycidol-ester series and an epoxy-ester series is reported. The most effective glycidol-ester inhibitor, compound 1, exhibited an I(50) of 1.2x10(-8) M, and the most effective epoxy-ester inhibitor, compound 11, exhibited an I(50) of 9.4x10(-8) M. The potency of the inhibitors was found to be dependent on the absolute configuration of the epoxide. In both series of inhibitors, the C-10 R-configuration was found to be significantly more potent that the corresponding C-10 S-configuration. A mechanism for epoxide hydration catalyzed by insect EH is also presented. PMID:10876120

Linderman, R J; Roe, R M; Harris, S V; Thompson, D M

2000-01-01

124

Enantioselective Synthesis of anti- and syn-homopropargyl alcohols via Chiral Br?nsted Acid Catalyzed asymmetric Allenylboration reactions  

PubMed Central

Chiral Brønsted acid catalyzed asymmetric allenylboration reactions are described. Under optimized conditions, anti-homopropargyl alcohols 2 are obtained in high yields with excellent diastereo- and enantioselectivities from stereochemically matched aldehyde allenylboration reactions with (M)-1 catalyzed by the chiral phosphoric acid (S)-4. The syn-isomers 3 can also be obtained in good diastereoselectivities and excellent enantioselectivities from the mismatched allenylboration reactions of aromatic aldehydes using (M)-1 in the presence of the enantiomeric phosphoric acid (R)-4. The stereochemistry of the methyl group introduced into 2 and 3 is controlled by the chirality of the allenylboronate (M)-1, whereas the configuration of the new hydroxyl stereocenter is controlled by the enantioselectivity of the chiral phosphoric acid catalyst used in these reactions. The synthetic utility of this methodology was further demonstrated in triple asymmetric syntheses of a variety of anti,anti-stereotriads, the direct synthesis of which has constituted a significant challenge using previous generations of aldol and crotylmetal reagents.

Chen, Ming; Roush, William R.

2012-01-01

125

Establishment of the absolute configuration of the bioactive marine alkaloid eudistomin X by stereospecific synthesis  

Microsoft Academic Search

A stereospecific synthesis of both enantiomers of the marine alkaloid eudistomin X using the amino acid chiral pool is achieved. Comparison of 1H and 13C NMR chemical shifts of the synthetic product as either the free base, mono-salt or disalt with those reported for the natural product established that the ascidian metabolite was originally characterised as a mono-salt and that

Rhys Finlayson; Amira Brackovic; Annabel Simon-Levert; Bernard Banaigs; Ronan F. O’Toole; Christopher H. Miller; Brent R. Copp

2011-01-01

126

Application of a Catalytic Asymmetric Povarov Reaction using Chiral Ureas to the Synthesis of a Tetrahyroquinoline Library  

PubMed Central

A 2328-membered library of 2,3,4-trisubstituted tetrahydroquinolines was produced using a combination of solution- and solid-phase synthesis techniques. A tetrahydroquinoline (THQ) scaffold was prepared via an asymmetric Povarov reaction using cooperative catalysis to generate three contiguous stereogenic centers. A matrix of 4 stereoisomers of the THQ scaffold was prepared to enable the development of stereo/structure-activity relationships (SSAR) upon biological testing. A sparse matrix design strategy was employed to select library members to be synthesized with the goal of generating a diverse collection of tetrahydroquinolines with physicochemical properties suitable for downstream discovery.

Gerard, Baudouin; O'Shea, Morgan Welzel; Donckele, Etienne; Kesavan, Sarathy; Akella, Lakshmi B.; Xu, Hao; Jacobsen, Eric N.; Marcaurelle, Lisa A.

2012-01-01

127

Preparation of anti-Vicinal Amino Alcohols: Asymmetric Synthesis of d-erythro-Sphinganine, (+)-Spisulosine, and d-ribo-Phytosphingosine  

PubMed Central

Two variations of the Overman rearrangement have been developed for the highly selective synthesis of anti-vicinal amino alcohol natural products. A MOM ether-directed palladium(II)-catalyzed rearrangement of an allylic trichloroacetimidate was used as the key step for the preparation of the protein kinase C inhibitor d-erythro-sphinganine and the antitumor agent (+)-spisulosine, whereas the Overman rearrangement of chiral allylic trichloroacetimidates generated by the asymmetric reduction of an ?,?-unsaturated methyl ketone allowed rapid access both to d-ribo-phytosphingosine and l-arabino-phytosphingosine.

2013-01-01

128

Asymmetric synthesis of protected ?-amino boronic acid derivatives with an air- and moisture-stable Cu(II) catalyst.  

PubMed

The asymmetric borylation of N-tert-butanesulfinyl imines with bis(pinacolato)diboron is achieved using a Cu(II) catalyst and provides access to synthetically useful and pharmaceutically relevant ?-amino boronic acid derivatives. The Cu(II)-catalyzed reaction is performed on the benchtop in air at room temperature using commercially available, inexpensive reagents at low catalyst loadings. A variety of N-tert-butanesulfinyl imines, including ketimines, react readily to provide ?-sulfinamido boronate esters in good yields and with high stereoselectivity. In addition, this transformation is applied to the straightforward, telescoped synthesis of ?-sulfinamido trifluoroborates. PMID:24684495

Buesking, Andrew W; Bacauanu, Vlad; Cai, Irene; Ellman, Jonathan A

2014-04-18

129

Kinase Mimic Catalysts for Asymmetric Synthesis of Phosphorylated Inositols and Cycloalkanols.  

National Technical Information Service (NTIS)

The present invention provides peptide-based phosphorylation catalysts (PBPC's) for the asymmetric monophosphorylation of cyclitols, particularly myo-inositols. The PBPC's of the invention effect a regio and enantioselective phosphorylation of a myo-inosi...

A. J. Morgan B. Sculimbrene S. J. Miller

2005-01-01

130

Synthesis of chiral sulfoximine-based thioureas and their application in asymmetric organocatalysis  

PubMed Central

Summary For the first time, chiral sulfoximine derivatives have been applied as asymmetric organocatalysts. In combination with a thiourea-type backbone the sulfonimidoyl moiety leads to organocatalysts showing good reactivity in the catalytic desymmetrization of a cyclic meso-anhydride and moderate enantioselectivity in the catalytic asymmetric Biginelli reaction. Straightforward synthetic routes provide the newly designed thiourea-sulfoximine catalysts in high overall yields without affecting the stereohomogeneity of the sulfur-containing core fragment.

Frings, Marcus; Thome, Isabelle

2012-01-01

131

A facile asymmetric synthesis of glycerol phospholipids via tritylglycidol prepared by the asymmetric epoxidation of allyl alcohol. Thiolester and thioether analogs of phosphatidylcholine.  

PubMed

Trityl-glycidol was synthesized by in situ derivatization of glycidol, which was prepared by the catalytic asymmetric epoxidation of allyl alcohol. Depending on the enantiomer of diisopropyl tartrate used with the titanium catalyst, either (R)- or (S)-trityl-glycidol was obtained in a "one pot" synthesis in about 50% overall yield. The optical purity, determined by NMR spectroscopy of a Mosher ester, was greater than 98% ee. Nucleophilic opening of the chiral epoxide with dodecyl mercaptan gave optically active 1-S-dodecyl-3-O-trityl-1-thio-glycerol, which was used to synthesize 1-S-dodecyl-2-O-decanoyl-thio-sn-glycero-3-phosphocholine. Opening of the epoxide with methyl xanthate gave a 1,2-trithiocarbonate derivative of trityl glycerol which can be used to synthesize 1,2-bis(S-decanoyl)-1,2-dithio-sn-glycero-3-phosphocholine. Opening of the epoxide with thiodecanoic acid gave 1-S-decanoyl-3-O-trityl-1-thio-glycerol which was used to synthesize 1-S-decanoyl-2-O-decanoyl-1-thio-sn-glycero-3-phosphocholine. PMID:2354542

Hendrickson, H S; Hendrickson, E K

1990-03-01

132

Asymmetric reactions in continuous flow  

PubMed Central

Summary An overview of asymmetric synthesis in continuous flow and microreactors is presented in this review. Applications of homogeneous and heterogeneous asymmetric catalysis as well as biocatalysis in flow are discussed.

Mak, Xiao Yin; Laurino, Paola

2009-01-01

133

Synthesis, properties, and reactivity of cocaine benzoylthio ester possessing the cocaine absolute configuration.  

PubMed

One aspect of immunopharmacotherapy for cocaine abuse involves the use of a catalytic monoclonal antibody (mAb) to degrade cocaine via hydrolysis of the benzoate ester. A cocaine benzoylthio ester analogue provides a means to implement high-throughput selection strategies to potentially isolate mAbs with high activity. The required analogue was synthesized starting from (-)-cocaine hydrochloride and possessed the cocaine absolute configuration. Key points in the preparation were the introduction of the sulfur atom at C-3 via a bromomagnesium thiolate addition to the exo face of anhydroecgonine, separation of C-2 diastereomers, recycling of a C-2 thio ester byproduct, and formation of the necessary C-2 methyl and C-3 benzoylthio esters. Effects resulting from the lower electronegativity and greater hydrophobicity of sulfur compared to oxygen were observed. These characteristics could result in interesting drug properties. Furthermore, the analogue was found to be a substrate for catalytic mAbs that hydrolyze cocaine as monitored by HPLC and also spectrophotometry by coupling cleavage of the benzoylthio ester to the disulfide exchange with Ellman's reagent. Screening antibody libraries with the new cocaine analogue using the spectroscopic assay provides an avenue for the high-throughput identification of catalysts that efficiently breakdown cocaine. PMID:11929256

Isomura, Shigeki; Hoffman, Timothy Z; Wirsching, Peter; Janda, Kim D

2002-04-10

134

Simple synthesis of amorphous NiWO4 nanostructure and its application as a novel cathode material for asymmetric supercapacitors.  

PubMed

This study reports a simple synthesis of amorphous nickel tungstate (NiWO4) nanostructure and its application as a novel cathode material for supercapacitors. The effect of reaction temperature on the electrochemical properties of the NiWO4 electrode was studied, and results demonstrate that the material synthesized at 70 °C (NiW-70) has shown the highest specific capacitance of 586.2 F g(-1) at 0.5 A g(-1) in a three-electrode system. To achieve a high energy density, a NiW-70//activated carbon asymmetric supercapacitor is successfully assembled by use of NiW-70 and activated carbon as the cathode and anode, respectively, and then, its electrochemical performance is characterized by cyclic voltammetry and galvanostatic charge-discharge measurements. The results show that the assembled asymmetric supercapacitor can be cycled reversibly between 0 and 1.6 V with a high specific capacitance of 71.1 F g(-1) at 0.25 A g(-1), which can deliver a maximum energy density of 25.3 Wh kg(-1) at a power density of 200 W kg(-1). Furthermore, this asymmetric supercapacitor also presented an excellent, long cycle life along with 91.4% specific capacitance being retained after 5000 consecutive times of cycling. PMID:23910723

Niu, Lengyuan; Li, Zhangpeng; Xu, Ye; Sun, Jinfeng; Hong, Wei; Liu, Xiaohong; Wang, Jinqing; Yang, Shengrong

2013-08-28

135

A facile asymmetric synthesis of (s)-14-methyl-1-octadecene, the sex pheromone of the peach leafminer moth.  

PubMed

An asymmetric synthesis of 14-methyl-1-octadecene, the sex pheromone of the peach leafminer moth has been achieved. The target molecule was synthesized in six linear steps and in 30.3% overall yield from commercially available hexanoyl chloride, (S)-4-benzyloxazolidin-2-one and 1,9-nonanediol. The hexanoyl chloride was connected with (S)-4-benzyloxazolidin-2-one, and with the induction of the chiral oxazolidinone auxiliary, after chiral methylation, LAH reduction and then tosylation gave the chiral key intermediate 5 in high stereoselectivity. 1,9-Nonanediol, was selectively brominated, THP protected and subjected to Li?CuCl?-mediated C-C coupling to afford a C12 intermediate. The target molecule, (S)-14-methyl-1-octadecene, was obtained after the two parts were subjected to a second Li?CuCl?-mediated C-C coupling. Our synthetic approach represents the first time a substrate-control asymmetric synthesis of (S)-14-methyl-1-octadecene has been reported. PMID:23652989

Zhang, Tao; Ma, Wei-Li; Li, Tian-Rui; Wu, Jia; Wang, Jun-Run; Du, Zhen-Ting

2013-01-01

136

The Construction of All-Carbon Quaternary Stereocenters by Use of Pd-Catalyzed Asymmetric Allylic Alkylation Reactions in Total Synthesis  

PubMed Central

All-carbon quaternary stereocenters have posed significant challenges in the synthesis of complex natural products. These important structural motifs have inspired the development of broadly applicable palladium-catalyzed asymmetric allylic alkylation reactions of unstabilized non-biased enolates for the synthesis of enantioenriched ?-quaternary products. This microreview outlines key considerations in the application of palladium-catalyzed asymmetric allylic alkylation reactions and presents recent total syntheses of complex natural products that have employed these powerful transformations for the direct, catalytic, enantioselective construction of all-carbon quaternary stereocenters.

Hong, Allen Y.

2014-01-01

137

Stereoselective terminal functionalization of small peptides for catalytic asymmetric synthesis of unnatural peptides  

PubMed Central

The asymmetric phase-transfer catalytic alkylation of peptides has been achieved by the use of designed C2-symmetric chiral quaternary ammonium bromide 1 as catalyst. Excellent stereoselectivities were uniformly observed in the alkylation with a variety of alkyl halides and the efficiency of the transmission of stereochemical information was not affected by the side-chain structure of the preexisting amino acid residues. This method also enables an asymmetric construction of noncoded ?,?-dialkyl-?-amino acid residues at the peptide terminal. Since this chirality can be efficiently transferred to the adjacent amino acid moiety, our approach provides a general procedure not only for the highly stereoselective terminal functionalization of peptides but also for the sequential asymmetric construction of unnatural oligopeptides, which should play a vital role in the peptide-based drug discovery process.

Maruoka, Keiji; Tayama, Eiji; Ooi, Takashi

2004-01-01

138

Transgenic rice grains expressing a heterologous ?-hydroxyphenylpyruvate dioxygenase shift tocopherol synthesis from the ? to the ? isoform without increasing absolute tocopherol levels.  

PubMed

We generated transgenic rice plants overexpressing Arabidopsis thaliana ?-hydroxyphenylpyruvate dioxygenase (HPPD), which catalyzes the first committed step in vitamin E biosynthesis. Transgenic grains accumulated marginally higher levels of total tocochromanols than controls, reflecting a small increase in absolute tocotrienol synthesis (but no change in the relative abundance of the ? and ? isoforms). In contrast, there was no change in the absolute tocopherol level, but a significant shift from the ? to the ? isoform. These data confirm HPPD is not rate limiting, and that increasing flux through the early pathway reveals downstream bottlenecks that act as metabolic tipping points. PMID:22361804

Farré, Gemma; Sudhakar, Duraialagaraja; Naqvi, Shaista; Sandmann, Gerhard; Christou, Paul; Capell, Teresa; Zhu, Changfu

2012-10-01

139

?-Ketophosphonates as Ester Surrogates: Isothiourea-Catalyzed Asymmetric Diester and Lactone Synthesis.  

PubMed

Isothiourea HBTM-2.1 catalyzes the asymmetric Michael addition/lactonization of aryl- and alkenylacetic acids using ?-keto-?,?-unsaturated phosphonates as ?,?-unsaturated ester surrogates, giving access to a diverse range of stereodefined lactones or enantioenriched functionalized diesters upon ring-opening. PMID:24734989

Smith, Siobhan R; Leckie, Stuart M; Holmes, Reuben; Douglas, James; Fallan, Charlene; Shapland, Peter; Pryde, David; Slawin, Alexandra M Z; Smith, Andrew D

2014-05-01

140

Catalytic Asymmetric Homologation of ?-Ketoesters with ?-Diazoesters: Synthesis of Succinate Derivatives with Chiral Quaternary Centers.  

PubMed

Y not? In the presence of the L/Y(OTf)3 catalyst, the first catalytic asymmetric homologation of ?-ketoesters with ?-alkyl-?-diazoesters through either a 1,2-aryl or 1,2-alkyl shift was accomplished. Highly functionalized succinate derivatives containing a quaternary stereocenter were obtained in excellent yield and enantioselectivity under mild reaction conditions. Tf=trifluoromethanesulfonyl. PMID:24039072

Li, Wei; Liu, Xiaohua; Tan, Fei; Hao, Xiaoyu; Zheng, Jianfeng; Lin, Lili; Feng, Xiaoming

2013-10-01

141

The synthesis of tenofovir and its analogues via asymmetric transfer hydrogenation.  

PubMed

A series of tenofovir analogues with potential antiviral and immunobiologically active compounds were synthesized through an asymmetric transfer hydrogenation reaction from achiral purine derivatives. Up to 97% ee and good to excellent yields were achieved under mild conditions through short reaction steps. The present report suggests an efficient process to acquire tenofovir and its analogues. PMID:24650095

Zhang, Qian; Ma, Bai-Wei; Wang, Qian-Qian; Wang, Xing-Xing; Hu, Xia; Xie, Ming-Sheng; Qu, Gui-Rong; Guo, Hai-Ming

2014-04-01

142

Design, synthesis and applications of chiral N-2-phenyl-2-propyl sulfinyl imines for Group-Assisted Purification (GAP) asymmetric synthesis  

PubMed Central

A new chiral (Rs)-2-phenyl-2-propyl sulfinamide has been designed and synthesized; its derived aldimines and ketimines have been applied for asymmetric addition reaction with allylmagnesium bromide. The reaction was conveniently performed at room temperature to give a series of homoallylic amines in high yields (up to quant) and diastereoselectivity (up to >99 % de). The pure products were obtained by relying on Group-Assisted Purification (GAP) chemistry to avoid traditional purification methods of column chromatography or recrystallization. The conversion of disulfide to (Rs)-thiosulfinate was also confirmed to be of the GAP chemistry in which washing crude product can generate pure enantiomer. The absolute stereochemistry has been determined by X-ray analysis.

Pindi, Suresh; Wu, Jianbin; Li, Guigen

2013-01-01

143

Two Remarkable Epimerizations Imperative for the Success of an Asymmetric Total Synthesis of (+)-Aigialospirol  

PubMed Central

Two remarkable epimerization processes were uncovered during our pursuit of an enantioselective synthesis of (+)-aigialospirol featuring a cyclic acetal tethered ring-closing metathesis. Through modeling, we were able to turn these two unexpected epimerizations to our advantage via modeling to ensure a successful and concise total synthesis, thereby firmly establishing cyclic acetal tethered RCM as a powerful strategy in natural product synthesis. Most importantly, calculations allowed us to fully understand the nature and the mechanistic course of these two epimerizations that were imperative to the total synthesis efforts.

Figueroa, Ruth; Feltenberger, John B.; Guevarra, Christle C.

2010-01-01

144

Highly Regio- and Enantioselective Synthesis of Polysubstituted 2H-Pyrroles via Pd-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Pyrroles.  

PubMed

A highly efficient synthesis of chiral polysubstituted 2H-pyrrole derivatives via a Pd-catalyzed intermolecular asymmetric allylic dearomatization reaction of pyrroles is presented. With the commercially available palladium precursor and chiral ligand, the polysubstituted 2H-pyrrole products containing a chiral quaternary carbon center were obtained with up to 97% ee and >95/5 regioselectivity. PMID:24758260

Zhuo, Chun-Xiang; Zhou, Yong; You, Shu-Li

2014-05-01

145

Asymmetric synthesis of the HMG-CoA reductase inhibitor atorvastatin calcium: an organocatalytic anhydride desymmetrization and cyanide-free side chain elongation approach.  

PubMed

An efficient asymmetric synthesis of atorvastatin calcium has been achieved from commercially available diethyl 3-hydroxyglutarate through a novel approach that involves an organocatalytic enantioselective cyclic anhydride desymmetrization to establish C(3) stereogenicity and cyanide-free assembly of C7 amino type side chain via C5+C2 strategy as the key transformations. PMID:24575888

Chen, Xiaofei; Xiong, Fangjun; Chen, Wenxue; He, Qiuqin; Chen, Fener

2014-03-21

146

Catalytic Asymmetric Synthesis of 3,3'-Diaryloxindoles as Triarylmethanes with a Chiral All-Carbon Quaternary Center: Phase-Transfer-Catalyzed SN Ar Reaction.  

PubMed

Catalytic asymmetric synthesis of unsymmetrical triarylmethanes with a chiral all-carbon quaternary center was achieved by using a chiral bifunctional quaternary phosphonium bromide catalyst in the SN Ar reaction of 3-aryloxindoles under phase-transfer conditions. The presence of a urea moiety in the chiral phase-transfer catalyst was important for obtaining high enantioselectivity in this reaction. PMID:24802937

Shirakawa, Seiji; Koga, Kenta; Tokuda, Takashi; Yamamoto, Kenichiro; Maruoka, Keiji

2014-06-10

147

A Stereoselective Synthesis of (-)-Viridiofungin A Utilizing a TiCl4-Promoted Asymmetric Multicomponent Reaction  

PubMed Central

A stereoselective synthesis of (?)-Viridiofungin A is described. The convergent synthesis utilized a unique highly diastereoselective multicomponent reaction between optically active phenyldihydrofuran and an ?-ketoester to provide two chiral centers including a quarternary carbon center in a single step. Other key steps include an acyloxycarbonum ion-mediated tetrahydrofuran ring-opening reaction and a Julia-Kocienski olefination.

Ghosh, Arun K.; Kass, Jorden

2012-01-01

148

Towards an asymmetric synthesis of the bacterial peptide deformylase (PDF) inhibitor fumimycin.  

PubMed

Studies towards the synthesis of the bacterial peptide deformylase (PDF) inhibitor fumimycin are reported. The synthetic approach features an organocatalytic access to the alpha,alpha-disubstituted amino acid unit and results in the synthesis of an advanced intermediate which already contains all functionalities of fumimycin. PMID:20024097

Hartmann, Caroline E; Gross, Patrick J; Nieger, Martin; Bräse, Stefan

2009-12-21

149

A stereoselective synthesis of (-)-viridiofungin A utilizing a TiCl(4)-promoted asymmetric multicomponent reaction.  

PubMed

A stereoselective synthesis of (-)-viridiofungin A is described. The convergent synthesis utilized a unique highly diastereoselective multicomponent reaction between optically active phenyldihydrofuran and an ?-ketoester to provide two chiral centers including a quarternary carbon center in a single step. Other key steps include an acyloxycarbonium ion-mediated tetrahydrofuran ring-opening reaction and a Julia-Kocienski olefination. PMID:22195730

Ghosh, Arun K; Kass, Jorden

2012-01-20

150

Synthesis of asymmetrically substituted terpyridines by palladium-catalyzed direct C-H arylation of pyridine N-oxides.  

PubMed

The synthesis of asymmetrically substituted 2,2':6',2''-terpyridines is reported. First, palladium-catalyzed C-H arylation of pyridine N-oxides with substituted bromopyridines gave 2,2'-bipyridine N-oxides, which were further arylated in a second step to form 2,2':6',2''-terpyridine N-oxides. Yields of up to 77% were obtained with N-oxides bearing an electron-withdrawing ethoxycarbonyl substituent in the 4-position. Pd(OAc)2 with either P(tBu)3 or P(o-tolyl)3 was used as the catalyst. Cyclometalated complexes derived from Pd(OAc)2 and these phosphines were also effective. K3PO4 as the base gave better results than K2CO3. Subsequent deoxygenation with H2 and Pd/C as the catalyst gave the asymmetrically substituted 2,2':6',2''-terpyridines in near quantitative yield. This reaction sequence significantly reduces the number of steps required in comparison with known cross-coupling methods and therefore allows convenient and scalable access to substituted terpyridines. PMID:24227705

Duric, Sasa; Sypaseuth, Fanni D; Hoof, Santina; Svensson, Emma; Tzschucke, C Christoph

2013-12-16

151

N,N-Diisopropyl-N-phosphonyl Imines Lead to Efficient Asymmetric Synthesis of Aziridine-2-Carboxylic Esters  

PubMed Central

Highly diastereoselective asymmetric synthesis of chiral aziridine-2-carboxylic esters is reported for 20 examples with good yields (51–87%) and excellent diastereoselectivities (>99:1 dr for most cases). The modified N-phosphonyl imines are proven to be superior to previous imine auxiliaries for the aza Darzens reaction by using secondary isopropyl to replace primary benzyl group for N,N-diamino protection. In the meanwhile, a special operation by slowly adding the pre-cooled imine solution at ?78 °C into the preformed ?-bromo lithium enolate mixture at this temperature in the presence of 4 Å molecular sieves was found to be crucial in terms of yields and diastereoselectivity. The present method can provide an easy and general access to ?-hydroxy ?-amino acids and other important amino building blocks.

Kattamuri, Padmanabha V.; Xiong, Yiwen; Pan, Yi; Li, Guigen

2013-01-01

152

Palladium-Catalyzed Asymmetric Alkylation in the Synthesis of Cyclopentanoid and Cycloheptanoid Core Structures Bearing All-Carbon Quaternary Stereocenters  

PubMed Central

General catalytic asymmetric routes toward cyclopentanoid and cycloheptanoid core structures embedded in numerous natural products have been developed. The central stereoselective transformation in our divergent strategies is the enantioselective decarboxylative alkylation of seven-membered ?-ketoesters to form ?-quaternary vinylogous esters. Recognition of the unusual reactivity of ?-hydroxyketones resulting from the addition of hydride or organometallic reagents enabled divergent access to ?-quaternary acylcyclopentenes through a ring contraction pathway or ?-quaternary cycloheptenones through a carbonyl transposition pathway. Synthetic applications of these compounds were explored through the preparation of mono-, bi-, and tricyclic derivatives that can serve as valuable intermediates for the total synthesis of complex natural products. This work complements our previous work with cyclohexanoid systems.

Hong, Allen Y.; Bennett, Nathan B.; Krout, Michael R.; Jensen, Thomas; Harned, Andrew. M.

2011-01-01

153

Catalytic asymmetric claisen rearrangement of gosteli-type allyl vinyl ethers: total synthesis of (-)-9,10-dihydroecklonialactone B.  

PubMed

The enantioselective synthesis of (-)-9,10-dihydroecklonialactone B is described. The catalytic asymmetric Claisen rearrangement of a Gosteli-type allyl vinyl ether was utilized to afford an acyclic ?-keto ester building block endowed with functionality amenable to the preparation of the carbocyclic target molecule by suitable postrearrangement transformations: A highly diastereoselective Corey-Bakshi-Shibata reduction of a ?-chiral ?-keto ester and a reductive homologation of an ?-hydroxy ester. A transprotection tactic by a chemoselective intramolecular 6-exo-trig iodoetherification enabled regioselective ring-closing alkene metatheses to afford the 5- as well as the 14-membered ring, however, with mixed success in terms of E/Z selectivity. PMID:24621347

Becker, Julia; Butt, Lena; von Kiedrowski, Valeska; Mischler, Elisabeth; Quentin, Florian; Hiersemann, Martin

2014-04-01

154

A General Strategy for the Chemoenzymatic Synthesis of Asymmetrically Branched N-Glycans  

PubMed Central

A systematic, efficient means of producing diverse libraries of asymmetrically branched N-glycans is needed to investigate the specificities and biology of glycan binding proteins. To that end, we describe a core pentasaccharide that at potential branching positions is modified by orthogonal protecting groups to allow selective attachment of unique saccharide moieties by chemical glycosylation. The appendages were selected in such a way that the antenna of the resulting deprotected compounds could be selectively extended by glycosyltransferases to give libraries of asymmetrical multi-antennary glycans. The power of the methodology was demonstrated by the preparation of a series of complex oligosaccharides that were printed as microarrays and screened for binding to lectins and influenza-virus hemagglutinins, which showed that recognition is modulated by presentation of minimal epitopes in the context of complex N-glycans.

Wang, Zhen; Chinoy, Zoeisha S.; Ambre, Shailesh G.; Peng, Wenjie; McBride, Ryan; de Vries, Robert P.; Glushka, John; Paulson, James C.; Boons, Geert-Jan

2013-01-01

155

Enantioselective Synthesis of Tryptophan Derivatives by a Tandem Friedel - Crafts Conjugate Addition/Asymmetric Protonation Reaction  

PubMed Central

The tandem Friedel–Crafts conjugate addition/asymmetric protonation reaction between 2-substituted indoles and methyl 2-acetamidoacrylate is reported. The reaction is catalyzed by (R)-3,3?-dibromo-BINOL in the presence of stoichiometric SnCl4, and is the first example of a tandem conjugate addition/asymmetric protonation reaction using a BINOL•SnCl4 complex as the catalyst. A range of indoles furnished synthetic tryptophan derivatives in good yields and high levels of enantioselectivity, even on preparative scale. The convergent nature of this transformation should lend itself to the preparation of unnatural tryptophan derivatives for use in a broad array of synthetic and biological applications.

Kieffer, Madeleine E.; Repka, Lindsay M.; Reisman, Sarah E.

2012-01-01

156

Asymmetric Synthesis of Diverse Glycolic Acid Scaffolds via Dynamic Kinetic Resolution of ?-Keto Esters  

PubMed Central

The dynamic kinetic resolution of ?-keto esters via asymmetric transfer hydrogenation has been developed as a technique for the highly stereoselective construction of structurally diverse ?-substituted-?-hydroxy carboxylic acid derivatives. Through the development of a privileged m-terphenylsulfonamide for (arene)RuCl(monosulfonamide) complexes with a high affinity for selective ?-keto ester reduction, excellent levels of chemo-, diastereo-, and enantiocontrol can be realized in the reduction of ?-aryl- and ?-chloro-?-keto esters.

Steward, Kimberly M.; Corbett, Michael T.; Goodman, C. Guy; Johnson, Jeffrey S.

2012-01-01

157

Synthesis of Several Cleistrioside and Cleistetroside Natural Products via a Divergent De Novo Asymmetric Approach  

PubMed Central

The de novo asymmetric syntheses of several partially acylated dodecanyl tri- and tetra-rhamnoside natural products (cleistriosides-5 & 6 and cleistetrosides-2 to 7) have been achieved (19 to 24 steps). The divergent route requires the use of three or less protecting groups. The asymmetry was derived via Noyori reduction of an acylfuran. The rhamno-stereochemistry was installed by a diastereoselective palladium-catalyzed glycosylation, ketone reduction and dihydroxylation.

Wu, Bulan; Li, Miaosheng; O'Doherty, George A.

2010-01-01

158

A two step synthesis of a key unit B precursor of cryptophycins by asymmetric hydrogenation  

PubMed Central

Summary A novel highly enantioselective two step access to a unit B precursor of cryptophycins in good yields from commercially available starting materials has been developed. The key step is an asymmetric hydrogenation using the commercially available [(COD)Rh-(R,R)-Et-DuPhos]BF4 catalyst. The synthetic route provides the advantage of less synthetic steps, proceeds with high yields and enantioselectivity, and avoids hazardous reaction conditions.

Sammet, Benedikt; Brax, Mathilde

2011-01-01

159

A two step synthesis of a key unit B precursor of cryptophycins by asymmetric hydrogenation.  

PubMed

A novel highly enantioselective two step access to a unit B precursor of cryptophycins in good yields from commercially available starting materials has been developed. The key step is an asymmetric hydrogenation using the commercially available [(COD)Rh-(R,R)-Et-DuPhos]BF(4) catalyst. The synthetic route provides the advantage of less synthetic steps, proceeds with high yields and enantioselectivity, and avoids hazardous reaction conditions. PMID:21448253

Sammet, Benedikt; Brax, Mathilde; Sewald, Norbert

2011-01-01

160

Efficient synthesis of carbazolespirooxindole skeletons via asymmetric Diels-Alder reaction of 3-vinylindoles and methyleneindolinones.  

PubMed

A highly efficient catalytic asymmetric Diels-Alder reaction between 3-vinylindoles and methyleneindolinones has been achieved using chiral N,N'-dioxide-Ni(ii) complexes as the catalysts. A wide variety of substrates were readily tolerated, generating exclusively the corresponding exo-carbazolespirooxindole derivatives in excellent yields with high enantiomeric excesses (up to 98% yield, >99?:?1 d.r., and 98% ee) under mild reaction conditions. PMID:24967917

Zheng, Haifeng; He, Peng; Liu, Yangbin; Zhang, Yulong; Liu, Xiaohua; Lin, Lili; Feng, Xiaoming

2014-08-14

161

Asymmetric Synthesis of Medium-Sized Rings by Intramolecular Au(I)-Catalyzed Cyclopropanation  

PubMed Central

An efficient method for the asymmetric gold(I)-catalyzed preparation of medium sized rings has been developed. The method provides 7- to 9-membered rings in excellent yield. High enantioselectivities can be achieved for 7- and 8-membered ring products employing chiral gold(I) complexes. The results provide insight into the mechanism, showing the fluxional nature of gold(I)-stabilized vinyl carbenoid intermediates.

Watson, Iain D. G.; Ritter, Stefanie; Toste, F. Dean

2009-01-01

162

Stereodivergent organocatalytic intramolecular Michael addition/lactonization for the asymmetric synthesis of substituted dihydrobenzofurans and tetrahydrofurans.  

PubMed

A stereodivergent asymmetric Lewis base catalyzed Michael addition/lactonization of enone acids into substituted dihydrobenzofuran and tetrahydrofuran derivatives is reported. Commercially available (S)-(-)-tetramisole hydrochloride gives products with high syn diastereoselectivity in excellent enantioselectivity (up to 99:1 d.r.syn/anti , 99?% eesyn ), whereas using a cinchona alkaloid derived catalyst gives the corresponding anti-diastereoisomers as the major product (up to 10:90 d.r.syn/anti , 99?% eeanti ). PMID:24989672

Belmessieri, Dorine; de la Houpliere, Alix; Calder, Ewen D D; Taylor, James E; Smith, Andrew D

2014-07-28

163

The Synthesis and Study on Optical Characteristics of the Asymmetrical Polymerizable Fluorescent Brighteners  

Microsoft Academic Search

A novel type of asymmetrical polymerizable fluorescent brighteners was synthesized using 4-nitro-4-aminostilbene-2,2?-disulfonic acid (ANSD) and cyanuric chloride as raw materials, through reduction reaction of sodium hydrosulfide and condensation reaction of variant amine compounds replacing the chlorine of triazine rings, containing another dibasic triazine compound grafted polyaminamide. The ultraviolet absorption and fluorescence emission properties, photoinduced isomerization phenomena and application properties of

Jing Liu; Guang-hua Zhang

2010-01-01

164

Synthesis of Chiral Exocyclic Amines by Asymmetric Hydrogenation of Aromatic Quinolin-3-amines.  

PubMed

Asymmetric hydrogenation of aromatic quinolin-3-amines was successfully developed with up to 94?% enantiomeric excess (ee). Introduction of the phthaloyl moiety to the amino group is crucial to eliminate the inhibition effect caused by the substrate and product, to activate the aromatic ring, and to improve the diastereoselectivity. This new methodology provides direct and facile access to chiral exocyclic amines. Notably, this report is the first on the highly enantioselective hydrogenation of aromatic amines. PMID:24827549

Cai, Xian-Feng; Guo, Ran-Ning; Chen, Mu-Wang; Shi, Lei; Zhou, Yong-Gui

2014-06-10

165

Asymmetric synthesis of pyrimidyl alkanol without adding chiral substances by the addition of diisopropylzinc to pyrimidine-5-carbaldehyde in conjunction with asymmetric autocatalysis  

Microsoft Academic Search

Enantiomerically enriched pyrimidyl alkanol with either S or R configuration was obtained stochastically from the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc without adding chiral substances in conjunction with subsequent asymmetric autocatalysis, leading to amplification of the enantiomeric excess.

Kenso Soai; Itaru Sato; Takanori Shibata; Soichiro Komiya; Masanobu Hayashi; Yohei Matsueda; Hikaru Imamura; Tadakatsu Hayase; Hiroshi Morioka; Hayami Tabira; Jun Yamamoto; Yasunori Kowata

2003-01-01

166

Desoxoprosopinine: A Model for the Total Asymmetric Synthesis of Prosopis Alkaloids.  

National Technical Information Service (NTIS)

This work develops a general synthesis of the Prosopis africana alkaloids, using a chiral auxiliary-mediated process. Enantiopure N- acyldihydropyridones were produced and used as intermediates, targeting (+)- desoxoprosopinine as a synthetic model for th...

M. J. Sandelier

1999-01-01

167

Synthesis of dicationic symmetrical and asymmetrical ionic liquids and their tribological properties as ultrathin films  

Microsoft Academic Search

A new series of novel dicationic symmetrical and asymmetrical ionic liquids (ILs) consisting of tributylalkyl phosphonium\\u000a and alkylimidazolium were synthesized. Their tribological properties in the form of spin-coated ultra-thin films sliding against\\u000a AISI-52100 steel ball were studied in a ball-on-plate configuration on a Universal Micro-tribometer-2MT tester, using perfluoropolyether\\u000a (PFPE) and normal ILs as comparisons. The best result is obtained for

Guiqin Yu; Shiqiang Yan; Feng Zhou; Xuqing Liu; Weimin Liu; Yongmin Liang

2007-01-01

168

Flexible and Asymmetric Ligand in Constructing Coordinated Complexes: Synthesis, Crystal Structures and Fluorescent Characterization  

PubMed Central

Flexible and asymmetric ligand L [L = 1-((pyridin-3-yl)methyl)-1H-benzotriazole], is used as a basic backbone to construct complicated metal-organic frameworks. Two new polymers, namely, [Ag2(L)2(NO3)2]n (1) and [Ag(L)(ClO4)]n (2), were synthesized and characterized by X-ray structure analysis and fluorescent spectroscopy. The complex 1 gives an “S” type double helical conformation, whereas complex 2 exhibits a 1D zigzag configuration. Different anions affect the silver coordination geometry and crystal packing topology.

Chen, Peng; Lin, Jianhua

2011-01-01

169

Concise methods for the synthesis of chiral polyoxazolines and their application in asymmetric hydrosilylation  

PubMed Central

Summary Seven polyoxazoline ligands were synthesized in high yield in a one-pot reaction by heating polycarboxylic acids or their esters and chiral ?-amino alcohols under reflux with concomitant removal of water or the alcohol produced in the reaction. The method is much simpler and more efficient in comparison to those methods reported in the literature. The compounds were used as chiral ligands in the rhodium-catalyzed asymmetric hydrosilylation of aromatic ketones, and the effects of the linkers and the substituents present on the oxazoline rings on the yield and enantioselectivity investigated. Compound 2 was identified as the best ligand of this family for the hydrosilylation of aromatic ketones.

Xu, Zun Le; Qiu, Sheng Xiang

2010-01-01

170

Synthesis, crystal structure and DNA cleavage activities of copper(II) complexes with asymmetric tridentate ligands  

Microsoft Academic Search

Two asymmetric tridentate copper(II) complexes, [Cu(dppt)Cl2]·0.25H2O (1) (dppt=3-(1,10-phenanthrolin-2-yl)-5,6-diphenyl-as-triazine) and [Cu(pta)Cl2] (2) (pta=3-(1,10-phenanthrolin-2-yl)-as-triazino[5,6-f]acenaphthylene), have been prepared and characterized by elemental analysis, IR and Fast atomic bombardment mass spectra. Complex 1 has also been structurally characterized. The complexes exist as distorted square pyramid with five co-ordination sites occupied by the tridentate ligand and the two chlorine anions. DNA interaction studies suggest that

Xiang-Li Wang; Hui Chao; Hong Li; Xian-Lan Hong; Liang-Nian Ji; Xiao-Yuan Li

2004-01-01

171

Asymmetric stereodivergent strategy towards aminocyclitols.  

PubMed

A concise asymmetric synthesis of aminocyclitols, such as diastereomeric 2-deoxystreptamine analogues and conduramine?A, is described. The Pd-catalyzed asymmetric desymmetrization of meso 1,4-dibenzolate enables the synthesis of highly oxidized cyclohexane architectures. These scaffolds can potentially be used to access new aminoglycoside antibiotics and enantiomerically pure ?-glucosidase inhibitors. PMID:24889256

Trost, Barry M; Malhotra, Sushant

2014-07-01

172

Asymmetric synthesis of cis-2,5-disubstituted pyrrolidine, the core scaffold of ?3-AR agonists.  

PubMed

A practical, enantioselective synthesis of cis-2,5-disubstituted pyrrolidine is described. Application of an enzymatic DKR reduction of a keto ester, which is easily accessed through a novel intramolecular N?C benzoyl migration, yields syn-1,2-amino alcohol in >99% ee and >99:1 dr. Subsequent hydrogenation of cyclic imine affords the cis-pyrrolidine in high diastereoselectivity. By integrating biotechnology into organic synthesis and isolating only three intermediates over 11 steps, the core scaffold of ?3-AR agonists is synthesized in 38% overall yield. PMID:23451898

Xu, Feng; Chung, John Y L; Moore, Jeffery C; Liu, Zhuqing; Yoshikawa, Naoki; Hoerrner, R Scott; Lee, Jaemoon; Royzen, Maksim; Cleator, Ed; Gibson, Andrew G; Dunn, Robert; Maloney, Kevin M; Alam, Mahbub; Goodyear, Adrian; Lynch, Joseph; Yasuda, Nobuyashi; Devine, Paul N

2013-03-15

173

Asymmetric, Organocatalytic, 3-Step Synthesis of ?-Hydroxy-(E)-?-?-Unsaturated Sulfones and Esters  

PubMed Central

Efficient and enantiocontrolled synthesis of ?-hydroxy-?,?-unsaturated sulfones and esters are reported through the reaction of enantioenriched ?-selenyl aldehydes with EWG-stabilized carbanions and then a one pot selenide oxidation, in situ epoxide formation, and final in situ epoxide opening.

Petersen, Kimberly S.

2009-01-01

174

Pd-catalyzed enantioselective C-H iodination: asymmetric synthesis of chiral diarylmethylamines.  

PubMed

An enantioselective C-H iodination reaction using a mono-N-benzoyl-protected amino acid has been developed for the synthesis of chiral diarylmethylamines. The reaction uses iodine as the sole oxidant and proceeds at ambient temperature and under air. PMID:24151991

Chu, Ling; Wang, Xiao-Chen; Moore, Curtis E; Rheingold, Arnold L; Yu, Jin-Quan

2013-11-01

175

A new asymmetric synthesis of (2 S,3 R,4 R,5 S)-trihydroxypipecolic acid  

Microsoft Academic Search

A novel four step synthesis of enantiomerically pure (2S,3R,4R,5S)-trihydroxypipecolic acid, starting from readily available materials, that is, condensation products of (R)-(?)-phenylglycinol with a mesotrihydroxylated glutaraldehyde, is described. The scope and limitations of the reaction have also been investigated.

Andriamihamina Tsimilaza; Tony Tite; Sabrina Boutefnouchet; Marie-Christine Lallemand; François Tillequin; Henri-Philippe Husson

2007-01-01

176

Asymmetric synthesis of indolines by catalytic enantioselective reduction of 3H-indoles.  

PubMed

A highly enantioselective metal-free reduction of 3H-indoles has been developed. This Brønsted acid catalyzed transfer hydrogenation of indole derivatives with Hantzsch dihydropyridine as the hydrogen source constitutes an efficient method for the synthesis of various optically active indolines with high enantioselectivities. PMID:20857958

Rueping, Magnus; Brinkmann, Claus; Antonchick, Andrey P; Atodiresei, Iuliana

2010-10-15

177

Synthesis and evaluation of a series of novel asymmetrical curcumin analogs for the treatment of inflammation.  

PubMed

Curcumin has been reported to possess multiple bioactivities, such as antioxidant, anticancer, and anti-inflammatory properties, however the clinical application of curcumin has been significantly limited by its instability and poor metabolism. Modification of curcumin has led to discovery and development of lots of novel therapeutic candidates. In recent years acute and chronic inflammation has been the focus of numerous studies in various diseases. Here, we synthesized a series of asymmetrical curcumin analogs with high in vitro chemical stability, and their anti-inflammatory activity was evaluated in LPS-stimulated macrophages. According to the bio-screening results and QSAR analysis, these analogs exhibited potent activities against LPS-induced TNF-? and IL-6 release. Among the analogs of the potent anti-inflammatory activity, compounds 3b8 and 3b9 exhibited significant protection and possess enhanced anti-inflammatory activity thereby attenuated the LPS-induced septic death in mice. PMID:24901832

Zhang, Yali; Zhao, Leping; Wu, Jianzhang; Jiang, Xin; Dong, Lili; Xu, Fengli; Zou, Peng; Dai, Yuanrong; Shan, Xiaoou; Yang, Shulin; Liang, Guang

2014-01-01

178

The synthesis and characterization of new iron coordination complexes utilizing an asymmetric coordinating chelate ligand  

SciTech Connect

A binuclear, unsymmetric coordinating ligand that is an effective metal chelator has been designed and synthesized. The new ligand has been shown to react readily with iron(II)/(III) forming a variety of coordination complexes. The binuclear complexes are of significant interest since they represent proof-of-principle for the development of coordinatively asymmetric, binuclear metal chelate compounds. Although this structural type of chelator now appears to be common in biological systems, it has not been previously described for inorganic coordination chemistry. The isolation of oxidation products will be helpful in establishing reaction mechanism(s) of these complexes with molecular oxygen. It is expected that this ligand and derivatives of it will play an important role in the development of bioinorganic complexes that aim to mimic enzyme active sites that function by substrate interaction at only one metal site of a multimetal active site.

Watkins, B.E.; Satcher, J.H.

1995-07-01

179

Asymmetric total synthesis of 6-Tuliposide B and its biological activities against tulip pathogenic fungi.  

PubMed

The structure-activity relationship was investigated to evaluate the antifungal activities of tuliposides and tulipalins against tulip pathogenic fungi. 6-Tuliposide B was effectively synthesized via the asymmetric Baylis-Hillman reaction. Tuliposides and tulipalins showed antifungal activities against most of the strains tested at high concentrations (2.5 mM), while Botrytis tulipae was resistant to tuliposides. Tulipalin formation was involved in the antifungal activity, tulipalin A showed higher inhibitory activity than 6-tuliposide B and tulipalin B. Both the tuliposides and tulipalins showed pigment-inducing activity against Gibberella zeae and inhibitory activity against Fusarium oxysporum f. sp tulipae. These activities were induced at a much lower concentration (0.05 mM) than the antifungal MIC values. PMID:21512240

Shigetomi, Kengo; Omoto, Shoko; Kato, Yasuo; Ubukata, Makoto

2011-01-01

180

Asymmetric synthesis of dihydroindanes by convergent alkoxide-directed metallacycle-mediated bond formation.  

PubMed

A convergent synthesis of highly substituted and stereodefined dihydroindanes is described from alkoxide-directed Ti-mediated cross-coupling of internal alkynes with substituted 4-hydroxy-1,6-enynes (substrates that derive from 2-directional functionalization of readily available epoxy alcohol derivatives). In addition to describing a new and highly stereoselective approach to bimolecular [2 + 2 + 2] annulation that delivers products not available with other methods in this area of chemical reactivity, evidence is provided to support annulation by way of regioselective alkyne-alkyne coupling, followed by metal-centered [4 + 2] rather than stepwise alkene insertion and reductive elimination. Overall, the reaction proceeds with exquisite stereochemical control and defines a convenient, convergent, and enantiospecific entry to fused carbocycles of great potential value in target-oriented synthesis and medicinal chemistry. PMID:22235773

Greszler, Stephen N; Reichard, Holly A; Micalizio, Glenn C

2012-02-01

181

Asymmetric Synthesis of Dihydroindanes by Convergent Alkoxide-Directed Metallacycle-Mediated Bond Formation  

PubMed Central

A convergent synthesis of highly substituted and stereodefined dihydroindanes is described from alkoxide-directed Ti-mediated cross-coupling of internal alkynes with substituted 4-hydroxy-1,6-enynes (substrates that derive from 2-directional functionalization of readily available epoxy alcohol derivatives). In addition to describing a new and highly stereoselective approach to bimolecular [2+2+2] annulation that delivers products not available with other methods in this area of chemical reactivity, evidence is provided to support annulation by way of regioselective alkyne–alkyne coupling, followed by metal-centered [4+2] rather than stepwise alkene insertion and reductive elimination. Overall, the reaction proceeds with exquisite stereochemical control and defines a convenient, convergent, and enantiospecific entry to fused carbocycles of great potential value in target-oriented synthesis and medicinal chemistry.

Greszler, Stephen N.; Reichard, Holly A.; Micalizio, Glenn C.

2012-01-01

182

Organocatalytic asymmetric synthesis of 3-chlorooxindoles bearing adjacent quaternary-tertiary centers.  

PubMed

A new methodology was developed for the synthesis of enantiomerically enriched 3,3-disubstituted 3-chlorooxindoles 3 via a Michael addition of 3-chloroxindoles to nitroolefins 2, catalyzed by chiral squaramide 10. Products with adjacent quaternary-tertiary centers were isolated in excellent yields (up to 99%), high diastereoselectivities (up to 11:1), and enantiomeric purities (up to 92%). This is the first example where 3-chloroxoindoles 1 have been used as nucleophiles in a highly stereoselective organocatalytic reaction. PMID:22958139

Noole, Artur; Järving, Ivar; Werner, Franz; Lopp, Margus; Malkov, Andrei; Kanger, Tõnis

2012-09-21

183

Dynamic kinetic asymmetric ring-opening/reductive amination sequence of racemic nitroepoxides with chiral amines: enantioselective synthesis of chiral vicinal diamines.  

PubMed

We report a highly diastereoselective synthesis of vicinal diamines by the treatment of nitroepoxides with primary amines and then a reducing agent. When using a chiral primary amine, racemic nitroepoxides are transformed into chiral diamines as a single enantiomers (>95:5 er) through a dynamic kinetic asymmetric transformation (DYKAT). The overall process is a one-pot procedure combining the exposure of nitroepoxides to chiral amines to afford diastereomeric mixtures of aminoimines and subsequent stereoselective imine reduction. PMID:23641667

Agut, Juan; Vidal, Andreu; Rodríguez, Santiago; González, Florenci V

2013-06-01

184

Enhancement of enantioselectivity by THF in asymmetric mo-catalyzed olefin metathesis. Catalytic enantioselective synthesis of cyclic tertiary ethers and spirocycles.  

PubMed

Mo-catalyzed enantioselective rearrangement of achiral cyclopentenyl tertiary ethers to chiral cyclohexenyl tertiary ethers are reported. These olefin metathesis transformations proceed efficiently and with high levels of enantioselectivity. A noteworthy feature of these reactions is that added tetrahydrofuran exerts a remarkably positive influence on the enantioselectivity of the metathesis-based rearrangement. The first examples of catalytic asymmetric synthesis of spirocyclic structures by enantioselective olefin metathesis are also disclosed. PMID:12207534

Teng, Xin; Cefalo, Dustin R; Schrock, Richard R; Hoveyda, Amir H

2002-09-11

185

Role of Ortho-Substituents on Rhodium-Catalyzed Asymmetric Synthesis of ?-Lactones by Intramolecular C-H Insertions of Aryldiazoacetates.  

PubMed

A rhodium-catalyzed asymmetric synthesis of ?-lactones via intramolecular C-H insertion into the ester group of aryldiazoacetates has been developed. The ?-lactones were synthesized in high yields and with high levels of diastereo- and enantioselectivity. Halo and trifluoromethyl substituents at the ortho position of the aryldiazoacetates enhance intramolecular C-H insertions over intermolecular reactions, allowing C-H insertion of even methyl C-H bonds. PMID:24840720

Fu, Liangbing; Wang, Hengbin; Davies, Huw M L

2014-06-01

186

Absolutely relativity  

NASA Astrophysics Data System (ADS)

The light speed constancy is proved here and then it is not a principle suppose it is a proposition proved absolutely based on the Galilean transformation and simultaneity and this is a book full from new discoveries along the absolutely proof for Lorentz transformation. Even in a page I have proved Lorentz transformation by the brawer constant point based on the Galilean transformation until to show easily it is possible to generate absolutely relativity and this is not ether theorem suppose we are upon the new discoveries all mathematical and complete, not a theory.

Lutephy, Mohsen

2012-03-01

187

ASYMMETRIC SYNTHESIS OF THE ABCD RING SYSTEM OF DAPHNILACTONE B VIA A TANDEM, DOUBLE INTRAMOLECULAR, [4+2]/[3+2] CYCLOADDITION STRATEGY†  

PubMed Central

An asymmetric synthesis of the ABCD ring system of daphnilactone B is described. The synthesis features a tandem, double intramolecular, [4+2]/[3+2] cycloaddition of a highly functionalized, enantiomerically enriched nitroalkene to generate a pentacyclic nitroso acetal. The cycloaddition establishes six contiguous stereogenic centers including the critical CD ring junction that bears two quaternary stereogenic centers. Hydrogenolysis of the nitroso acetal followed by amide reduction and cyclization provided the AB rings. The methyl substituent on the A ring was installed in the correct configuration via hydrogenation of an exocyclic olefin in the final step.

Nguyen, Son T.; Baiazitov, Ramil Y.

2009-01-01

188

Asymmetric synthesis from terminal alkenes by diboration/cross-coupling cascades  

PubMed Central

Amongst prospective starting materials for organic synthesis, terminal (monosubstituted) alkenes are ideal. In the form of ?-olefins, they are manufactured on enormous scale and they are the core product features from many organic chemical reactions. While their latent reactivity can easily enable hydrocarbon chain extension, alkenes also have the attractive feature of being stable in the presence of many acids, bases, oxidants and reductants. In spite of these impressive attributes, relatively few catalytic enantioselective transformations have been developed that transform aliphatic ?-olefins in >90% ee and, with the exception of site-controlled isotactic polymerization of ?-olefins,1 none of these processes result in chain-extending C-C bond formation to the terminal carbon.2, 3, 4, 5, 6 Herein, we describe a strategy that directly addresses this gap in synthetic methodology and present a single-flask catalytic enantioselective conversion of terminal alkenes into a range of chiral products. These reactions are enabled by an unusual neighboring group participation effect that accelerates Pd-catalyzed cross-coupling of 1,2-bis(boronates) relative to nonfunctionalized alkyl boronate analogs. In tandem with enantioselective diboration, this reactivity feature connects abundant alkene starting materials to a diverse array of chiral products. Importantly with respect to synthesis utility, the tandem diboration/cross-coupling reaction (DCC reaction) generally provides products in high yield and high selectivity (>95:5 enantiomer ratio), employs low loadings (1–2 mol %) of commercially available catalysts and reagents, it offers an expansive substrate scope, and can address a broad range of alcohol and amine synthesis targets, many of which cannot be easily addressed with current technology.

Mlynarski, Scott N.; Schuster, Christopher H.; Morken, James P.

2013-01-01

189

Enantioselective synthesis of the fragrance trans-magnolione under asymmetric phase transfer catalysis  

Microsoft Academic Search

The stereoselective synthesis of the fragrance trans-magnolione (1) through conjugate Michael addition of alkyl acetoacetates to 2-pentyl-2-cyclopentenone (2) under solid\\/liquid phase transfer catalysis (PTC) is reported. Under optimized conditions, the 1,4-addition of tert-butyl acetoacetate to enone 2 catalyzed by N-9-anthracenylmethylquininium chloride (4) affords, after hydrolysis and decarboxylation, trans-(2S,3S)-1 with 85\\/15 d.r. and 74% ee. The use of the pseudo-enantiomeric catalyst

Stefano Superchi; Mariangela Nardiello; Maria Irene Donnoli; Patrizia Scafato; Rita Menicagli; Carlo Rosini

2005-01-01

190

Directed ortho,ortho'-dimetalation of hydrobenzoin: Rapid access to hydrobenzoin derivatives useful for asymmetric synthesis  

PubMed Central

Summary A variety of ortho,ortho'-disubstituted hydrobenzoin derivatives are readily accessible through a directed ortho,ortho'-dimetalation strategy in which the alcohol functions in hydrobenzoin are deprotonated by n-BuLi and the resulting lithium benzyl alkoxides serve as directed metalation groups. The optimization and scope of this reaction are discussed, and the utility of this process is demonstrated in the one-pot preparation of a number of chiral diols as well as a short synthesis of the chiral ligand Vivol.

Cho, Inhee; Meimetis, Labros; Belding, Lee; Katz, Michael J; Dudding, Travis

2011-01-01

191

Asymmetric catalytic oxidative cleavage of polycyclic systems: the synthesis of atropisomeric diazonanes and diazecanes.  

PubMed

Oxidative cleavage of internal double bonds in polycyclic systems can give access to compounds containing medium- to large-sized rings. In this example, the nine- and ten-membered ring containing compounds that resulted from the mCPBA-mediated (mCPBA=meta-chloroperoxybenzoic acid) oxidative cleavage reaction were shown to exhibit atropisomerism. The reaction of the polycyclic system with catalytic amounts of ruthenium tetraoxide followed by diol cleavage achieved the same synthetic goal. Use of the Nishiyama-Beller ruthenium-based catalysts enabled the synthesis of optically-enriched samples, providing the first example of an atropselective oxidative cleavage reaction. PMID:21469229

Jones, Alan M; Liu, Gu; Lorion, Magali M; Patterson, Stephen; Lebl, Tomas; Slawin, Alexandra M Z; Westwood, Nicholas J

2011-05-01

192

Development of a large scale asymmetric synthesis of the glucocorticoid agonist BI 653048 BS H3PO4.  

PubMed

The development of a large scale synthesis of the glucocorticoid agonist BI 653048 BS H3PO4 (1·H3PO4) is presented. A key trifluoromethyl ketone intermediate 22 containing an N-(4-methoxyphenyl)ethyl amide was prepared by an enolization/bromine-magnesium exchange/electrophile trapping reaction. A nonselective propargylation of trifluoromethyl ketone 22 gave the desired diastereomer in 32% yield and with dr = 98:2 from a 1:1 diastereomeric mixture after crystallization. Subsequently, an asymmetric propargylation was developed which provided the desired diastereomer in 4:1 diastereoselectivity and 75% yield with dr = 99:1 after crystallization. The azaindole moiety was efficiently installed by a one-pot cross coupling/indolization reaction. An efficient deprotection of the 4-methoxyphenethyl group was developed using H3PO4/anisole to produce the anisole solvate of the API in high yield and purity. The final form, a phosphoric acid cocrystal, was produced in high yield and purity and with consistent control of particle size. PMID:23544738

Reeves, Jonathan T; Fandrick, Daniel R; Tan, Zhulin; Song, Jinhua J; Rodriguez, Sonia; Qu, Bo; Kim, Soojin; Niemeier, Oliver; Li, Zhibin; Byrne, Denis; Campbell, Scot; Chitroda, Ashish; DeCroos, Phil; Fachinger, Thomas; Fuchs, Victor; Gonnella, Nina C; Grinberg, Nelu; Haddad, Nizar; Jäger, Burkhard; Lee, Heewon; Lorenz, Jon C; Ma, Shengli; Narayanan, Bikshandarkoil A; Nummy, Larry J; Premasiri, Ajith; Roschangar, Frank; Sarvestani, Max; Shen, Sherry; Spinelli, Earl; Sun, Xiufeng; Varsolona, Richard J; Yee, Nathan; Brenner, Michael; Senanayake, Chris H

2013-04-19

193

Asymmetric synthesis of drug-like spiro[chroman-3,3'-indolin]-2'-ones through aminal-catalysis.  

PubMed

Asymmetric synthesis of drug-like functionalized spiro[chroman-3,3'-indolin]-2'-ones 5 containing three contiguous stereocenters with high diastereo- and enantioselectivities was achieved using the reflexive-Michael (r-M) reaction of unmodified hydroxyenals 1 with various (E)-3-alkylideneindolin-2-ones 2 in the presence of (R)-DPPOTMS/AcOH (R)-3/4b as a catalyst at room temperature. Chiral spiro[chroman-3,3'-indolin]-2'-ones 5 were transformed into the functionalized spiranes 7, 9, and 10 in good yields with high selectivity through Wittig, TCRA, acetal protection and reduction reactions, respectively. Supporting evidence for the reaction pathway through the formation of the important catalytic species of "aminals" was observed through NMR and ESI-HRMS analysis of an ongoing reaction between 1 and (R)-3 in CDCl3 and also shown by the structural requirement in hydroxyenals 1 to generate the “aminals” with (R)-3 through controlled experiments. PMID:24297104

Ramachary, Dhevalapally B; Prasad, M Shiva; Laxmi, S Vijaya; Madhavachary, R

2014-01-28

194

Asymmetric synthesis of highly substituted azapolycyclic compounds via 2-alkenyl sulfoximines: potential scaffolds for peptide mimetics.  

PubMed

The application of metalated, enantiomerically pure acyclic and cyclic 2-alkenyl sulfoximines for the synthesis of highly substituted aza(poly)cyclic ring systems is described. The method relies on a one-pot combination of a reagent-controlled allyl transfer reaction to alpha- or beta-amino aldehydes, followed by a Michael-type cyclization of the intermediate vinyl sulfoximines generated in the first step. The sulfur-free target compounds are preferentially obtained by samarium iodide treatment of the sulfonimidoyl substituted heterocycles. In addition to this methodological work, initial results on the biological activity of selected examples are reported. Furthermore, a concept for the transformation of peptidic lead structures into non-peptide mimetics is described, and the relevance of the new approach to highly substituted azaheterocycles in this context is discussed. PMID:16551111

Reggelin, Michael; Junker, Bernd; Heinrich, Timo; Slavik, Stefan; Bühle, Philipp

2006-03-29

195

Asymmetric synthesis from terminal alkenes by cascades of diboration and cross-coupling.  

PubMed

Terminal, monosubstituted alkenes are ideal prospective starting materials for organic synthesis because they are manufactured on very large scales and can be functionalized via a broad range of chemical transformations. Alkenes also have the attractive feature of being stable in the presence of many acids, bases, oxidants and reductants. In spite of these attributes, relatively few catalytic enantioselective transformations have been developed that transform aliphatic ?-olefins into chiral products with an enantiomeric excess greater then 90 per cent. With the exception of site-controlled isotactic polymerization of ?-olefins, none of these catalytic enantioselective processes results in chain-extending carbon-carbon bond formation to the terminal carbon. Here we describe a strategy that directly addresses this gap in synthetic methodology, and present a single-flask, catalytic enantioselective conversion of terminal alkenes into a number of chiral products. These reactions are facilitated by a neighbouring functional group that accelerates palladium-catalysed cross-coupling of 1,2-bis(boronates) relative to non-functionalized alkyl boronate analogues. In tandem with enantioselective diboration, this reactivity feature transforms alkene starting materials into a diverse array of chiral products. We note that the tandem diboration/cross-coupling reaction generally provides products in high yield and high selectivity (>95:5 enantiomer ratio), uses low loadings (1-2 mol per cent) of commercially available catalysts and reagents, offers an expansive substrate scope, and can address a broad range of alcohol and amine synthesis targets, many of which cannot be easily addressed with current technology. PMID:24352229

Mlynarski, Scott N; Schuster, Christopher H; Morken, James P

2014-01-16

196

Synthesis of enantiopure 2-aryl-2-methoxypropionic acids and determination of their absolute configurations by X-ray crystallography.  

PubMed

Racemic 2-aryl-2-methoxypropionic acids were enantioresolved by the use of (S)-(-)-phenylalaninol 4. For instance, racemic 2-methoxy-2-phenylpropionic acid (+/-)-7 was condensed with phenylalaninol (S)-(-)-4 yielding a diastereomeric mixture of amides, which was easily separated by HPLC on silica gel affording the first-eluted amide (-)-13a and the second-eluted amide (+)-13b: alpha = 3.19, Rs = 3.49. The absolute configuration of amide (-)-13a was determined to be (R;S) by X-ray crystallography by reference to the S configuration of the phenylalaninol moiety. Amide (R;S)-(-)-13a was converted to oxazoline (R;S)-(-)-14a, from which enantiopure 2-methoxy-2-phenylpropionic acid (R)-(-)-7 was recovered. Other 2-aryl-2-methoxypropionic acids, (R)-(-)-8, (R)-(-)-9, (R)-(+)-10, (R)-(-)-11, and (R)-(-)-12, were similarly prepared in enantiopure forms with the use of phenylalaninol (S)-(-)-4, and their absolute configurations were clearly determined by X-ray crystallography or by chemical correlation. PMID:17559106

Sekiguchi, Satoshi; Naito, Junpei; Taji, Hiromi; Kasai, Yusuke; Sugio, Akinori; Kuwahara, Shunsuke; Watanabe, Masataka; Harada, Nobuyuki

2008-03-01

197

Catalytic asymmetric diaziridination.  

PubMed

The first catalytic enantio- and diastereoselective synthesis of diaziridines is presented. Aziridination of N-tosyl aldimines applying modified hydroxylamine under asymmetric phase-transfer catalysis furnished optically active diaziridines. The diaziridines are formed as a single diastereomer in up to 96% ee, containing two orthogonal N-protecting groups, which can be deprotected selectively. PMID:23488989

Lykke, Lennart; Halskov, Kim Søholm; Carlsen, Bjørn Dreiø; Chen, Victor X; Jørgensen, Karl Anker

2013-03-27

198

Synthesis of six-membered spirocyclic oxindoles with five consecutive stereocenters in an asymmetric organocatalytic one-pot Michael/Michael/aldol addition sequence.  

PubMed

An asymmetric organocatalytic one-pot synthesis of six-membered spirocyclic oxindoles has been successfully developed through a relay Michael/Michael/aldol addition reaction catalyzed by the combination of readily available diphenylprolinol silyl ether and bifunctional quinine thiourea. The one-pot protocol affords the highly substituted spirocyclic oxindoles in high yields and perfect enantioselectivities. More importantly, through judicious choice of the organocatalysts employed, this reaction could be readily adapted to predominantly afford an alternative major diastereomer of the product. PMID:23469872

Zhou, Bing; Yang, Yaxi; Shi, Jingjing; Luo, Zhi; Li, Yuanchao

2013-04-01

199

Asymmetric decarboxylative 1,4-addition of malonic Acid half thioesters to vinyl sulfones: highly enantioselective synthesis of 3-monofluoromethyl-3-arylpropanoic esters.  

PubMed

An asymmetric decarboxylative 1,4-addition of malonic acid half thioesters (MAHTs) to 2-aryl-substituted vinyl sulfones has been developed, yielding adducts with excellent enantioselectivity (up to 97?% ee). In view of tuning pKa values, a quinine-based benzyl-substituted thiourea was designed and demonstrated as the most efficient catalyst. The enantioselective synthesis of 3-monofluorinated analogues of 3-methyl indanone and (+)-turmerone has been accomplished from decarboxylative 1,4-addition adducts with satisfactory results. PMID:24591040

Qiao, Baokun; Liu, Qian; Liu, Hongjun; Yan, Lin; Jiang, Zhiyong

2014-05-01

200

Copper-Catalyzed Asymmetric Synthesis and Comparative Aldose Reductase Inhibition Activity of (+)/(-)-1,2-Benzothiazine-1,1-dioxide Acetic Acid Derivatives.  

PubMed

A copper catalyst system for the asymmetric 1,4-hydrosilylation of the ?,?-unsaturated carboxylate class was developed by which synthesis of (+)- and (-)-enantiomers of 1,2-benzothiazine-1,1-dioxide acetates has been achieved with a good yield and an excellent level of enantioselectivity. A comparative structure-activity relationship study yielded the following order of aldose reductase inhibition activity: (-)-enantiomers > racemic > (+)-enantiomers. Further, a molecular docking study suggested that the (-)-enantiomer had significant binding affinity and thus increased inhibition activity. PMID:24785266

Parveen, Shagufta; Hussain, Saghir; Qin, Xiangyu; Hao, Xin; Zhu, Shaojuan; Rui, Miao; Zhang, Shuzhen; Fu, Fengyan; Ma, Bing; Yu, Qun; Zhu, Changjin

2014-06-01

201

Copper(II)-catalyzed enantioselective hydrosilylation of halo-substituted alkyl aryl and heteroaryl ketones: asymmetric synthesis of (R)-fluoxetine and (S)-duloxetine.  

PubMed

A set of reaction conditions has been established to facilitate the non-precious copper-catalyzed enantioselective hydrosilylation of a number of structurally diverse ?-, ?- or ?-halo-substituted alkyl aryl ketones and ?-, ?- or ?-halo-substituted alkyl heteroaryl ketones under air to afford a broad spectrum of halo alcohols in high yields and good to excellent enantioselectivities (up to 99% ee). The developed procedure has been successfully applied to the asymmetric synthesis of antidepressant drugs (R)-fluoxetine and (S)-duloxetine, which highlighted its synthetic utility. PMID:24382650

Zhou, Ji-Ning; Fang, Qiang; Hu, Yi-Hu; Yang, Li-Yao; Wu, Fei-Fei; Xie, Lin-Jie; Wu, Jing; Li, Shijun

2014-02-14

202

Asymmetric synthesis of novel (1H-benzo[d]imidazol -2-ylthio)- and (di-n-butylamino-2-ylthio)acetamides.  

PubMed

Asymmetric synthesis of novel (1H-benzo[d]imidazol-2-ylthio)- and (di-n-butylamino)acetamides is described. The o-nitrobenzyl oxime ethers were reduced with borane in the presence of oxazaborolidenes derived from norephedrine or diphenylvalinol and diphenylphosphinic amide was reduced with modified sodium tetrahydroborate catalyzed with beta-ketoiminato cobalt(II) complex to the corresponding amines with high yields and moderate enantiomeric excess. Reaction of amines with 2-chloroacetyl chloride and next with thiobenzimidazole or n-dibutylamine gave corresponding products with high yields. PMID:23614279

?aczkowski, Krzysztof Z

2013-01-01

203

An enantioselective strategy for the total synthesis of (S)-tylophorine via catalytic asymmetric allylation and a one-pot DMAP-promoted isocyanate formation/Lewis acid catalyzed cyclization sequence.  

PubMed

A new asymmetric total synthesis of a phenanthroindolizidine alkaloid (S)-tylophorine is reported, which features a catalytic asymmetric allylation of aldehydes and an unexpected one-pot DMAP promoted isocyanate formation and Lewis acid catalyzed intramolecular cyclization reaction. In addition, White's direct C-H oxidation catalyst system converting monosubstituted olefins to linear allylic acetates was also employed for late-stage transformation. PMID:24756131

Su, Bo; Zhang, Hui; Deng, Meng; Wang, Qingmin

2014-06-14

204

Total synthesis and absolute configuration of minalemine A, a guanidine peptide from the marine tunicate Didemnum rodriguesi.  

PubMed

The total synthesis of the 3S,2S and 3R,2S diastereomers (1a and 1b) of minalemine A and the identification of the natural compound as the 3R,2S isomer is described. The key step in the synthesis is the preparation of the two enantiomers of the beta-amino diacid 3-(N-carboxymethyl)-aminodecanoic acid (Ncma), which were obtained by stereoselective alkylation with allyl bromide of two nonanoic acid imides bearing chiral oxazolidinones as chiral auxiliaries. Natural minalemine A shows identical 1H NMR and very similar 13C NMR spectra compared to the two synthetic diastereomers. Sufficient differences in their chromatographic behavior to allow conclusive identification were not found. However, the corresponding N-2-naphthoyl amides presented quite distinct circular dichroism spectra (CD), and these confirmed the 3R,2S configuration for the natural minalemines and the R configuration for the constituent beta-amino diacid, Ncma. PMID:11397155

Expósito, A; Fernández-Suárez, M; Iglesias, T; Muñoz, L; Riguera, R

2001-06-15

205

Total synthesis of (+)-crotogoudin.  

PubMed

Fellowship of the ring: The first total synthesis of (+)-crotogoudin, a 3,4-seco-atisane diterpenoid natural product, is reported. Asymmetric access to the bicyclo[2.2.2]octane core is achieved through a desymmetrization of a meso-diketone with baker's yeast (LG=leaving group, PG=protecting group). A SmI2 -induced radical cyclopropane-opening/annulation/elimination cascade affords the suitably decorated tetracyclic structure of (+)-crotogoudin. The synthesis led to revision of the reported optical rotation of the natural product and to assignment of its absolute configuration as an ent-atisane (5R,10R). PMID:24039092

Breitler, Simon; Carreira, Erick M

2013-10-11

206

Cloning, overexpression, and characterization of a high enantioselective nitrilase from Sphingomonas wittichii RW1 for asymmetric synthesis of (R)-phenylglycine.  

PubMed

In this study, a high (R)-enantioselective nitrilase gene from Sphingomonas wittichii RW1 was cloned and overexpressed in Escherichia coli BL21 (DE3). The recombinant nitrilase was purified to homogeneity with a molecular weight of 40 kDa. The pH and temperature optima were shown to be pH 8.0 and 40 °C, respectively. The purified nitrilase was most active toward succinonitrile, approximately 30-fold higher than that for phenylglycinonitrile. Using the E. coli BL21/ReSWRW1 whole cells as biocatalysts, the kinetic resolution for asymmetric synthesis of (R)-phenylglycine was investigated at pH 6.0. A yield of 46 % was obtained with 95 % enantiomeric excess (ee), which made it a promising biocatalyst for synthesis of (R)-phenylglycine. PMID:24664232

Qiu, Jian; Su, Er-Zheng; Wang, Hua-Lei; Cai, Wen-Wen; Wang, Wei; Wei, Dong-Zhi

2014-05-01

207

Synthesis and absolute configuration of a new 3,4-dihydro-beta-carboline-type alkaloid, 3,4-dehydro-5(S)-5-carboxystrictosidine, isolated from Peruvian Uña de Gato (Uncaria tomentosa).  

PubMed

The structure including the absolute configuration of a new glucoalkaloid, 3,4-dehydro-5(S)-5-carboxystrictosidine, isolated from Peruvian Uña de Gato (Cat's Claw, original plant: Uncaria tomentosa), was confirmed by synthesis starting from secologanin and L-tryptophan. PMID:12372867

Kitajima, Mariko; Yokoya, Masashi; Takayama, Hiromitsu; Aimi, Norio

2002-10-01

208

All-catalytic, efficient, and asymmetric synthesis of alpha,omega-diheterofunctional reduced polypropionates via "one-pot" Zr-catalyzed asymmetric carboalumination-Pd-catalyzed cross-coupling tandem process.  

PubMed

A highly efficient method for the synthesis of stereochemically pure (>/=99% ee and >50/1 dr) alpha,omega-diheterofunctional reduced polypropionates has been developed. The essential features of the method are represented by the conversion of inexpensive styrene into 2-methyl-4-phenyl-1-pentanol (1) in 50% yield over two steps from styrene via Zr-catalyzed asymmetric carboalumination (ZACA) reaction in the presence of (NMI)2ZrCl2 and Pd-catalyzed vinylation of the in situ generated isoalkylalanes in the presence of Zn(OTf)2 and a catalytic amount of Pd(DPEphos)Cl2. This ZACA-Pd-catalyzed vinylation may be repeated as needed without purification. After the final ZACA reaction, oxidation with O2 provides alpha-hydroxy-omega-phenyl reduced polypropionates, which can be fully or partially purified by chromatography. After acetylation, Ru-catalyzed oxidative cleavage of the Ph ring, and reduction with BH3.THF, the second chromatographic purification provides stereoisomerically pure alpha,omega-diheterofunctional reduced polypropionates (e.g., 9 and 11) that can be further converted to key intermediates 6 and 7 for the synthesis of ionomycin (4) and borrelidin (5), respectively, by known reactions. PMID:15740104

Novak, Tibor; Tan, Ze; Liang, Bo; Negishi, Ei-Ichi

2005-03-01

209

Asymmetric synthesis of ?-alkenyl homoallylic primary amines via 1,2-addition of Grignard reagent to ?,?-unsaturated phosphonyl imines  

PubMed Central

A series of chiral N-phosphonyl protected ?-alkenyl homoallylic primary amines were synthesized by asymmetric addition of allylmagnesium bromide Grignard reagent towards chiral ?,?-unsaturated imines. Only 1,2-adduct was obtained for all the imines with good yields and excellent diastereoselectivities. The chiral auxiliary could be easily removed under simple conditions, giving free multiple functionalized primary amines.

Xiong, Yiwen; Mei, Haibo; Xie, Chen; Han, Jianlin; Li, Guigen; Pan, Yi

2013-01-01

210

Organocatalytic asymmetric domino Michael-Henry reaction for the synthesis of substituted bicyclo[3.2.1]octan-2-ones.  

PubMed

The first organocatalytic asymmetric reaction between 1,4-cyclohexanedione and nitroalkenes has been studied, affording bicyclo[3.2.1]octane derivatives containing four continuous stereogenic centres. The products were obtained through a domino Michael-Henry process as a single diastereoisomer with excellent enantioselectivities. PMID:23392546

Tsakos, Michail; Elsegood, Mark R J; Kokotos, Christoforos G

2013-03-18

211

One-Pot Asymmetric Nitro-Mannich/Hydroamination Cascades for the Synthesis of Pyrrolidine Derivatives: Combining Organocatalysis and Gold Catalysis  

PubMed Central

The highly enantioselective preparation of trisubstituted pyrrolidine derivatives employing a one-pot nitro-Mannich/hydroamination cascade is reported. This cascade approach utilizes an asymmetric bifunctional organocatalytic nitro-Mannich reaction followed by a gold-catalyzed allene hydroamination reaction. The products are afforded in good yields and excellent diastereo- and enantioselectivities.

2014-01-01

212

Non-cross-linked polystyrene-supported 2-imidazolidinone chiral auxiliary: synthesis and application in asymmetric alkylation reactions  

PubMed Central

Summary Asymmetric alkylation reactions using non-cross-linked polystyrene (NCPS)-supported 2-imidazolidinone chiral auxiliaries were successfully investigated with excellent diastereocontrol (>99% de). The recovery and the recycling of this soluble polymer-supported chiral auxiliary were achieved in order to produce highly optical pure carboxylic acids.

Nguyen, Quynh Pham Bao

2013-01-01

213

Chiral bidentate phosphabenzene-based ligands: synthesis, coordination chemistry, and application in Rh-catalyzed asymmetric hydrogenations  

Microsoft Academic Search

Novel hydroxy-functionalized phosphabenzenes were synthesized, which provide the possibility to prepare chiral phosphabenzene–phosphites. These systems act as bidentate ligands toward rhodium centers and the corresponding metal complexes were applied in the rhodium-catalyzed asymmetric hydrogenation of prochiral substrates.

Christian Müller; Leire Guarrotxena López; Huub Kooijman; Anthony L. Spek; Dieter Vogt

2006-01-01

214

LDL Cholesterol Upregulates Synthesis of Asymmetrical Dimethylarginine in Human Endothelial Cells Involvement of S-Adenosylmethionine-Dependent Methyltransferases  

Microsoft Academic Search

Asymmetrical dimethylarginine (ADMA) is an endogenous nitric oxide synthase inhibitor. It is formed by protein arginine N-methyltransferases (PRMTs), which utilize S-adenosylmethionine as methyl group donor. ADMA plasma concentration is elevated in hypercholesterolemia, leading to endothelial dysfunction and producing proathero- genic changes of endothelial cell function. Four different isoforms of human PRMTs have been identified. Because the release of ADMA from

Rainer H. Boger; Karsten Sydow; Jurgen Borlak; Thomas Thum; Henrike Lenzen; Bibiana Schubert; Dimitrios Tsikas; Stefanie M. Bode-Boger

215

Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives  

Microsoft Academic Search

This paper reports the performance of asymmetric polyethersulfone ultrafiltration flat sheet membranes with polyethylene glycol (PEG) of different molecular weight as additives. The membranes were prepared by phase inversion process from casting solution containing polyethersulfone (PES) as polymer, N,N-dimethylformamide (DMF) as solvent and PEG of different molecular weights namely as PEG 200, PEG 400 and PEG 600 as additives. The

Ani Idris; Norashikin Mat Zain; M. Y. Noordin

2007-01-01

216

Catalytic asymmetric addition of aldehydes to oxocarbenium ions: a dual catalytic system for the synthesis of chromenes.  

PubMed

A synergistic catalytic system for the first asymmetric addition of aldehydes to in situ generated prochiral oxocarbenium ions has been developed. The dual catalytic protocol allows the simultaneous activation of both electrophile and nucleophile and provides access to a variety of valuable chiral 2H-chromenes with excellent enantioselectivities. PMID:22913599

Rueping, Magnus; Volla, Chandra M R; Atodiresei, Iuliana

2012-09-01

217

A gram-scale route to phlegmarine alkaloids: rapid total synthesis of (-)-cermizine B.  

PubMed

The synthesis of the Lycopodium alkaloid (-)-cermizine B (), which establishes its absolute configuration, is achieved by combining asymmetric organocatalysis and an uninterrupted eight-step reaction sequence, followed by a final reduction step. This "pot-economy" strategy provides access to the cis-phlegmarine stereoparent embedded in for the first time, rapidly and on a gram-scale. PMID:24727855

Bradshaw, Ben; Luque-Corredera, Carlos; Bonjoch, Josep

2014-06-10

218

Asymmetric synthesis of ( R)- and ( S)-4-methyloctanoic acids. A new route to chiral fatty acids with remote stereocenters  

Microsoft Academic Search

The enantioselective synthesis of both enantiomers of 4-methyloctanoic acid, one major aggregation pheromone component of the rhinoceros beetles of the genus Oryctes and an important aroma compound, is described. The key step of the synthesis is based on a stereospecific alkylation with an alcohol-protected alkyl iodide using a pseudoephedrine derivative as a chiral auxiliary followed by subsequent removal of the

Lourdes Muñoz; Ma Pilar Bosch; Gloria Rosell; Angel Guerrero

2009-01-01

219

Synthesis of polyethylene glycol supported chiral monosulfonamide and its application in asymmetric transfer hydrogenation of prochiral ketones  

Microsoft Academic Search

A new polyethylene glycol supported chiral monosulfonamide was conveniently synthesized from (R,R)-1,2-diphenylethylenediamine. The ruthenium catalyst prepared from polyethylene glycol supported chiral monosulfonamide with\\u000a [RuCl2(p-cymene)]2 was used in the asymmetric transfer hydrogenation of prochiral ketones in neat water with good to excellent conversions and\\u000a enantioselectivities. The catalyst could be easily recovered and reused several times.

Zhongqiang Zhou; Yong Sun

2010-01-01

220

Synthesis of Polymer-supported Chiral Lithium Amide Bases and Application in Asymmetric Deprotonation of Prochiral Cyclic Ketones  

PubMed Central

Polymer-supported chiral amines were effectively prepared from amino acid derivatives and Merrifield resin. Treatment of polymer-supported amines with n-butyllithium gave the corresponding polymer-suppported chiral lithium amide bases, which were tested in the asymmetric deprotonation reactions of prochiral ketones. The trimethylsilyl enol ethers were obtained in up to 82% ee at room temperature. The polymer-supported chiral lithium amides can be readily recycled and reused without any significant loss of reactivity or selectivity.

Ma, Lili; Williard, Paul G.

2007-01-01

221

Copper-catalyzed asymmetric hydroboration of ?-dehydroamino acid derivatives: facile synthesis of chiral ?-hydroxy-?-amino acids.  

PubMed

The Cu-catalyzed asymmetric conjugate hydroboration reaction of ?-substituted ?-dehydroamino acid derivatives has been established, affording enantioenriched syn- and anti-?-boronate-?-amino acid derivatives with excellent combined yields (83-99%, dr ? 1:1) and excellent enantioselectivities (92-98% ee). The hydroboration products were expediently converted into valuable ?-hydroxy-?-amino acid derivatives, which were widely used in the preparation of chiral drugs and bioactive molecules. PMID:24528372

He, Zhi-Tao; Zhao, Yi-Shuang; Tian, Ping; Wang, Chuan-Chuan; Dong, Han-Qing; Lin, Guo-Qiang

2014-03-01

222

Synthesis of C 3- and C 2-symmetric tris- and bis-sulfoxide ligands by asymmetric oxidation  

Microsoft Academic Search

A series of tris- and bis-sulfoxides were synthesized by multiple asymmetric oxidation of the corresponding sulfides. High enantioselectivities were obtained based on Horeau-type amplification of selectivity. Naphthyl-substituted sulfoxides allowed for higher selectivity and greater ease of purification. These sulfoxides were tested as ligands in rhodium-catalyzed olefin hydroacylation and 1,4-addition of phenyl boronic acid to 2-cyclohexen-1-one. While no enantioinduction was observed

Peter K. Dornan; Priscilla L. Leung; Vy M. Dong

2011-01-01

223

Synthesis of an azido precursor to (2S,5R)-5-hydroxylysine using an asymmetric organocatalytic chlorination/reduction sequence.  

PubMed

An efficient, robust, and scalable synthesis of an azido precursor to the modified amino acid (2S,5R)-5-hydroxylysine was developed on the basis of the use of a highly stereoselective organocatalytic ?-chlorination-reduction protocol. The final Fmoc-protected (2S,5R)-6-azido-5-hydroxylysine derivative can be used in solid-phase peptide synthesis, providing access to proteins that contain large quantities of post-translationally modified lysine (e.g., collagens). PMID:24175670

Johannes, Manuel; Brimble, Margaret A

2013-12-20

224

One-step synthesis of graphene nanoribbon-MnO2 hybrids and their all-solid-state asymmetric supercapacitors  

NASA Astrophysics Data System (ADS)

Three-dimensional (3D) hierarchical hybrid nanomaterials (GNR-MnO2) of graphene nanoribbons (GNR) and MnO2 nanoparticles have been prepared via a one-step method. GNR, with unique features such as high aspect ratio and plane integrity, has been obtained by longitudinal unzipping of multi-walled carbon nanotubes (CNTs). By tuning the amount of oxidant used, different mass loadings of MnO2 nanoparticles have been uniformly deposited on the surface of GNRs. Asymmetric supercapacitors have been fabricated with the GNR-MnO2 hybrid as the positive electrode and GNR sheets as the negative electrode. Due to the desirable porous structure, excellent electrical conductivity, as well as high rate capability and specific capacitances of both the GNR and GNR-MnO2 hybrid, the optimized GNR//GNR-MnO2 asymmetric supercapacitor can be cycled reversibly in an enlarged potential window of 0-2.0 V. In addition, the fabricated GNR//GNR-MnO2 asymmetric supercapacitor exhibits a significantly enhanced maximum energy density of 29.4 W h kg-1 (at a power density of 12.1 kW kg-1), compared with that of the symmetric cells based on GNR-MnO2 hybrids or GNR sheets. This greatly enhanced energy storage ability and high rate capability can be attributed to the homogeneous dispersion and excellent pseudocapacitive performance of MnO2 nanoparticles and the high electrical conductivity of the GNRs.Three-dimensional (3D) hierarchical hybrid nanomaterials (GNR-MnO2) of graphene nanoribbons (GNR) and MnO2 nanoparticles have been prepared via a one-step method. GNR, with unique features such as high aspect ratio and plane integrity, has been obtained by longitudinal unzipping of multi-walled carbon nanotubes (CNTs). By tuning the amount of oxidant used, different mass loadings of MnO2 nanoparticles have been uniformly deposited on the surface of GNRs. Asymmetric supercapacitors have been fabricated with the GNR-MnO2 hybrid as the positive electrode and GNR sheets as the negative electrode. Due to the desirable porous structure, excellent electrical conductivity, as well as high rate capability and specific capacitances of both the GNR and GNR-MnO2 hybrid, the optimized GNR//GNR-MnO2 asymmetric supercapacitor can be cycled reversibly in an enlarged potential window of 0-2.0 V. In addition, the fabricated GNR//GNR-MnO2 asymmetric supercapacitor exhibits a significantly enhanced maximum energy density of 29.4 W h kg-1 (at a power density of 12.1 kW kg-1), compared with that of the symmetric cells based on GNR-MnO2 hybrids or GNR sheets. This greatly enhanced energy storage ability and high rate capability can be attributed to the homogeneous dispersion and excellent pseudocapacitive performance of MnO2 nanoparticles and the high electrical conductivity of the GNRs. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr06650a

Liu, Mingkai; Tjiu, Weng Weei; Pan, Jisheng; Zhang, Chao; Gao, Wei; Liu, Tianxi

2014-03-01

225

Synthesis and Asymmetric Diels-Alder Reactions of Enantiopure 3-(Alkylsulfinyl)-1-methoxy-1,3-butadienes.  

PubMed

Simple syntheses of enantiopure (E)- and (Z)-3-(alkylsulfinyl)-1-methoxy-1,3-butadienes 2and 3 were provided by the addition of (1S)-isoborneol-10-sulfenic and (S)-phenyl-2-hydroxyethanesulfenic acids 8 to (E)- and (Z)-1-methoxybut-1-en-3-ynes (9) and (10). These additions proceeded with asymmetric induction, the extent of which depended upon the nature of the chiral hydroxyalkyl group. Cycloadditions of methyl acrylate to the enantiopure dienes proceeded with complete regioselectivity and very high stereoselectivity when catalyzed by lithium perchlorate or zinc chloride in dichloromethane. The chirality at sulfur controlled the diastereofacial selectivity in these Diels-Alder cycloadditions. PMID:11671762

Aversa, Maria C.; Barattucci, Anna; Bonaccorsi, Paola; Giannetto, Placido; Jones, David Neville

1997-06-27

226

Synthesis, structures, electrochemistry and catalytic activities of copper(II) and palladium(II) complexes with asymmetric tetraazacycloannulenes  

Microsoft Academic Search

Copper(II) and palladium(II) complexes with 15-membered asymmetric 5,9-dihydro-2,4,10,12-tetramethyl-1,5,9,13-monobenzotetraazacyclo[15]tetradecine have been synthesized and characterized. The electrochemical behaviors of the complexes showed a reduction and two one-electron irreversible oxidation waves in given potential ranges due to the metal ion and macrocycle ring, respectively. The electrocatalytic reduction of dioxygen on glassy carbon electrodes electropolymerized by such 15-membered and 14-membered tetraazaannulene complexes occurred at

E. H. Kim; D. I. Kim; H. S. Lee; H. G. Na; J. C. Byun; J. H. Choi; Y. C. Park

2007-01-01

227

Stereospecific asymmetric N-heterocyclic carbene (NHC)-catalyzed redox synthesis of trifluoromethyl dihydropyranones and mechanistic insights.  

PubMed

N-heterocyclic carbene (NHC)-catalyzed redox asymmetric hetero-Diels-Alder reactions of ?-aroyloxyaldehydes with ?-trifluoromethyl enones generates synthetically useful dihydropyranones containing a stereogenic trifluoromethyl substituent in good yields (up to 81%) and excellent diastereoselectivity and enantioselectivity (up to >95:5 dr and >99% ee). The process is stereospecific, with use of either (E)- or (Z)-?-trifluoromethyl enones forming syn- or anti-dihydropyranone products, respectively. Mechanistic studies through in situ kinetic analysis of the reaction reveal key differences in reactivity between chiral NHC precursor 1 and an achiral NHC precursor. PMID:23957790

Davies, Alyn T; Taylor, James E; Douglas, James; Collett, Christopher J; Morrill, Louis C; Fallan, Charlene; Slawin, Alexandra M Z; Churchill, Gwydion; Smith, Andrew D

2013-09-20

228

Solid-phase synthesis of chiral 3,4-diazaphospholanes and their application to catalytic asymmetric allylic alkylation  

PubMed Central

Functionalized chiral diazaphospholanes ligate to a variety of transition metals, yielding chiral, catalytically active, metal complexes. Previous work has established that amino acid derivatization of the carboxyl groups of (R,R)-N,N?-phthaloyl-2,3-(2-carboxyphenyl)-phenyl-3,4-diazaphospholane (1) yields phosphines that are excellent ligands for palladium-catalyzed asymmetric allylic alkylation reactions. Alanine functionalization is particularly effective for allylic alkylation of 1,3-dimethylallyl acetate. Standard Merrifield resins and amino acid coupling methods are used to synthesize the bead-attached phosphine having the topology bead-linker-lAla-(R,R)-1-lAla-OMe, as a 1:1 mixture of linkage isomers. Use of this supported phosphine in Pd-catalyzed asymmetric allylic alkylation yields 92% enantiomeric excess, matching prior solution-phase results. A 20-member collection of amino acid-functionalized phosphines on beads with the topology bead-linker-AA2-AA1-1-AA1-AA2 was synthesized by using parallel solid-state methods and screened for efficacy in allylic alkylation. Resulting enantioselectivities indicate that the AA1 position has the strongest effect on the reaction. Catalyst activities can vary widely with the nature of the phosphine ligand and the reaction conditions. Meaningful analysis of intrinsic catalytic activities awaits identification of the structure and abundance of the active catalyst.

Landis, Clark R.; Clark, Thomas P.

2004-01-01

229

Solid-phase synthesis of chiral 3,4-diazaphospholanes and their application to catalytic asymmetric allylic alkylation.  

PubMed

Functionalized chiral diazaphospholanes ligate to a variety of transition metals, yielding chiral, catalytically active, metal complexes. Previous work has established that amino acid derivatization of the carboxyl groups of (R,R)-N,N'-phthaloyl-2,3-(2-carboxyphenyl)-phenyl-3,4-diazaphospholane (1) yields phosphines that are excellent ligands for palladium-catalyzed asymmetric allylic alkylation reactions. Alanine functionalization is particularly effective for allylic alkylation of 1,3-dimethylallyl acetate. Standard Merrifield resins and amino acid coupling methods are used to synthesize the bead-attached phosphine having the topology bead-linker-LAla-(R,R)-1-LAla-OMe, as a 1:1 mixture of linkage isomers. Use of this supported phosphine in Pd-catalyzed asymmetric allylic alkylation yields 92% enantiomeric excess, matching prior solution-phase results. A 20-member collection of amino acid-functionalized phosphines on beads with the topology bead-linker-AA(2)-AA(1)-1-AA(1)-AA(2) was synthesized by using parallel solid-state methods and screened for efficacy in allylic alkylation. Resulting enantioselectivities indicate that the AA(1) position has the strongest effect on the reaction. Catalyst activities can vary widely with the nature of the phosphine ligand and the reaction conditions. Meaningful analysis of intrinsic catalytic activities awaits identification of the structure and abundance of the active catalyst. PMID:14985503

Landis, Clark R; Clark, Thomas P

2004-04-13

230

A photoisomerization-coupled asymmetric Stetter reaction: application to the total synthesis of three diastereomers of (-)-cephalimysin A  

PubMed Central

The total synthesis of 8-epi, 8,9-epi, and 9-epi-(?)-cephalimysin A is described. Our catalytic enantioselective synthesis takes advantage of a novel tandem photoisomerization/Stetter reaction. The approach provides rapid access to the desired spirofuranone lactam core in good yield and excellent enantioselectivity. A late stage oxidation strategy allows for flexible access to three of the four diastereomers of cephalimysin A. Access to the epimers provides further support for the correction of the initially proposed relative stereochemistry of cephalimysin A.

Lathrop, Stephen P.

2013-01-01

231

Synthesis and gene transfection activity of cyclen-based cationic lipids with asymmetric acyl-cholesteryl hydrophobic tails.  

PubMed

A series of novel 1,4,7,10-tetraazacyclododecane (cyclen)-based cationic lipids with asymmetric double hydrophobic tails (cholesteryl and long aliphatic chains) were designed and synthesized. Lysine was chosen as a linking moiety in the molecular backbone. The liposomes formed from 8 and dioleoylphosphatidylethanolamine (DOPE) could bind and condense plasmid DNA into nanoparticles under a low N/P ratio. These nano-scaled lipoplexes have low cytotoxicity, and might efficiently transfect A549 cells. In vitro transfection results revealed that all cationic lipids showed a comparable or better transfection efficiency (TE) than commercially available Lipofectamine 2000. The length and saturation degree of the aliphatic chain would affect their gene transfection performance, and the linoleic acid-containing 8e could give the best TE. PMID:24752704

Liu, Bao-Quan; Yi, Wen-Jing; Zhang, Ji; Liu, Qiang; Liu, Yan-Hong; Fan, Sheng-Di; Yu, Xiao-Qi

2014-06-01

232

Asymmetric synthesis of a (2Z,7E)-cyclononadiene by an intramolecular cycloalkylation and insight to its conformational properties  

PubMed

Here, we report a novel synthesis of a monosubstituted, enantio- and diastereomerically enriched trans-cyclononadienol. The reaction consists of an enantioselective (-)-sparteine-mediated allylic lithiation of an achiral 7-chlorononadienyl carbamate and a subsequent stereospecific intramolecular allyllithium-allyl chloride coupling. The stereochemical course of the cyclization has been determined, and the high configurative stability of the chiral nine-membered carbocycle has been investigated by kinetic measurements and rationalized by computational calculations. PMID:10956510

Deiters; Muck-Lichtenfeld; Frohlich; Hoppe

2000-08-10

233

Asymmetric synthesis of trisubstituted tetrahydrothiophenes bearing a quaternary stereocenter via double Michael reaction involving dynamic kinetic resolution.  

PubMed

The stereoselective synthesis of highly functionalized tetrahydrothiophenes bearing three contiguous stereocenters, one of them quaternary, can be achieved by reacting trans-?-cyano-?,?-unsaturated ketones and trans-tert-butyl 4-mercapto-2-butenoate in the presence of a readily available amine thiourea. The products are obtained in high yield, good diastereoselectivity, and excellent enantioselectivity. The overall formation of tetrahydrothiophenes occurs via a cascade double Michael reaction involving a highly efficient process of dynamic kinetic resolution. PMID:23786495

Meninno, Sara; Croce, Gianluca; Lattanzi, Alessandra

2013-07-01

234

Frequency-domain analysis of absolute gravimeters  

NASA Astrophysics Data System (ADS)

An absolute gravimeter is analysed as a linear time-invariant system in the frequency domain. Frequency responses of absolute gravimeters are derived analytically based on the propagation of the complex exponential signal through their linear measurement functions. Depending on the model of motion and the number of time-distance coordinates, an absolute gravimeter is considered as a second-order (three-level scheme) or third-order (multiple-level scheme) low-pass filter. It is shown that the behaviour of an atom absolute gravimeter in the frequency domain corresponds to that of the three-level corner-cube absolute gravimeter. Theoretical results are applied for evaluation of random and systematic measurement errors and optimization of an experiment. The developed theory agrees with known results of an absolute gravimeter analysis in the time and frequency domains and can be used for measurement uncertainty analyses, building of vibration-isolation systems and synthesis of digital filtering algorithms.

Svitlov, S.

2012-12-01

235

Synthesis and biological evaluation of asymmetric indole curcumin analogs as potential anti-inflammatory and antioxidant agents.  

PubMed

Abstract A series of asymmetric indole curcumin analogs were synthesized and evaluated as possible inhibiters of pro-inflammatory enzymes such as COX-2, pro-inflammatory cytokines as TNF-? and IL-6, trypsin and ?-glucuronidase. They were also tested for antioxidant activities. The results showed that compounds 5e and 5h were found to be the most potent inhibitors of COX-2 (83.33%, 82.50%) and ?-glucuronidase (67.80%, 64.12%). All the synthesized compounds exhibited promising activity against IL-6 in a range of 71-100% at 10 µM concentration. Compounds 5f, 5h, 5e, 5c and 5d showed significant inhibition against TNF-? (28-51%) and IL-6 (87-98%) with low toxicity (45-51%) against CCK-8 cells. With few exceptions, all other compounds were found to be good to excellent inhibitors of IL-6 and moderate inhibitors of TNF-?; however, the toxicity profiles of these compounds need to be ameliorated in further optimization studies. Amongst the tested compounds, 5c, 5b, 5j and 5g were found to possess excellent reducing activity and 5b, 5c and 5h were moderate DPPH (1,1-diphenyl-2-picryl hydrazine) radical scavengers. PMID:23356406

Bandgar, Babasaheb P; Kinkar, Santosh N; Chavan, Hemant V; Jalde, Shivkumar S; Shaikh, Rafik U; Gacche, Rajesh N

2014-02-01

236

1: Mass asymmetric fission barriers for {sup 98}Mo; 2: Synthesis and characterization of actinide-specific chelating agents  

SciTech Connect

Excitation functions have been measured for complex fragment emission from the compound nucleus {sup 98}Mo, produced by the reaction of {sup 86}Kr with {sup 12}C. Mass asymmetric fission barriers have been obtained by fitting the excitation functions with a transition state formalism. The extracted barriers are {approximately} 5.7 MeV higher, on average, than the calculations of the Rotating Finite Range Model (RFRM). These data clearly show an isospin dependence of the conditional barriers when compared with the extracted barriers from {sup 90}Mo and {sup 94}Mo. Eleven different liquid/liquid extractants were synthesized based upon the chelating moieties 3,2-HOPO and 3,4-HOPO; additionally, two liquid/liquid extractants based upon the 1,2-HOPO chelating moiety were obtained for extraction studies. The Pu(IV) extractions, quite surprisingly, yielded results that were very different from the Fe(III) extractions. The first trend remained the same: the 1,2-HOPOs were the best extractants, followed closely by the 3,2-HOPOs, followed by the 3,4-HOPOs; but in these Pu(IV) extractions the 3,4-HOPOs performed much better than in the Fe(III) extractions. 129 refs.

Veeck, A.C. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Livermore National Lab., CA (United States). Glenn T. Seaborg Inst. for Transactinium Science]|[Lawrence Berkeley National Lab., CA (United States). Nuclear Science Div.

1996-08-01

237

Synthesis of Silyloxy Dienes by Silylene Tranfer to Divinyl Ketones: Application to the Asymmetric Sythesis of Substituted Cyclohexanes  

PubMed Central

Silver-catalyzed silylene transfer to divinyl ketones provided 2-silyloxy-1,3-dienes with control of stereochemistry and regioselectivity. The products participated in Diels–Alder reactions with electron-deficient alkenes and imines to form six-membered ring products diastereoselectively. Cycloaddition reactions with alkenes bearing chiral auxiliaries provided access to chiral, non-racemic cyclohexenes. The methodology therefore represents a synthesis of diastereomerically and enantiomerically pure products in a single flask. The highly substituted cyclohexene products could be functionalized stereoselectively to provide cyclohexanols after oxidation of the carbon–silicon bond.

Ventocilla, Christian C.; Woerpel, K. A.

2012-01-01

238

Total Synthesis of (R)-Dihydroactinidiolide and (R)-Actinidiolide Using Asymmetric Catalytic Hetero-Diels-Alder Methodology.  

PubMed

The total synthesis of the naturally occurring bicyclic lactones (R)-dihydroactinidiolide and (R)-actinidiolide is presented. The key step in the syntheses is the copper(II)-bisoxazoline-catalyzed hetero-Diels-Alder reaction of a cyclic diene with ethyl glyoxylate giving the hetero-Diels-Alder product in high yield and with very high regio-, diastereo-, and enantioselectivity. The total syntheses proceed via an intermediate, which also has the potential for a series of other natural products. The structure of the key intermediate is confirmed by X-ray analysis. PMID:11674051

Yao, Sulan; Johannsen, Mogens; Hazell, Rita G.; Jørgensen, Karl Anker

1998-01-01

239

Asymmetric synthesis of ?-amino acids via homologation of Ni(II) complexes of glycine Schiff bases; Part 1: alkyl halide alkylations.  

PubMed

Alkylations of chiral or achiral Ni(II) complexes of glycine Schiff bases constitute a landmark in the development of practical methodology for asymmetric synthesis of ?-amino acids. Straightforward, easy preparation as well as high reactivity of these Ni(II) complexes render them ready available and inexpensive glycine equivalents for preparing a wide variety of ?-amino acids, in particular on a relatively large scale. In the case of Ni(II) complexes containing benzylproline moiety as a chiral auxiliary, their alkylation proceeds with high thermodynamically controlled diastereoselectivity. Similar type of Ni(II) complexes derived from alanine can also be used for alkylation providing convenient access to quaternary, ?,?-disubstituted ?-amino acids. Achiral type of Ni(II) complexes can be prepared from picolinic acid or via recently developed modular approach using simple secondary or primary amines. These Ni(II) complexes can be easily mono/bis-alkylated under homogeneous or phase-transfer catalysis conditions. Origin of diastereo-/enantioselectivity in the alkylations reactions, aspects of practicality, generality and limitations of this methodology is critically discussed. PMID:23832533

Sorochinsky, Alexander E; Aceña, José Luis; Moriwaki, Hiroki; Sato, Tatsunori; Soloshonok, Vadim A

2013-10-01

240

Assignment of relative configuration of desoxypropionates by 1H NMR spectroscopy: method development, proof of principle by asymmetric total synthesis of xylarinic acid A and applications.  

PubMed

The determination of the relative configuration of 1,3-dimethyl-substituted alkyl chains is possible by interpretation of (1)H NMR shift differences. Additionally, assignments are feasible in a variety of deuterated solvents, because the corresponding shift differences are not significantly influenced by the solvent. The trends for ?? values depending on functional groups adjacent to the stereogenic centers are shown. Based on a thorough comparison with literature data, the relative configuration of natural products can be predicted. For this purpose, we derived an empirical rule for the ranges in which ?? values usually occur. Furthermore, we were able to proof the validity of our method by the successful prediction of the relative configuration for the polyketide natural product xylarinic acid A, which was confirmed by the asymmetric total synthesis of its enantiomer. Based on the proposed simple analysis of published (1)H NMR data and the determination of the relevant chemical-shift differences, we predicted the relative configurations of several previously unassigned natural products. PMID:22544461

Schmidt, Yvonne; Lehr, Konrad; Colas, Lucie; Breit, Bernhard

2012-06-01

241

Direct amino acid-catalyzed cascade biomimetic reductive alkylations: application to the asymmetric synthesis of Hajos-Parrish ketone analogues.  

PubMed

A direct amino acid-catalyzed chemo- and enantioselective process for the double cascade synthesis of highly substituted 2-alkyl-cyclopentane-1,3-diones, 2-alkyl-3-methoxy-cyclopent-2-enones and Hajos-Parrish (H-P) ketone analogs is presented via reductive alkylation chemistry. For the first time, we have developed a single-step alkylation of cyclopentane-1,3-dione with aldehydes/ketones and a Hantzsch ester through an organocatalytic reductive alkylation strategy. A direct combination of amino acid-catalyzed cascade olefination-hydrogenation and cascade Robinson annulations of cyclopentane-1,3-dione, aldehydes/ketones, a Hantzsch ester and methyl vinyl ketone furnished the highly functionalized H-P ketone analogues in good to high yields and with excellent enantioselectivities. Many of the reductive alkylation products have shown direct applications in pharmaceutical chemistry. PMID:18972048

Ramachary, Dhevalapally B; Kishor, Mamillapalli

2008-11-21

242

Asymmetric catalysis on the nanoscale: the organocatalytic approach to helicenes.  

PubMed

The first asymmetric organocatalytic synthesis of helicenes is reported. A novel SPINOL-derived phosphoric acid, bearing extended ?-substituents, catalyzes the asymmetric synthesis of helicenes through an enantioselective Fischer indole reaction. A variety of azahelicenes and diazahelicenes could be obtained with good to excellent yields and enantioselectivities. PMID:24737692

Kötzner, Lisa; Webber, Matthew J; Martínez, Alberto; De Fusco, Claudia; List, Benjamin

2014-05-12

243

Absolute Theory of Relativity  

NASA Astrophysics Data System (ADS)

We redo Einstein's thought experiment with atomic clocks from the Special Theory of Relativity. Herein we consider an absolute time and an absolute space but no ultimate speed, and we call it Absolute Theory of Relativity (ATR). Our ATR is free from time dilation, space contraction, relative simultaneity, and relativistic paradoxes.

Smarandache, Florentin

2012-03-01

244

Absolute compression test  

Microsoft Academic Search

This patent describes a method for measuring the absolute compression of each cylinder in an internal combustion engine having known ideal performance parameters and consists of the following steps: sensing the absolute angular speed of the crankshaft of an engine during at least one noncombustive engine cycle; relating particular subcyclic absolute speed variations to particular cylinders; calculating the work of

Timmerman

1986-01-01

245

Asymmetric Conjugate Addition of Alkylzirconium Reagents to ?,?-Unsaturated Lactones.  

PubMed

The asymmetric synthesis of ?-substituted lactones by catalytic asymmetric conjugate addition of alkyl groups to ?,?-unsaturated lactones is reported. The method uses alkylzirconium nucleophiles prepared in situ from alkenes and the Schwartz reagent. Enantioselective additions to 6- and 7-membered lactones proceed at rt, tolerate a wide variety of functional groups, and are readily scalable. The method was used in a formal asymmetric synthesis of mitsugashiwalactone. PMID:24893570

Maciver, Eleanor E; Maksymowicz, Rebecca M; Wilkinson, Nancy; Roth, Philippe M C; Fletcher, Stephen P

2014-06-20

246

Asymmetric Synthesis of Nonracemic Primary Amines via Spiroborate Catalyzed Reduction of pure (E)- and (Z)-O-Benzyloximes: Applications toward the Synthesis of Calcimimetic Agents  

PubMed Central

Highly enantiopure (1-aryl)- and (1-naphthyl)-1-ethylamines were synthesized by the borane-mediated reduction of single-isomeric (E)- and (Z)-O-benzyloxime ethers using the stable spiroborate ester derived from (S)-diphenyl valinol and ethylene glycol as the chiral catalyst. Primary (R)-arylethylamines were prepared by the reduction of pure (Z)-ethanone oxime ethers in up to 99% ee using 15% of catalyst. Two convenient and facile approaches to the synthesis of new and known calcimimetic analogs employing enantiopure (1-naphthalen-1-yl)ethylamine as chiral precursor are described.

Ou, Wenhua; Espinosa, Sandraliz; Melendez, Hector J.; Farre, Silvia M.; Alvarez, Jaime L.; Torres, Valerie; Martinez, Ileanne; Santiago, Kiara M.; Ortiz-Marciales, Margarita

2013-01-01

247

Asymmetric cycloaddition of ?,?-unsaturated ?-ketoesters with electron-rich alkenes catalyzed by a chiral Er(OTf)3/N,N'-dioxide complex: highly enantioselective synthesis of 3,4-dihydro-2H-pyrans.  

PubMed

The asymmetric inverse-electron-demand hetero-Diels-Alder (HDA) reactions of ?,?-unsaturated ?-ketoesters with electron-rich alkenes were investigated, with an N,N'-dioxide/erbium(III) complex employed as the catalyst. Quantitative conversion of the ?,?-unsaturated ?-ketoesters and excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >99:1 d.r.) were observed for a broad range of substrates by using a 0.5-0.05 mol% catalyst loading under mild reaction conditions. The reaction could be scaled up to the gram scale with the same results. In addition, this was the first application of Er(OTf)(3) to the asymmetric inverse-electron-demand HDA reaction and it behaved as an efficient catalyst. Moreover, the synthetic utility of this methodology was demonstrated with the synthesis of key intermediates of some natural products. PMID:21647990

Zhu, Yin; Xie, Mingsheng; Dong, Shunxi; Zhao, Xiaohu; Lin, Lili; Liu, Xiaohua; Feng, Xiaoming

2011-07-11

248

Synthesis of enantiopure sulfonimidamides and elucidation of their absolute configuration by comparison of measured and calculated CD spectra and X-ray crystal structure determination.  

PubMed

Straightforward syntheses of enantiopure N-benzoyl- and N-tert-butyloxycarbonyl-protected sulfonimidamides, which can be used as building blocks in newly designed catalysts, are presented. Key synthetic step is a dynamic resolution of a racemic sulfinic acid sodium salt. All subsequent transformations proceed stereospecifically. The absolute configurations at the sulfur atoms of both sulfonimidamides were determined by comparison of measured and calculated CD spectra. An X-ray crystal structure determination of a sulfonimidoylguanidine derivative confirmed this result. PMID:19938003

Worch, Christin; Atodiresei, Iuliana; Raabe, Gerhard; Bolm, Carsten

2010-01-11

249

Absolute nuclear material assay  

DOEpatents

A method of absolute nuclear material assay of an unknown source comprising counting neutrons from the unknown source and providing an absolute nuclear material assay utilizing a model to optimally compare to the measured count distributions. In one embodiment, the step of providing an absolute nuclear material assay comprises utilizing a random sampling of analytically computed fission chain distributions to generate a continuous time-evolving sequence of event-counts by spreading the fission chain distribution in time.

Prasad, Manoj K. (Pleasanton, CA); Snyderman, Neal J. (Berkeley, CA); Rowland, Mark S. (Alamo, CA)

2012-05-15

250

Absolute nuclear material assay  

DOEpatents

A method of absolute nuclear material assay of an unknown source comprising counting neutrons from the unknown source and providing an absolute nuclear material assay utilizing a model to optimally compare to the measured count distributions. In one embodiment, the step of providing an absolute nuclear material assay comprises utilizing a random sampling of analytically computed fission chain distributions to generate a continuous time-evolving sequence of event-counts by spreading the fission chain distribution in time.

Prasad, Manoj K. (Pleasanton, CA) [Pleasanton, CA; Snyderman, Neal J. (Berkeley, CA) [Berkeley, CA; Rowland, Mark S. (Alamo, CA) [Alamo, CA

2010-07-13

251

Applications of organocatalytic asymmetric synthesis to drug prototypes--dual action and selective inhibitors of n-nitric oxide synthase with activity against the 5-HT1D/1B subreceptors.  

PubMed

The scope of MacMillan's organocatalytic asymmetric conjugate addition reaction of indoles and electron-rich aromatics to ?,?-unsaturated aldehydes has been extended to the use of 3-amino crotonaldehydes as substrates. The aromatics used include indoles as well as an aniline and a furan. The scope and effect of the groups on nitrogen (R, R') has also been studied. The method has been applied to the concise synthesis of an advanced precursor to S-(+)-1, a drug prototype for the treatment of migraine headaches. PMID:21271688

Hanessian, Stephen; Stoffman, Eli; Mi, Xueling; Renton, Paul

2011-03-01

252

Enantioselective synthesis of tacrine–huperzine A hybrids. Preparative chiral MPLC separation of their racemic mixtures and absolute configuration assignments by X-ray diffraction analysis  

Microsoft Academic Search

A new synthesis of racemic 7-substituted bicyclo[3.3.1]non-6-en-3-ones, rac-4, whose key-step involves the reaction of a vinyl triflate, rac-7, with an organometallic reagent, has been developed. This procedure has been applied to the enantioselective synthesis of (+)- and (?)-7-ethylbicyclo[3.3.1]non-6-en-3-one, (+)- and (?)-4b, from which both enantiomers of the cholinesterase inhibitor, tacrine–huperzine A hybrid, 9b, have been obtained. Rac-9b and its related

Pelayo Camps; Joan Contreras; Jordi Morral; Diego Muñoz-Torrero; Xavier Solans

1998-01-01

253

Synthesis of Both Enantiomers of Methyl 3-methyloctanoate, Key Component for the Scent of African Orchids and Determination of their Absolute Configuration  

Microsoft Academic Search

Synthesis of enantiomers of methyl 3-methyloctanoate 2 was achieved starting from enantiomers of methyl 3-hydroxy-2-methylpropanoate 3 in 9 steps. (S)-Enantiomer was shown to be identical with the naturally occurring isomer by GC analysis using a capillary column with chiral stationary phase. Interestingly, unnatural (R)-enantiomer has stronger floral note than natural (S)-enantiomer.

Takeshi Kitahara; Kang Seung Hyun; Shigeyuki Tamogami; Roman Kaiser

1994-01-01

254

Synthesis and structure revision of the C43-C67 part of amphidinol 3.  

PubMed

Stereoselective synthesis of the C43-C67 part of amphidinol 3 (AM3) and its C51-epimer was achieved starting from a common intermediate corresponding to the tetrahydropyran moiety of AM3, via asymmetric oxidations and Julia-Kocienski olefination. By comparing NMR data of the synthetic specimens with those of AM3, the absolute configuration at C51 of AM3 was revised from R to S. PMID:23692417

Ebine, Makoto; Kanemoto, Mitsunori; Manabe, Yoshiyuki; Konno, Yosuke; Sakai, Ken; Matsumori, Nobuaki; Murata, Michio; Oishi, Tohru

2013-06-01

255

Synthesis of (3S,3?S)- and meso-Stereoisomers of Alloxanthin and Determination of Absolute Configuration of Alloxanthin Isolated from Aquatic Animals  

PubMed Central

In order to determine the absolute configuration of naturally occurring alloxanthin, a HPLC analytical method for three stereoisomers 1a–c was established by using a chiral column. Two authentic samples, (3S,3?S)- and meso-stereoisomers 1b and 1c, were chemically synthesized according to the method previously developed for (3R,3?R)-alloxanthin (1a). Application of this method to various alloxanthin specimens of aquatic animals demonstrated that those isolated from shellfishes, tunicates, and crucian carp are identical with (3R,3?R)-stereoisomer 1a, and unexpectedly those from lake shrimp, catfish, biwa goby, and biwa trout are mixtures of three stereoisomers of 1a–c.

Yamano, Yumiko; Maoka, Takashi; Wada, Akimori

2014-01-01

256

Absolute hardness: companion parameter to absolute electronegativity  

SciTech Connect

For neutral and charged species, atomic and molecular, a property called absolute hardness eta is defined. Let E(N) be a ground-state electronic energy as a function of the number of electrons N. As is well-known, the derivative of E(N) with respect to N, keeping nuclear charges Z fixed, is the chemical potential ..mu.. or the negative of the absolute electronegativity chi: ..mu.. = (deltaE/deltaN)/sub Z/ = /sup -/chi. The corresponding second derivative is hardness: 2eta = (delta..mu../deltaN)/sub Z/ = (deltachi/deltaN)/sub Z/ = (delta/sup 2/E/deltaN/sup 2/)/sub Z/. Operational definitions of chi and eta are provided by the finite difference formulas (the first due to Mulliken) chi = 1/2(I+A), eta = 1/2(I-A), where I and A are the ionization potential and electron affinity of the species in question. Softness is the opposite of hardness: a low value of eta means high softness. The principle of hard and soft acids and bases is derived theoretically by making use of the hypothesis that extra stability attends bonding of A to B when the ionization potentials of A and B in the molecule (after charge transfer) are the same. For bases B, hardness is identified as the hardness of the species B/sup +/. Tables of absolute hardness are given for a number of free atoms, Lewis acids, and Lewis bases, and the values are found to agree well with chemical facts. 1 figure, 3 tables.

Parr, R.G.; Pearson, R.G.

1983-12-28

257

Absolute biological needs.  

PubMed

Absolute needs (as against instrumental needs) are independent of the ends, goals and purposes of personal agents. Against the view that the only needs are instrumental needs, David Wiggins and Garrett Thomson have defended absolute needs on the grounds that the verb 'need' has instrumental and absolute senses. While remaining neutral about it, this article does not adopt that approach. Instead, it suggests that there are absolute biological needs. The absolute nature of these needs is defended by appeal to: their objectivity (as against mind-dependence); the universality of the phenomenon of needing across the plant and animal kingdoms; the impossibility that biological needs depend wholly upon the exercise of the abilities characteristic of personal agency; the contention that the possession of biological needs is prior to the possession of the abilities characteristic of personal agency. Finally, three philosophical usages of 'normative' are distinguished. On two of these, to describe a phenomenon or claim as 'normative' is to describe it as value-dependent. A description of a phenomenon or claim as 'normative' in the third sense does not entail such value-dependency, though it leaves open the possibility that value depends upon the phenomenon or upon the truth of the claim. It is argued that while survival needs (or claims about them) may well be normative in this third sense, they are normative in neither of the first two. Thus, the idea of absolute need is not inherently normative in either of the first two senses. PMID:23586876

McLeod, Stephen

2014-07-01

258

BF 3·Et 2O catalyzed diastereoselective nucleophilic reactions of 3-silyloxypiperidine N, O-acetal with silyl enol ether and application to the asymmetric synthesis of (+)-febrifugine  

Microsoft Academic Search

The asymmetric BF3·Et2O catalyzed nucleophilic reactions of 3-silyloxypiperidine N,O-acetal 10 with silyl enol ethers derived from ketones are described. (+)-Febrifugine 1, an antimalarial alkaloid, was successfully synthesized based on this nucleophilic substitution. In addition, N,O-acetal 10 was synthesized from l-benzyl glutamate in 11 steps.

Ru-Cheng Liu; Wei Huang; Jing-Yi Ma; Bang-Guo Wei; Guo-Qiang Lin

2009-01-01

259

Asymmetric organocatalytic desymmetrization of 4,4-disubstituted cyclohexadienones at high pressure: a new powerful strategy for the synthesis of highly congested chiral cyclohexenones.  

PubMed

A highly diastereoselective and enantioselective method for the asymmetric desymmetrization of 4,4-disubstituted cyclohexadienones using the Michael addition reaction of malonates under catalysis with the primary amine-thiourea conjugate catalyst and PPY at high pressure was developed. PMID:24887588

Miyamae, Naomu; Watanabe, Naruhisa; Moritaka, Maya; Nakano, Keiji; Ichikawa, Yoshiyasu; Kotsuki, Hiyoshizo

2014-07-16

260

Organocatalytic tandem Morita-Baylis-Hillman-Michael reaction for asymmetric synthesis of a drug-like oxa-spirocyclic indanone scaffold.  

PubMed

A novel multicatalytic MBH-Michael tandem reaction has been developed for the asymmetric assembly of ninhydrin, nitroolefins and aldehydes into a structurally complex oxa-spirocyclic indanone backbone. Successive iodocyclization allowed us to convert these multifunctional allylic products into fused chiral natural product mimics. PMID:23955114

Li, Xiang; Yang, Lei; Peng, Cheng; Xie, Xin; Leng, Hai-Jun; Wang, Biao; Tang, Zheng-Wei; He, Gu; Ouyang, Liang; Huang, Wei; Han, Bo

2013-10-01

261

Simultaneous axial screw dislocation-mediated growth and radial layer-by-layer deposition for controlled synthesis of asymmetric axial ZnO nanospindles.  

PubMed

Single-component nanostructures with axial asymmetry were successfully synthesized in organic solvents via a new type of growth model. Asymmetric axial ZnO nanospindles with a hexagonal cross-section were produced by a growth model consisting of simultaneous axial screw dislocation-mediated growth and radial layer-by-layer deposition. The growth process of ZnO nanospindles is explained by comprehensively characterizing and monitoring the products at different reaction time intervals. Hexagonal discs containing dislocations were first generated at a reaction time of 2.5 min. When the reaction time continued to increase, the nanodiscs grew along the ?0002? direction. Half-nanospindles were formed at mid-reaction stage when the growth rate of [0001] was greater than [000-1]. Finally, the asymmetric nanospindles were obtained at 40 min. Further, the length of the asymmetric axial ZnO spindles can be precisely tuned by the adjustment of reaction temperature. Thus, the growth model presented here can synthesize a new category of one-dimensional asymmetric nanostructures. PMID:24970315

Yang, Hong-Jie; He, Sheng-Yan; Tuan, Hsing-Yu

2014-07-10

262

Asymmetric synthesis of ?-amino acids via homologation of Ni(II) complexes of glycine Schiff bases. Part 2: aldol, Mannich addition reactions, deracemization and (S) to (R) interconversion of ?-amino acids.  

PubMed

This review provides a comprehensive treatment of literature data dealing with asymmetric synthesis of ?-amino-?-hydroxy and ?,?-diamino acids via homologation of chiral Ni(II) complexes of glycine Schiff bases using aldol and Mannich-type reactions. These reactions proceed with synthetically useful chemical yields and thermodynamically controlled stereoselectivity and allow direct introduction of two stereogenic centers in a single operation with predictable stereochemical outcome. Furthermore, new application of Ni(II) complexes of ?-amino acids Schiff bases for deracemization of racemic ?-amino acids and (S) to (R) interconversion providing additional synthetic opportunities for preparation of enantiomerically pure ?-amino acids, is also reviewed. Origin of observed diastereo-/enantioselectivity in the aldol, Mannich-type and deracemization reactions, generality and limitations of these methodologies are critically discussed. PMID:24043459

Sorochinsky, Alexander E; Aceña, José Luis; Moriwaki, Hiroki; Sato, Tatsunori; Soloshonok, Vadim

2013-11-01

263

Synthesis, structure and DNA-binding studies of tetranuclear copper(II) complexes bridged by asymmetrical N , N? -bis(substituted)oxamides  

Microsoft Academic Search

Two tetranuclear copper(II) complexes bridged by asymmetrical N,N?-bis(substituted)oxamides have been synthesized and characterized as [Cu4(dmapob)2(Me2bpy)2](pic)2·6H2O (1) and [Cu4(oxbe)2(dabt)2](pic)2 (2), where H3dmapob and H3oxbe denote N-benzoato-N?-[3-(dimethylamino)propyl]oxamido and N-benzoato-N?-(2-aminoethyl)oxamide, respectively; and Me2bpy, dabt, and pic represent 4,4?-dimethyl-2,2?-bipyridine 2,2?-diamino-4,4?-bithiazole, and 2,4,6-trinitrophenol, respectively.\\u000a Complex 1 was characterized by elemental analyses, IR and electronic spectra, and single-crystal X-ray diffraction. Its structure consists\\u000a of two asymmetrical

Jin-Ju Nie; Yan-Tuan Li; Xiao-Wen Li; Zhi-Yong Wu; Cui-Wei Yan

2011-01-01

264

Terminating Catalytic Asymmetric Heck Cyclizations by Stereoselective Intramolecular Capture of ?3-Allylpalladium Intermediates: Total Synthesis of (-)-Spirotryprostatin B and Three Stereoisomers  

PubMed Central

A catalytic intramolecular Heck reaction, followed by capture of the resulting ?3-allylpalladium intermediate by a tethered diketopiperazine, is the central step in a concise synthetic route to (?)-spirotryprostatin B and three stereoisomers. This study demonstrates that an acyclic, chiral ?3-allylpalladium fragment generated in a catalytic asymmetric Heck cyclization can be trapped by even a weakly nucleophilic diketopiperazine more rapidly than it undergoes diastereomeric equilibration.

Overman, Larry E.; Rosen, Mark D.

2010-01-01

265

Asymmetric Catalytic aza-Morita-Baylis-Hillman Reaction for the Synthesis of 3-Substituted-3-Aminooxindoles with Chiral Quaternary Carbon Centers  

PubMed Central

Asymmetric catalytic aza-Morita-Baylis-Hillman (aza-MBH) reaction of isatin-derived ketimines with MVK has been established by using chiral amino and phosphino catalysts. The reaction resulted in biomedically important 3-substituted 3-amino-2-oxindoles in good yields (>80% for most cases) and excellent enantioselectivity (90–99%ee). Twenty-eight cases assembled with chiral quaternary stereogenic centers have been examined under convenient systems.

Hu, Fang-Le; Wei, Yin; Shi, Min; Pindi, Suresh; Li, Guigen

2013-01-01

266

Asymmetric ?-amination of 4-substituted pyrazolones catalyzed by a chiral Gd(OTf)3/N,N'-dioxide complex: highly enantioselective synthesis of 4-amino-5-pyrazolone derivatives.  

PubMed

The asymmetric ?-amination of 4-substituted pyrazolones with azodicarboxylates was investigated for the first time, employing an N,N'-dioxide gadolinium(III) complex as the catalyst. The novel transformations exhibited high yield, and 4-amino-5-pyrazolone derivatives bearing a chiral quaternary center were obtained in excellent yields (up to 99%) and enantioselectivities (90%-97% ee) for a broad scope of 5-pyrazolones by using 1 mol % or only 0.05 mol % of catalyst. PMID:21214254

Yang, Zhigang; Wang, Zhen; Bai, Sha; Liu, Xiaohua; Lin, Lili; Feng, Xiaoming

2011-02-18

267

Phosphine-Olefin Ligands Based on a Planar-Chiral (?-Arene)chromium Scaffold: Design, Synthesis, and Application in Asymmetric Catalysis.  

PubMed

The NMR and X-ray crystallographic studies clarified that planar-chiral alkenylene-bridged (phosphino-?-arene)(phosphine)chromium complexes 3 were capable of coordinating to a rhodium(I) cation in a bidentate fashion at the (?-arene)-bound phosphorus atom and at the olefin moiety. The P-olefin chelate coordination of 3 constructs the effective chiral environment at the metal center, and thus, these rhodium complexes display high performances in various rhodium-catalyzed asymmetric 1,4- and 1,2-addition reactions with arylboron nucleophiles. The control experiments demonstrated that the (?(2)-olefin)-Rh interaction as well as the bridging structure in 3 played the pivotal roles in the high enantioselectivity of the Rh-catalyzed asymmetric reactions. To enhance the synthetic utilities of these phosphine-olefin ligands, an enantiospecific and scalable synthetic method was developed. The novel synthetic method is flexible in terms of the substituent variation, and a library of the planar-chiral (arene)chromium-based phosphine-olefin ligands was established by the combinatorial approach. Among the newly prepared ligand library, compound 3g, which is with a bis(3,5-dimethylphenyl)phosphino group on the ?(6)-arene ring, was found to be a far better chiral ligand in the rhodium-catalyzed asymmetric reactions showing excellent enantioselectivity and high yields. PMID:24932944

Ogasawara, Masamichi; Tseng, Ya-Yi; Arae, Sachie; Morita, Tomotaka; Nakaya, Takeshi; Wu, Wei-Yi; Takahashi, Tamotsu; Kamikawa, Ken

2014-07-01

268

Absolute configuration of franganine.  

PubMed

The absolute configuration of franganine (1), a cyclopeptide alkaloid isolated from the methanol root bark extract of Discaria americana, was established on the basis of detailed NMR spectroscopic data and X-ray diffraction analysis of its salt (2). PMID:22680778

Caro, Miguel S B; de Oliveira, Leonardo H; Ilha, Vinicius; Burrow, Robert A; Dalcol, Ionara I; Morel, Ademir F

2012-06-22

269

Verifying Chandra Absolute Times  

Microsoft Academic Search

Calibrating the absolute time accuracy of Chandra requires requires the efforts of many people. Here we present some attempts to verify this accuracy. HRC observations of the Crab indicated that the Chandra clock is within 0.2 msec of being correct. Checking ACIS requires correcting for the time delays caused by the continuous clocking readout. A method is presented to correct

Allyn Tennant

2001-01-01

270

Absolute Low Speed Anemometer  

Microsoft Academic Search

An absolute anemometer suitable for measurements in gases under a wide variety of conditions is described. The instrument, based on an idea first used by Kovasznay, utilizes the temperature fluctuations in the wake of a sinusoidally heated fine wire as tracer. A second hot-wire detects the fluctuations, and the phase change between two successive positions of this wire can be

R. E. Walker; A. A. Westenberg

1956-01-01

271

Absolute ultraviolet stellar fluxes  

NASA Technical Reports Server (NTRS)

A study, made to calibrate the ultraviolet spectra of stars to within an absolute accuracy of 10%, is summarized. Data were taken by a 13 inch UVR telescope onboard several rocket flights. Calibrated and existing ground based data are in reasonable agreement. Calibrated results are presented in graphical form.

Evans, D. C.

1972-01-01

272

Enantio- and diastereoselective synthesis of piperidines by coupling of four components in a "one-pot" sequence involving diphenylprolinol silyl ether mediated Michael reaction.  

PubMed

An efficient, asymmetric, four-component, one-pot synthesis of highly substituted piperidines with excellent diastereo- and enantioselectivity was established through the diphenylprolinol silyl ether mediated Michael reaction of aldehyde and nitroalkene, followed by the domino aza-Henry reaction/hemiaminalization reaction and a Lewis acid mediated allylation or cyanation reaction. All carbons of the piperidine ring are substituted with different groups, and its five contiguous stereocenters are completely controlled in both relative and absolute senses. PMID:20853903

Urushima, Tatsuya; Sakamoto, Daisuke; Ishikawa, Hayato; Hayashi, Yujiro

2010-10-15

273

Absolute beam brightness detector  

SciTech Connect

In generally accepted emittance measurement, main attention is concentrated on emittance areas {epsilon}{sub x}, {epsilon}{sub y} occupied by desired part of ion beam in transverse phase space and shape of these areas. The absolute beam phase density (brightness) as usually is not measured directly and the average beam brightness B is calculated from a beam intensity I and the transverse emittances. In the ion source and low energy beam transport (LEBT) optimization, it is important to preserve the beam brightness because some aberration of ion optic and beam instabilities can decrease the brightness of the central part of ion beam significantly. For these brightness measurements, it is convenient to use an absolute beam brightness detector with the brightness determination from one short considered in this article.

Dudnikov, Vadim [Muons, Inc., Batavia, Illinois 60510 (United States)

2012-02-15

274

Template synthesis and crystal structure of an asymmetric square-planar complex: Pyridine2,6-dicarbaldehyde-bis( S -methylisothiosemicarbazonato)nickel(II) iodide  

Microsoft Academic Search

The asymmetric complex [Ni(HL)]I (where H2\\u000a L is pyridine-2,6-dicarbaldehyde-bis(S-methylisothiosemicarbazone)) is synthesized by the [2 + 1] template condensation of S-methylisothiosemicarbazide hydroiodide with pyridine-2,6-dicarbaldehyde in the presence of nickel(II) acetate. The crystal\\u000a structure of the [Ni(HL)]I complex (where HL is C11H14N7S2) is determined using X-ray diffraction. The square-planar coordination of the nickel(II) central atom is provided by four\\u000a N donor atoms

J. I. Gradinaru; S. T. Malinovskii; M. A. Popovici; M. Gdaniec

2005-01-01

275

The Direct Catalytic Asymmetric Aldol Reaction  

PubMed Central

Asymmetric aldol reactions are a powerful method for the construction of carbon-carbon bonds in an enantioselective fashion. Historically this reaction has been performed in a stoichiometric fashion to control the various aspects of chemo-, diastereo-, regio- and enantioselectivity, however, a more atom economical approach would unite high selectivity with the use of only a catalytic amount of a chiral promoter. This critical review documents the development of direct catalytic asymmetric aldol methodologies, including organocatalytic and metal-based strategies. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets

Brindle, Cheyenne S.

2013-01-01

276

Asymmetric allylation of ?-ketoester-derived N-benzoylhydrazones promoted by chiral sulfoxides/N-oxides Lewis bases: highly enantioselective synthesis of quaternary ?-substituted ?-allyl-?-amino acids.  

PubMed

Chiral sulfoxides/N-oxides (R)-1 and (R,R)-2 are effective chiral promoters in the enantioselective allylation of ?-keto ester N-benzoylhydrazone derivatives 3a-g to generate the corresponding N-benzoylhydrazine derivatives 4a-g, with enantiomeric excesses as high as 98%. Representative hydrazine derivatives 4a-b were subsequently treated with SmI2, and the resulting amino esters 5a-b with LiOH to obtain quaternary ?-substituted ?-allyl ?-amino acids 6a-b, whose absolute configuration was assigned as (S), with fundament on chemical correlation and electronic circular dichroism (ECD) data. PMID:23846830

Reyes-Rangel, Gloria; Bandala, Yamir; García-Flores, Fred; Juaristi, Eusebio

2013-09-01

277

Asymmetric inter- and intramolecular cyclopropanation of alkenes catalyzed by chiral ruthenium porphyrins. Synthesis and crystal structure of a chiral metalloporphyrin carbene complex.  

PubMed

Extensive investigations of asymmetric intermolecular cyclopropanation of terminal alkenes with diazoacetates catalyzed by ruthenium porphyrin [Ru(P*)(CO)(EtOH)] (1, H2P = 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene-9-yl]porphyrin) and the application of catalyst 1 to asymmetric intramolecular cyclopropanation of allylic or homoallylic diazoacetates are described. The intermolecular cyclopropanation of styrene and its derivatives with ethyl diazoacetate afforded the corresponding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up to 36 and extremely high catalyst turnovers of up to 1.1 x 10(4). Examination of the effects of temperature, diazoacetate, solvent, and substituent in the intermolecular cyclopropanation reveals that (i) both enantioselectivity and trans selectivity increase with decreasing temperature, (ii) sterically encumbered diazoacetates N2CHCO2R, such as R = Bu(t), and donor solvents, such as diethyl ether and tetrahydrofuran, are beneficial to the trans selectivity, and (iii) electron-donating para substituents on styrene accelerate the cyclopropanations, with the log(k(X)/k(H)) vs sigma(+) plot for para-substituted styrenes p-X-C6H4CH=CH2 (X = MeO, Me, Cl, CF3) exhibiting good linearity with a small negative rho(+) value of -0.44 +/- 0.09. In the case of intramolecular cyclopropanation, complex 1 promoted the decomposition of a series of allylic diazoacetates to form the cyclopropyl lactones in up to 85% ee, contributing the first efficient metalloporphyrin catalyst for an asymmetric intramolecular cyclopropanation. Both the inter- and intramolecular cyclopropanations were proposed to proceed via a reactive chiral ruthenium carbene intermediate. The enantioselectivities in these processes were rationalized on the basis of the X-ray crystal structures of closely related stable chiral carbene complexes [Ru(P*)(CPh2)] (2) and [Ru(P*)(C(Ph)CO2CH2CH=CH2)] (3) obtained from reactions of complex 1 with N2CPh2 and N2C(Ph)CO2CH2CH=CH2, respectively. PMID:11457174

Che, C M; Huang, J S; Lee, F W; Li, Y; Lai, T S; Kwong, H L; Teng, P F; Lee, W S; Lo, W C; Peng, S M; Zhou, Z Y

2001-05-01

278

Electronic Absolute Cartesian Autocollimator  

NASA Technical Reports Server (NTRS)

An electronic absolute Cartesian autocollimator performs the same basic optical function as does a conventional all-optical or a conventional electronic autocollimator but differs in the nature of its optical target and the manner in which the position of the image of the target is measured. The term absolute in the name of this apparatus reflects the nature of the position measurement, which, unlike in a conventional electronic autocollimator, is based absolutely on the position of the image rather than on an assumed proportionality between the position and the levels of processed analog electronic signals. The term Cartesian in the name of this apparatus reflects the nature of its optical target. Figure 1 depicts the electronic functional blocks of an electronic absolute Cartesian autocollimator along with its basic optical layout, which is the same as that of a conventional autocollimator. Referring first to the optical layout and functions only, this or any autocollimator is used to measure the compound angular deviation of a flat datum mirror with respect to the optical axis of the autocollimator itself. The optical components include an illuminated target, a beam splitter, an objective or collimating lens, and a viewer or detector (described in more detail below) at a viewing plane. The target and the viewing planes are focal planes of the lens. Target light reflected by the datum mirror is imaged on the viewing plane at unit magnification by the collimating lens. If the normal to the datum mirror is parallel to the optical axis of the autocollimator, then the target image is centered on the viewing plane. Any angular deviation of the normal from the optical axis manifests itself as a lateral displacement of the target image from the center. The magnitude of the displacement is proportional to the focal length and to the magnitude (assumed to be small) of the angular deviation. The direction of the displacement is perpendicular to the axis about which the mirror is slightly tilted. Hence, one can determine the amount and direction of tilt from the coordinates of the target image on the viewing plane.

Leviton, Douglas B.

2006-01-01

279

Pd-catalyzed asymmetric allylic etherification using chiral biphenol-based diphosphinite ligands and its application for the formal total synthesis of (-)-galanthamine.  

PubMed

A library of novel chiral biphenol-based diphosphinite (BOP) ligands was designed and created. These BOP ligands were applied to a Pd-catalyzed intermolecular allylic etherification reaction, which provided a key intermediate for the formal total synthesis of (-)-galanthamine with 97% ee in 97% yield. PMID:23472673

Zang, Yang; Ojima, Iwao

2013-04-19

280

Stereoselectivity in ene reactions with 1O2: matrix effects in polymer supports, photo-oxygenation of organic salts and asymmetric synthesis.  

PubMed

The ene reaction of chiral allylic alcohols is applied as a tool for the investigation of intrapolymer effects by means of the stereoselectivity of the singlet-oxygen addition. The diastereo selectivity strongly depends on the structure of the polymer, the substrate loading degree and also on the degree of conversion demonstrating additional supramolecular effects evolving during the reaction. The efficiency and the stability of polymer-bound sensitizers were evaluated by the ene reaction of singlet oxygen with citronellol. The ene reaction with chiral ammonium salts of tiglic acid was conducted under solution phase conditions or in polystyrene beads under chiral contact ion-pair conditions. The products thus obtained precipitate during the photoreaction as ammonium salts. Moderate asymmetric induction was observed for this procedure for the first time. PMID:16674271

Griesbeck, Axel G; Bartoschek, Anna; Neudörfl, Jörg; Miara, Claus

2006-01-01

281

Synthesis, spectral, photolysis and electrochemical studies of mononuclear copper(II) complex with a new asymmetric tetradentate ligand: Application as copper nanoparticle precursor  

NASA Astrophysics Data System (ADS)

A copper(II) complex with asymmetric tetradentate Schiff base ligand, obtained by the single condensation of 1,2-diaminopropane with 2-hydroxy-5-methoxy benzaldehyde was prepared. The ligand and complex were characterized by their IR, UV-Vis, FT-IR, NMR spectra and CV. Crystal structures of the mononuclear copper complex have been obtained by X-ray diffraction studies which revealed to be distorted square planner coordination geometry. The spectral data confirm coordination of ligand to copper ion center. The redox properties of complex at different scan rates exhibit grossly similar features consisting of an electrochemically pseudo-reversible Cu(II)/Cu(I) reduction at ca. -0.97 V and pseudo-reversible Cu(I)/Cu(II) oxidation at ca. -0.81 V. The copper nanoparticles with average size of 73 nm were formed by thermal reduction of copper complex in the presence of triphenylphosphine.

Habibi, Mohammad Hossein; Mikhak, Maryam

2012-10-01

282

Zn(II), Ni(II), Cu(II) and Pb(II) complexes of tridentate asymmetrical Schiff base ligands: synthesis, characterization, properties and biological activity.  

PubMed

New asymmetrical tridentate Schiff base ligands were synthesized using 1,2-phenylenediamine, 4-methyl-1,2-phenylenediamine, 2-hydroxy-1-napthaldehyde, 9-anthracenecarboxaldehyde. Schiff base ligands and their metal complexes were synthesised and characterized by using FT-IR, (1)H NMR, (13)C NMR, UV-Vis, XRD, ESR, elemental analysis and fluorescence studies. The antimicrobial activity of the ligands and their metal complexes were studied against Staphylococcus aureus ATCC 29213, S. aureus ATCC 25923, Streptococcus mutans RSHM 676, Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853. The determination of the antibacterial activity was done using the broth microdilution methods. In general, it has been determined that the studied compounds have MIC values similar to Gram-positive and Gram-negative bacteria. It has been found that Ni, Pb, Zn derivatives of HL1A and ZnL(2)A has lower MIC values than ampicillin for P. aeruginosa ATCC 27853 strain. PMID:23274224

?ahin, Mustafa; Koçak, Nuriye; Erdenay, Damla; Arslan, U?ur

2013-02-15

283

Synthesis of iridium and rhodium complexes with new chiral phosphine-NHC ligands based on 1,1'-binaphthyl framework and their application in asymmetric hydrogenation.  

PubMed

The first series of chiral phosphine-imidazole carbene ligands based on a 1,1'-binaphthyl framework were synthesized from (R)-2-amine-2'-(diphenylphosphino)-1,1'-binaphthyl (1) in a four-step pathway. After deprotonation of these phosphine-imidazolium salts with LiO(t)Bu, and subsequent complexation with [Ir(COD)Cl]2 and anion exchange with NaBArF, phosphine-carbene chelated iridium complexes (R)-6a and (R)-6b were obtained. Their structures have been characterized by NMR and X-ray diffraction analysis. The NHC-phosphine rhodium complex (R)-6c has been also obtained by a similar synthetic method. These iridium complexes have been applied to catalyze the asymmetric hydrogenation of alkenes to give the corresponding products in moderate to excellent conversion (up to 99%) and moderate enantioselectivities under mild conditions (up to 61% ee). PMID:23900600

Gu, Peng; Zhang, Jun; Xu, Qin; Shi, Min

2013-10-01

284

Asymmetric cell division  

Microsoft Academic Search

Asymmetric cell division is a conserved mechanism for partitioning information during mitosis. Over the past several years, significant progress has been made in our understanding of how cells establish polarity during asymmetric cell division and how determinants, in the form of localized proteins and mRNAs, are segregated. In particular, genetic studies in Drosophila and Caenorhabditis elegans have linked cell polarity,

Fabrice Roegiers; Yuh Nung Jan

2004-01-01

285

ABSOLUTE POLARIMETRY AT RHIC.  

SciTech Connect

Precise and absolute beam polarization measurements are critical for the RHIC spin physics program. Because all experimental spin-dependent results are normalized by beam polarization, the normalization uncertainty contributes directly to final physics uncertainties. We aimed to perform the beam polarization measurement to an accuracy Of {Delta}P{sub beam}/P{sub beam} < 5%. The absolute polarimeter consists of Polarized Atomic Hydrogen Gas Jet Target and left-right pairs of silicon strip detectors and was installed in the RHIC-ring in 2004. This system features proton-proton elastic scattering in the Coulomb nuclear interference (CNI) region. Precise measurements of the analyzing power A{sub N} of this process has allowed us to achieve {Delta}P{sub beam}/P{sub beam} = 4.2% in 2005 for the first long spin-physics run. In this report, we describe the entire set up and performance of the system. The procedure of beam polarization measurement and analysis results from 2004-2005 are described. Physics topics of AN in the CNI region (four-momentum transfer squared 0.001 < -t < 0.032 (GeV/c){sup 2}) are also discussed. We point out the current issues and expected optimum accuracy in 2006 and the future.

OKADA; BRAVAR, A.; BUNCE, G.; GILL, R.; HUANG, H.; MAKDISI, Y.; NASS, A.; WOOD, J.; ZELENSKI, Z.; ET AL.

2007-09-10

286

NOVA: Absolute Zero  

NSDL National Science Digital Library

On a hot day, some might wish they could get the temperature down a bit. They might not wish it to be as cold as, say absolute zero, but there are many scientists who are interested in doing just that. For those who are curious, absolute zero clocks in at around minus 460 degrees Fahrenheit. This engaging website is meant to serve as a complementary resource to the two-part series that recently aired on NOVA on this engaging topic. Visitors can start by watching a short preview of the program, and then continuing on to look over some of the special interactive features on the site. All told, there are ten different features, including "A Sense of Scale", "How Low Can You Go?", and "Milestones in Cold Research". The "Milestones in Cold Research" is a great place to start, as it's an interactive timeline that chronicles the "netherworld of extraordinarily low temperatures" as investigated by everyone from Galileo to current researchers. Of course, there are also more playful features here, such as "The Ice Trade", which asks users to dispatch ships loaded with natural ice to Florida, Brazil, and India.

287

Theoretical study of the asymmetric phase-transfer mediated epoxidation of chalcone catalyzed by chiral crown ethers derived from monosaccharides  

NASA Astrophysics Data System (ADS)

The synthesis of a novel, optically active crown ether derived from ?- D-altropyranoside is described. A significantly different asymmetric induction was generated by the ?- D-glucopyranoside-, ?- D-mannopyranoside- and ?- D-altropyranoside-based chiral crown catalysts in the epoxidation of trans-chalcone with tert-butyl hydroperoxide under phase-transfer catalytic conditions. It was shown that absolute configuration of the crown-ring fused carbon atoms of the monosaccharides has a great impact on the enantioselectivity. The asymmetric induction could be well explained by considering the possible mechanistic pathway. Molecular modeling (MCMM) and subsequent DFT calculations - in accordance with the experimental results - indicate that the use of glucopyranoside-based catalyst 1 and that of mannopyranoside-based crown ether 2 results in the preferred formation of the opposite antipodes (2 R,3 S and 2 S,3 R, respectively) of the corresponding epoxyketone. At the same time, practically no asymmetric induction was proved if altropyranoside-based crown 3 is applied as the catalyst. The computational results are in qualitative agreement with the experimental data.

Makó, Attila; Menyhárd, Dóra K.; Bakó, Péter; Keglevich, György; T?ke, László

2008-12-01

288

A concise asymmetric synthesis of (2S,3S,7S)-3,7-dimethylpentadecan-2-yl Acetate and propionate, the sex pheromones of pine sawflies.  

PubMed

(2S,3S,7S)-3,7-Dimethylpentadecan-2-yl acetate (2) and its propionate analogue (3) are the main sex pheromones of all Neodiprion species and Diprion similes, respectively. Starting from (S)-malic acid and employing a highly chemo-, regio-, and stereoselective tandem ester reduction-epoxide formation-reductive epoxide-opening reaction protocol, an efficient total synthesis of (2S,3S,7S)-2 and -3 is reported herein. PMID:15153034

Huang, Pei-Qiang; Lan, Hong-Qiao; Zheng, Xiao; Ruan, Yuan-Ping

2004-05-28

289

Absolute Equilibrium Entropy  

NASA Technical Reports Server (NTRS)

The entropy associated with absolute equilibrium ensemble theories of ideal, homogeneous, fluid and magneto-fluid turbulence is discussed and the three-dimensional fluid case is examined in detail. A sigma-function is defined, whose minimum value with respect to global parameters is the entropy. A comparison is made between the use of global functions sigma and phase functions H (associated with the development of various H-theorems of ideal turbulence). It is shown that the two approaches are complimentary though conceptually different: H-theorems show that an isolated system tends to equilibrium while sigma-functions allow the demonstration that entropy never decreases when two previously isolated systems are combined. This provides a more complete picture of entropy in the statistical mechanics of ideal fluids.

Shebalin, John V.

1997-01-01

290

New phosphine-diamine and phosphine-amino-alcohol tridentate ligands for ruthenium catalysed enantioselective hydrogenation of ketones and a concise lactone synthesis enabled by asymmetric reduction of cyano-ketones  

PubMed Central

Enantioselective hydrogenation of ketones is a key reaction in organic chemistry. In the past, we have attempted to deal with some unsolved challenges in this arena by introducing chiral tridentate phosphine-diamine/Ru catalysts. New catalysts and new applications are presented here, including the synthesis of phosphine-amino-alcohol P,N,OH ligands derived from (R,S)-1-amino-2-indanol, (S,S)-1-amino-2-indanol and a new chiral P,N,N ligand derived from (R,R)-1,2-diphenylethylenediamine. Ruthenium pre-catalysts of type [RuCl2(L)(DMSO)] were isolated and then examined in the hydrogenation of ketones. While the new P,N,OH ligand based catalysts are poor, the new P,N,N system gives up to 98% e.e. on substrates that do not react at all with most catalysts. A preliminary attempt at realising a new delta lactone synthesis by organocatalytic Michael addition between acetophenone and acrylonitrile, followed by asymmetric hydrogenation of the nitrile functionalised ketone is challenging in part due to the Michael addition chemistry, but also since Noyori pressure hydrogenation catalysts gave massively reduced reactivity relative to their performance for other acetophenone derivatives. The Ru phosphine-diamine system allowed quantitative conversion and around 50% e.e. The product can be converted into a delta lactone by treatment with KOH with complete retention of enantiomeric excess. This approach potentially offers access to this class of chiral molecules in three steps from the extremely cheap building blocks acrylonitrile and methyl-ketones; we encourage researchers to improve on our efforts in this potentially useful but currently flawed process.

2012-01-01

291

Synthesis, photophysical and charge-transporting properties of a novel asymmetric indolo [3,2-b]carbazole derivative containing benzothiazole and diphenylamino moieties.  

PubMed

A novel asymmetric donor-?-donor-?-acceptor compound, 2-benzothiazolyl-8-diphenylamino-5,11-dihexylindolo[3,2-b]carbazole (BDDAICZ), has been successfully synthesized by introducing a benzothiazole moiety (as an electron-acceptor) and a diphenylamino moiety (as an electron-donor) to 2-position and 8-position of indolo[3,2-b]carbazole moiety (as a skeleton and an electron-donor), and characterized by elemental analysis, (1)H NMR, (13)C NMR and MS. The thermal, electrochemical properties of BDDAICZ were characterized by thermogravimetric analysis combined with electrochemistry. The absorption and emission spectra of BDDAICZ was experimentally determined in several solvents and computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated absorption and emission wavelengths are coincident with the measured data. The ionization potential (IP), the electron affinity (EA) and reorganization energy of BDDAICZ were also investigated using density functional theory (DFT). Charge-transporting properties of BDDAICZ were characterized by OLEDs devices fabricated by using it as charge-transport layers. The results show that BDDAICZ has excellent thermal stability, electrochemical stability and hole-transporting properties, indicating its potential application as a hole-transporting material in OLEDs devices. PMID:24973792

Shi, Heping; Yuan, Jiandong; Dong, Xiuqing; Cheng, Fangqin

2014-12-10

292

Synthesis and characterization of new chiral Cu(ii)-N4 complexes and their application in the asymmetric aza-Henry reaction.  

PubMed

Cu(ii) Schiff base complexes Cu(ii)- and Cu(ii)- based on 2-acetyl pyridine with both (1R,2R)-1,2-diaminocyclohexane and (1S,2S)-1,2-diaminocyclohexane were synthesized in a single step. Subsequent reduction of ligands and with NaBH4 followed by complexation with Cu(OTf)2 resulted in generation of two more additional chiral centers in complexes Cu(ii)- and Cu(ii)-. The ligands and their corresponding complexes were well characterized by several spectral techniques like (1)H-NMR, (13)C-NMR, LC-MS, CD, UV-Vis spectroscopy and microanalysis. The respective Cu(ii) complexes derived from ligands and were investigated using both the solution and solid state EPR spectra. The particular orientation of the reduced complex with Cu(OTf)2 was confirmed by the X-ray crystal structure of the corresponding complex. All the catalytic protocols were applied in the asymmetric aza-Henry reaction to evaluate the catalytic properties of the Cu(ii) complexes in the present study. PMID:24988542

Das, Anjan; Kureshy, Rukhsana I; Maity, Nabin Ch; Subramanian, P S; Khan, Noor-Ul H; Abdi, Sayed H R; Suresh, E; Bajaj, Hari C

2014-07-22

293

Asymmetric synthesis of pochonin?E and F, revision of their proposed structure, and their conversion to potent Hsp90 inhibitors.  

PubMed

A concise and modular synthesis of pochonin?E and F, and their epimers at C-6 established the correct stereochemistry of these two natural products. Several members of the pochonin family have been shown to bind the heat shock protein?90 (Hsp90), which has been the focus of intense drug discovery efforts. Pochonin?E and F as well as their epimers were derivatized into the corresponding pochoximes and further modified at the C-6 position. Molecular dynamics simulations, docking studies, and Hsp90 affinity measurements were performed to evaluate the impact of these modifications. PMID:22696415

Karthikeyan, Ganesan; Zambaldo, Claudio; Barluenga, Sofia; Zoete, Vincent; Karplus, Martin; Winssinger, Nicolas

2012-07-16

294

Asymmetric catalysis: An enabling science  

PubMed Central

Chirality of organic molecules plays an enormous role in areas ranging from medicine to material science, yet the synthesis of such entities in one enantiomeric form is one of the most difficult challenges. The advances being made stem from the convergence of a broader understanding of theory and how structure begets function, the developments in the interface between organic and inorganic chemistry and, most notably, the organic chemistry of the transition metals, and the continuing advancements in the tools to help define structure, especially in solution. General themes for designing catalysts to effect asymmetric induction are helping to make this strategy more useful, in general, with the resultant effect of a marked enhancement of synthetic efficiency.

Trost, Barry M.

2004-01-01

295

Divergent total synthesis of the Lycopodium alkaloids huperzine A, huperzine B, and huperzine U.  

PubMed

Huperzine A, huperzine B, and huperzine U are congeners isolated from the Chinese herb Huperzia serrata (= Lycopodium serratum ) in minuscule amounts. The most efficient total synthesis of huperzine A, the first asymmetric total syntheses of huperzine B, and the first total synthesis of huperzine U have been achieved efficiently in overall yields of 17%, 10%, and 9%, respectively, each spanning 10-13 steps from (R)-pulegone. The featured steps include palladium-catalyzed Buchwald-Hartwig coupling and Heck cyclization reactions and an Ir-catalyzed olefin isomerization reaction. This work has established the absolute configurations of huperzine B and huperzine U and revealed that natural huperzine A, huperzine B, and huperzine U possess the same set of absolute stereochemistries, thus providing support for the potential role of huperzine B and huperzine U in the biosynthesis of huperzine A. PMID:24299147

Ding, Rui; Fu, Jian-Guo; Xu, Guang-Qiang; Sun, Bing-Feng; Lin, Guo-Qiang

2014-01-01

296

Asymmetric reductive amination.  

PubMed

Asymmetric reductive amination (ARA) affords synthetically valuable chiral amines straightforwardly. This chapter reviews the recent advances made in the area, focusing on ARA by hydrogenation, transfer hydrogenation, organocatalytic reduction, and biocatalytic reduction. PMID:24158548

Wang, Chao; Xiao, Jianliang

2014-01-01

297

Analyzing Asymmetric Operations.  

National Technical Information Service (NTIS)

Current mathematical combat models and simulations have limited usefulness in analyzing asymmetrical operations, which exploit differences between friendly and enemy forces. The tools required to analyze these operations quantitatively are sparse and the ...

G. Bustamante

2000-01-01

298

Asymmetric Wiener Control.  

National Technical Information Service (NTIS)

A one-dimensional Wiener control problem with integral discounted quadratic cost function and asymmetric bounds on the control is considered, with infinite horizon. The optimal control is explicitly found. Bellman equations and Ito integrals are used to s...

H. Weiner

1983-01-01

299

Evolution of the total synthesis of (-)-okilactomycin exploiting a tandem oxy-cope rearrangement/oxidation, a Petasis-Ferrier union/rearrangement, and ring-closing metathesis.  

PubMed

An effective, asymmetric total synthesis of the antitumor antibiotic (-)-okilactomycin (1), as well as assignment of the absolute configuration, has been achieved exploiting a convergent strategy. Highlights of the synthesis include a diastereoselective oxy-Cope rearrangement/oxidation sequence to install the C(1) and C(13) stereogenic centers, a Petasis-Ferrier union/rearrangement to construct the highly functionalized tetrahydropyranone inscribed within the 13-membered macrocycle ring, employing for the first time a sterically demanding acetal, an intramolecular chemoselective acylation to access an embedded bicyclic lactone, and an efficient ring-closing metathesis (RCM) reaction to generate the macrocyclic ring. PMID:19170499

Smith, Amos B; Bosanac, Todd; Basu, Kallol

2009-02-18

300

Evolution of the Total Synthesis of (-)-Okilactomycin Exploiting a Tandem Oxy-Cope Rearrangement/Oxidation, the Petasis-Ferrier Union/Rearrangement and Ring Closing Metathesis  

PubMed Central

An effective, asymmetric total synthesis of the antitumor antibiotic (-)-okilactomycin (1) and assignment of the absolute configuration, has been achieved exploiting a convergent strategy. Highlights of the synthesis include: a diastereoselective oxy-Cope rearrangement/oxidation sequence to install the C(1) and C(13) stereogenic centers; a Petasis-Ferrier union/rearrangement to construct the highly functionalized tetrahydropyranone inscribed within the thirteen membered macrocycle ring, employing for the first time a sterically demanding acetal; an intramolecular chemoselective acylation to access an embedded bicyclic lactone; and an efficient ring closing metathesis (RCM) reaction to generate the macrocyclic ring.

Smith, Amos B.; Bosanac, Todd; Basu, Kallol

2009-01-01

301

Synthesis of alpha-amino acids via asymmetric phase transfer-catalyzed alkylation of achiral nickel(II) complexes of glycine-derived Schiff bases.  

PubMed

Achiral, diamagnetic Ni(II) complexes 1 and 3 have been synthesized from Ni(II) salts and the Schiff bases, generated from glycine and PBP (7) and PBA (11), respectively, in MeONa/MeOH solutions. The requisite carbonyl-derivatizing agents pyridine-2-carboxylic acid(2-benzoyl-phenyl)-amide 7 (PBP) and pyridine-2-carboxylic acid(2-formyl-phenyl)-amide 11 (PBA) were readily prepared from picolinic acid and o-aminobenzophenone or picolinic acid and methyl o-anthranilate, respectively. The structure of 1 was established by X-ray crystallography. Complexes 1 and 3 were found to undergo C-alkylation with alkyl halides under PTC conditions in the presence of beta-naphthol or benzyltriethylammonium bromide as catalysts to give mono- and bis-alkylated products, respectively. Decomposition of the complexes with aqueous HCl under mild conditions gave the required amino acids, and PBP and PBA were recovered. Alkylation of 1 with highly reactive alkyl halides, carried out under the PTC conditions in the presence of 10% mol of (S)- or (R)-2-hydroxy-2'-amino-1,1'-binaphthyl 31a (NOBIN) and/or its N-acyl derivatives and by (S)- or (R)-2-hydroxy-8'-amino-1,1'-binaphthyl 32a (iso-NOBIN) and its N-acyl derivatives, respectively, gave rise to alpha-amino acids with high enantioselectivities (90-98.5% ee) in good-to-excellent chemical yields at room temperature within several minutes. An unusually large positive nonlinear effect was observed in these reactions. The Michael addition of acrylic derivatives 37 to 1 was conducted under similar conditions with up to 96% ee. The (1)H NMR and IR spectra of a mixture of the sodium salt of NOBIN and 1 indicated formation of a complex between the two components. Implications of the association and self-association of NOBIN for the observed sense of asymmetric induction and nonlinear effects are discussed. PMID:14558835

Belokon, Yuri N; Bespalova, Natalia B; Churkina, Tatiana D; Císarová, Ivana; Ezernitskaya, Marina G; Harutyunyan, Syuzanna R; Hrdina, Radim; Kagan, Henri B; Kocovský, Pavel; Kochetkov, Konstantin A; Larionov, Oleg V; Lyssenko, Konstantin A; North, Michael; Polásek, Miroslav; Peregudov, Alexander S; Prisyazhnyuk, Vladimir V; Vyskocil, Stepán

2003-10-22

302

Absolute Norms on C n  

Microsoft Academic Search

In this paper, we study the absolute normalized norms on Cn. We show that the set of all absolute normalized norms on Cn is in one-to-one correspondence with the set ?n of all continuous convex functions on ?n with some suitable conditions, where ?n={(s1,…,sn?1)?Rn?1; s1+s2+···+sn?1?1, si?0 (1?i?n?1)}. As some applications, we show that an absolute norm on Cn is strictly

Kichi-Suke Saito; Mikio Kato; Yasuji Takahashi

2000-01-01

303

Discrimination of cryptochirality in chiral isotactic polystyrene by asymmetric autocatalysis† †Electronic supplementary information (ESI) available: The asymmetric autocatalysis induced by bis(phenol) 5. See DOI: 10.1039/b912813a Click here for additional data file.  

PubMed Central

Chiral isotactic polystyrenes induce the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde, affording the enantiomerically enriched pyrimidyl alkanol with the corresponding absolute configuration to that of cryptochiral polystyrenes in conjunction with asymmetric autocatalysis.

Kawasaki, Tsuneomi; Hohberger, Christiane; Araki, Yuko; Hatase, Kunihiko; Beckerle, Klaus

2009-01-01

304

Absolute Zero: Science Educator's Guide  

NSDL National Science Digital Library

This guide provides recommendations for curricular modules on low temperature physics. Designed for teachers and informal educators of middle school students. this guide complements the Absolute Zero Community Education Outreach Guide. Suggestions on leading discussions, increasing student participation, and the use of inquiry are included. This material is related to a two-part public broadcasting special, Absolute Zero, produced by Meridian Productions and Windfall Films. Absolute Zero is underwritten by the National Science Foundation and the Alfred P. Sloan Foundation and is based largely on Tom Shachtmanâs acclaimed book, Absolute Zero and the Conquest of Cold.

2008-09-18

305

Synthesis, spectroscopic characterization and antimicrobial activity of binuclear metal complexes of a new asymmetrical Schiff base ligand: DNA binding affinity of copper(II) complexes.  

PubMed

The 1:1 condensation of o-acetoacetylphenol and 1,2-diaminopropane under condition of high dilution gives the mono-condensed Schiff base, (E)-3-(1-aminopropan-2-ylimino)-1-(2-hydroxyphenyl)butan-1-one. The mono-condensed Schiff base has been used for further condensation with isatin to obtain the new asymmetrical dicompartmental Schiff base ligand, (E)-3-(2-((E)-4-(2-hydroxyphenyl)-4-oxobutan-2-ylideneamino) propylimino)indolin-2-one (H3L) with a N2O3 donor set. Reactions of the ligand with metal salts give a series of new binuclear complexes. The ligand and its metal complexes were characterized by elemental analyses, IR, (1)H and (13)C NMR, electronic, ESR and mass spectra, conductivity and magnetic susceptibility measurements as well as thermal analyses. The analytical and spectroscopic tools showed that the complexes can be formulated as: [(HL)(VO)2(SO4)(H2O)]·4H2O, [(HL)Fe2Cl4(H2O)3]·EtOH, [(HL)Fe2(ox)Cl2(H2O)3]·2H2O, [(L)M2(OAc)(H2O)m]·nH2O; M=Co, Ni or Cu, m=4, 0 and n=2, 3, [(HL)Cu2Cl]Cl·6H2O and [(L)(UO2)2(OAc)(H2O)3]·6H2O. The metal complexes exhibited octahedral geometrical arrangements except copper complexes that exhibited tetrahedral geometries and uranyl complex in which the metal ion is octa-coordinated. The Schiff base and its metal complexes were evaluated for antimicrobial activity against Gram positive bacteria (Staphylococcus aureus), Gram negative bacteria (Escherichia coli) and fungi (Candida albicans and Aspergillus flavus). The ligand and some of its complexes were found to be biologically active. The DNA-binding properties of the copper complexes (6 and 7) have been investigated by electronic absorption, fluorescence and viscosity measurements. The results obtained indicate that these complexes bind to DNA via an intercalation binding mode with an intrinsic binding constant, Kb of 1.34×10(4) and 2.5×10(4) M(-1), respectively. PMID:23988527

Shebl, Magdy

2014-01-01

306

Absolute Identification by Relative Judgment  

ERIC Educational Resources Information Center

In unidimensional absolute identification tasks, participants identify stimuli that vary along a single dimension. Performance is surprisingly poor compared with discrimination of the same stimuli. Existing models assume that identification is achieved using long-term representations of absolute magnitudes. The authors propose an alternative…

Stewart, Neil; Brown, Gordon D. A.; Chater, Nick

2005-01-01

307

Asymmetric reduction of steroidal 20-ketones: chemical synthesis of corticosteroid derivatives containing the 20 alpha, 21-diol and 17 alpha, 20 alpha, 21-triol side chains.  

PubMed

A method is presented for the chemical synthesis of corticosteroid derivatives containing the 20 alpha, 21-diol and 17 alpha, 20 alpha, 21-triol side chains. The ketol side chains of cortisol, corticosterone, 11-deoxycortisol, and 11-deoxycorticosterone were reduced at C-20 with sodium borohydride in a two-phase system consisting of aqueous calcium chloride and an organic phase of chloroform or ethyl acetate. Stereoselectivity of reduction was 92% alpha-oriented for cortisol and 79% alpha-oriented for 11-deoxycortisol at -27 degrees. The 20 alpha-form diminished relative to the 20 beta-form with increasing temperature. For the 17-deoxy steroids, reduction to the 20 alpha-form was 23% for 11-deoxycorticosterone and 41% for corticosterone. The 20 alpha/20 beta ratios of 17-deoxy steroids were unchanged between 0 degree and -27 degrees. Calcium ions increased the solubility of corticosteroids in the aqueous phase. We propose that calcium ions affect the stereochemistry of reduction by forming a bidentate complex with the side chains of 17 alpha-hydroxy steroids, fixing them in an orientation favorable to 20 alpha-reduction, and by altering the phase partition of the steroids. PMID:6680932

Han, C A; Monder, C

1983-12-01

308

Asymmetric Synthesis, Structure, and Reactivity of Unexpectedly Stable Spiroepoxy-?-Lactones Including Facile Conversion to Tetronic Acids: Application to (+)-Maculalactone A  

PubMed Central

A novel class of small spirocyclic heterocycles, spiroepoxy-?-lactones (1,4-dioxaspiro[2.3]-hexan-5-ones), is described that exhibit a number of interesting reactivity patterns. These spiroheterocycles, including an optically active series, are readily synthesized by epoxidation of ketene dimers (4-alkylidene-2-oxetanones) available from homo- or heteroketene dimerization. An analysis of bond lengths in these systems by X-ray crystallography and comparison to data for known spirocycles and those determined computationally, suggest that anomeric effects in these systems may be more pronounced due to their rigidity and may contribute to their surprising stability. The synthetic utility of spiroepoxy-?-lactones was explored and one facile rearrangement identified under several conditions provides a 3-step route from acid chlorides to optically active tetronic acids, ubiquitous heterocycles in bioactive natural products. The addition of various nucleophiles to these spirocycles leads primarily to addition at C5 and C2. The utility of an optically active spiroepoxy-?-lactone was demonstrated in the concise, enantioselective synthesis of the anti-fouling agent, (+)-maculalactone A, which proceeds in 5 steps from hydrocinnamoyl chloride by way of a tetronic acid intermediate.

Duffy, Richard J.; Morris, Kay A.; Vallakati, Ravikrishna; Zhang, Wei; Romo, Daniel

2009-01-01

309

Synthesis, structures, and magnetic properties of novel mononuclear, tetranuclear, and 1D chain Mn(III) complexes involving three related asymmetrical trianionic ligands.  

PubMed

The manganese(III) complexes studied in this report derive from asymmetrical trianionic ligands abbreviated H(3)L(i) (i = 4-6). These ligands are obtained through reaction of salicylaldehyde with "half-units", the latter resulting from monocondensation of different diamines with phenylsalicylate,. Upon deprotonation, L(i) (i = 4-6) possess an inner N(2)O(2) coordination site with one amido, one imine, and two phenoxo functions, and an outer amido oxygen donor. The trianionic character of such ligands yields original neutral complexes with the L/Mn stoichiometry. The crystal and molecular structures of three complexes have been determined at 190 K (1) or 180 K (2 and 3). Complex 1 crystallizes in the triclinic space group P (No. 2): a = 7.8582(14) A, b = 10.9225(16) A, c = 12.4882(18) A, alpha = 67.231(14) degrees, beta = 72.134(14) degrees, gamma = 82.589(13) degrees, V = 940.6(3) A(3), Z = 2. Complex 2 crystallizes in the orthorhombic space group Pbcn (Nuomicron. 60): a = 23.8283(15) A, b = 11.1605(7) A, c = 26.152(2) A, V = 6954.8(8) A(3), Z = 8, while complex 3 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 11.7443(14) A, b = 7.5996(10) A, c = 18.029(2) A, beta = 100.604(10) degrees, V = 1581.6(3) A(3), Z = 4. Owing to hydrogen bonds and pi-pi stackings, the mononuclear neutral molecules of 1 are arranged in a 2D network while complexes 2 and 3 are tetranuclear and polymeric (1D chain) species, respectively, owing to the bridging ability of the oxygen atom of the amido function. The experimental magnetic susceptibilities of complexes 2 and 3 indicate the occurrence of similarly weak Mn(III)-Mn(III) antiferromagnetic interactions (J = -1.1 cm(-1)). Single ion zero-field splitting of manganese(III) must be taken into account for satisfactorily fitting the data by exact calculation of the energy levels associated to the spin Hamiltonian through diagonalization of the full matrix for axial symmetry in 2 (J = - 1.1 cm(-1), D(1) = 2.2 cm(-1), D(2) = -2.8 cm(-1)), D(1) and D(2) being associated to the six- and five-coordinate Mn ions, respectively. A weaker antiferromagnetic interaction (J = - 0.2 cm(-1)) operates through pi-pi stacking in complex 1. Complex 3 is a weak ferromagnet (ordering temperature approximately 7 K) as a result of the spin canting originating from the crystal packing. PMID:15074994

Costes, Jean-Pierre; Dahan, Françoise; Donnadieu, Bruno; Rodriguez Douton, Maria-Jesus; Fernandez Garcia, Maria-Isabel; Bousseksou, Azzedine; Tuchagues, Jean-Pierre

2004-04-19

310

Organocatalytic asymmetric fluorination of ?-chloroaldehydes involving kinetic resolution  

PubMed Central

Summary In a previous study it was shown that the enantioselective ?-fluorination of racemic ?-chloroaldehydes with a chiral organocatalyst yielded the corresponding ?-chloro-?-fluoroaldehydes with high enantioselectivity. It was also revealed that kinetic resolution of the starting aldehydes was involved in this asymmetric fluorination. This paper describes the determination of the absolute stereochemistry of a resulting ?-chloro-?-fluoroaldehyde. Some information about the substrate scope and a possible reaction mechanism are also described which shed more light on the nature of this asymmetric fluorination reaction.

Okimi, Takuya; Abe, Yoshiyuki; Narayama, Akira; Nakamura, Nami; Iwasa, Seiji

2014-01-01

311

Determination of the Absolute Stereochemistry of Secondary Alcohols: An Advanced Organic Chemistry Experiment for Undergraduate Students.  

ERIC Educational Resources Information Center

Describes experiments which can be completed in five four-hour laboratory sessions, including two synthesis (alpha-phenylbutyric and alpha-phenylbutyric acid anhydride) and determining the absolute stereochemistry of secondary alcohols using the synthetic products. (JN)

Bandaranayake, Wickramasinghe M.

1980-01-01

312

Synthesis and assignment of absolute configuration of (-)-oleocanthal: a potent, naturally occurring non-steroidal anti-inflammatory and anti-oxidant agent derived from extra virgin olive oils.  

PubMed

[structure: see text] Effective total syntheses and the assignment of absolute configurations of both the (+)- and (-)-enantiomers of oleocanthal 1 (a.k.a. deacetoxy ligstroside aglycon), the latter derived from extra virgin olive oils and known to be responsible for the back of the throat irritant properties of olive oils, have been achieved. The absolute and relative stereochemistry of the naturally occurring enantiomer (-)-1 proved to be 3S,4E. Both syntheses begin with d-(-)-ribose, proceed in 12 steps, and are achieved with an overall yield of 7%. Both enantiomers proved to be non-steroidal anti-inflammatory and anti-oxidant agents. PMID:16235961

Smith, Amos B; Han, Qiang; Breslin, Paul A S; Beauchamp, Gary K

2005-10-27

313

Enantioselective Photocycloaddition of 3-Hydroxyflavones: Total Syntheses and Absolute Configuration Assignment of (+)-Ponapensin and (+)-Elliptifoline  

PubMed Central

We have previously reported development of biomimetic, asymmetric [3+2] photocycloadditions between 3-hydroxyflavones and cinnamate dipolarophiles to access (?)-rocaglamide and related natural products. Herein, we describe enantioselective syntheses of aglain cycloadducts leading to the first total syntheses and absolute configuration assignments of the aglain natural products (+)-ponapensin and (+)-elliptifoline.

Lajkiewicz, Neil J.; Roche, Stephane P.; Gerard, Baudouin

2012-01-01

314

Improved synthesis of structural analogues of (-)-epicatechin gallate for modulation of staphylococcal ?-lactam resistance?  

PubMed Central

The high-yielding synthesis of enantiomerically pure epicatechin gallate analogues where the A and/or B-ring hydroxylation is reduced or altered has been achieved by optimising routes to the catechin stereochemistry. The B-ring analogues were synthesised by using an electrophilic ring closure onto an enantiomerically enriched epoxide as a key step. The A and B-ring hydroxyl-deleted analogues were synthesised through a Mitsunobu cyclisation. For the B-ring analogues, the anti- (catechin) stereochemistry was converted to the syn- (epicatechin) stereochemistry by a known oxidation/reduction protocol. Absolute stereochemistry was derived from either a Sharpless epoxidation or asymmetric dihydroxylation.

Anderson, James C.; Grounds, Helen; Reeves, Suzanna; Taylor, Peter W.

2014-01-01

315

Cu(I)-NHC catalyzed asymmetric silyl transfer to unsaturated lactams and amides.  

PubMed

The first asymmetric silylation of unsaturated lactams and amides using Cu(I)-NHC catalysts and PhMe2SiBpin has been accomplished. The method has been exploited in an expedient asymmetric synthesis of the (R)-enantiomer of the nootropic drug oxiracetam. PMID:24350760

Pace, Vittorio; Rae, James P; Procter, David J

2014-01-17

316

Globalization and Asymmetrical Warfare.  

National Technical Information Service (NTIS)

Globalization is having a tremendous effect on the ability of terrorist and criminal organizations to act on a global scale. These organizations are using asymmetrical means to target U.S. interests at home and abroad. The events of September 11th were th...

W. J. Hartman

2002-01-01

317

Absolute gravity measurements in Indonesia  

NASA Astrophysics Data System (ADS)

For the purposes of the calibration of the superconducting gravimeter in Bandung and the establishment of the absolute gravity points, we have carried out absolute gravity measurements for the first time in Indonesia in November, 2002. We have been conducting a superconducting gravimeter (SG TT-70 #08) observation in Bandung since December 1997, under the cooperation between Kyoto University and the Volcanological Survey of Indonesia. It is one of the GGP observation points and the unique SG point near the equator. Hence the SG data are expected not only for the studies of solid earth dynamics but also for the studies of the fluid envelope (ocean, atmosphere, land water) in equatorial regions. However, the SG is a relative gravimeter and it inevitably requires calibration by means of an absolute gravimeter to ensure the scale factor and to determine instrumental drifts. Moreover, there was no absolute gravity point in Indonesia so far, therefore the realization of absolute gravity measurements in Indonesia had been strongly desired. We have carried out absolute gravity measurements in Bandung during Nov. 10 -19 by means of a FG5 (#210), and we obtained more than 29,000 effective drops. The gravity value newly determined at the gravity base point in Bandung is 977976701.9 uGal (1.e-8 m/s**2) and the scale factor for the SG is 52.281 uGal/V, although both values are still tentative and might be slightly revised in the future. We have also established another absolute gravity point in Yogyakarta near Merapi volcano. The absolute gravity measurements in Yogyakarta have been carried out during Nov. 22-26 and a tentative gravity value obtained is 978203091.9 uGal.

Fukuda, Y.; Higashi, T.; Takemoto, S.; Abe, M.; Sjafra, D.; Dendi, K.; Andan, A.; Doi, K.; Imanishi, Y.; Arguino, G.

2003-04-01

318

Cryogenic, Absolute, High Pressure Sensor  

NASA Technical Reports Server (NTRS)

A pressure sensor is provided for cryogenic, high pressure applications. A highly doped silicon piezoresistive pressure sensor is bonded to a silicon substrate in an absolute pressure sensing configuration. The absolute pressure sensor is bonded to an aluminum nitride substrate. Aluminum nitride has appropriate coefficient of thermal expansion for use with highly doped silicon at cryogenic temperatures. A group of sensors, either two sensors on two substrates or four sensors on a single substrate are packaged in a pressure vessel.

Chapman, John J. (Inventor); Shams. Qamar A. (Inventor); Powers, William T. (Inventor)

2001-01-01

319

Mn-salen catalyst, competitor of enzymes, for asymmetric epoxidation  

Microsoft Academic Search

Asymmetric epoxidation of simple olefins using (salen)manganese(III) complexes as catalysts has made a great advances in the last half decade and now finds wide application in organic synthesis. In this article, we describe the scope of the reaction, and the principal achievements to date are presented in Tables. The mechanistic aspect of the reaction is also discussed briefly.

Tsutomu Katsuki

1996-01-01

320

Absolute configuration of naturally occurring glabridin.  

PubMed

The title compound {systematic name: 4-[(3R)-8,8-dimethyl-3,4-dihydro-2H-pyrano[2,3-f]chromen-3-yl]benzene-1,3-diol, commonly named glabridin}, C20H20O4, is a species-specific biomarker from the roots Glycyrrhiza glabra L. (European licorice, Fabaceae). In the present study, this prenylated isoflavan has been purified from an enriched CHCl3 fraction of the extract of the root, using three steps of medium-pressure liquid chromatography (MPLC) by employing HW-40F, Sephadex LH-20 and LiChroCN as adsorbents. Pure glabridin was crystallized from an MeOH-H2O mixture (95:5?v/v) to yield colorless crystals containing one molecule per asymmetric unit (Z' = 1) in the space group P212121. Although the crystal structure has been reported before, the determination of the absolute configuration remained uncertain. Stereochemical analysis, including circular dichroism, NMR data and an X-ray diffraction data set with Bijvoet differences, confirms that glabridin, purified from its natural source, is found only in a C3 R configuration. These results can therefore be used as a reference for the assignment of the configuration and enantiopurity of any isolated or synthetic glabridin sample. PMID:24192160

Simmler, Charlotte; Fronczek, Frank R; Pauli, Guido F; Santarsiero, Bernard D

2013-11-01

321

Asymmetric hydrogenations (Nobel lecture).  

PubMed

The start of the development of catalysts for asymmetric hydrogenation was the concept of replacing the triphenylphosphane ligand of the Wilkinson catalyst with a chiral ligand. With the new catalysts, it should be possible to hydrogenate prochiral olefins. Knowles and his co-workers were convinced that the phosphorus atom played a central role in this selectivity, as only chiral phosphorus ligands such as (R,R)-DIPAMP, whose stereogenic center lies directly on the phosphorus atom, lead to high enantiomeric excesses when used as catalysts in asymmetric hydrogenation reactions. This hypothesis was disproven by the development of ligands with chiral carbon backbones. Although the exact mechanism of action of the phosphane ligands is not incontrovertibly determined to this day, they provide a simple entry to a large number of chiral compounds. PMID:19746594

Knowles, William S

2002-06-17

322

Asymmetrically Driven Implosions  

Microsoft Academic Search

Modelling of the implosion dynamics of a capsule irradiated by an asymmetric x-ray drive is a taxing problem for radiation-hydrocodes. We present preliminary experimental data along with detailed modelling of capsule implosions in which a one-sided x-ray source geometry was adopted. A thin-shelled low-Z pellet was suspended in a gold cavity (or `secondary hohlraum') attached to a standard HELEN hohlraum

A. M. Dunne; M. J. Edwards; J. M. Foster; P. A. Rosen

1997-01-01

323

Asymmetric dipolar ring  

DOEpatents

A device having a dipolar ring surrounding an interior region that is disposed asymmetrically on the ring. The dipolar ring generates a toroidal moment switchable between at least two stable states by a homogeneous field applied to the dipolar ring in the plane of the ring. The ring may be made of ferroelectric or magnetic material. In the former case, the homogeneous field is an electric field and in the latter case, the homogeneous field is a magnetic field.

Prosandeev, Sergey A. (Fayetteville, AR); Ponomareva, Inna V. (Tampa, FL); Kornev, Igor A. (Ill-de-France, FR); Bellaiche, Laurent M. (Fayetteville, AR)

2010-11-16

324

Asymmetric acoustic gratings  

NASA Astrophysics Data System (ADS)

The unidirectional transmission of acoustic waves is realized by a simple geometrically asymmetric steel grating structure. This exotic phenomenon stems from the one-way diffraction effect induced by the different periods of the slits on the both surfaces of the sample. And the frequency range of unidirectional transmission is simply determined by the structure periods. The experimental results agree well with the theoretical simulation. This remarkable effect is expected potential applications in ultrasonic devices, such as acoustic rectifiers and acoustic diodes.

He, Zhaojian; Peng, Shasha; Ye, Yangtao; Dai, Zhongwei; Qiu, Chunyin; Ke, Manzhu; Liu, Zhengyou

2011-02-01

325

Asymmetrically driven implosions  

SciTech Connect

Techniques to achieve uniform near-spherical symmetry of radiation drive on a capsule in a laser-heated hohlraum have received detailed attention in the context of inertial confinement fusion. However, much less attention has been paid to the understanding of the hohlraum physics in cases where the radiation drive departs significantly from spherical symmetry. A series of experiments has been carried out to study the implosion dynamics of a capsule irradiated by a deliberately asymmetric x-ray drive. The experimental data provide a sensitive test of radiation transport in hohlraums in which drive symmetry is modulated by asymmetric laser beam timing and the use of wall materials of different albedos. Data from foam ball and thin-shell capsule experiments are presented together with modeling using consecutively linked Lagrangian and Eulerian calculational schemes. The thin-shell capsules exhibit much stronger sensitivity to early-time asymmetry than do the foam balls, and this sensitivity results in the formation of a well-defined polar jet. These data are shown to challenge computational modeling in this highly asymmetric convergent regime. All of the experiments detailed were carried out at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] at the Laboratory for Laser Energetics in Rochester, NY.

Vaughan, K.; McAlpin, S.; Foster, J. M.; Stevenson, R. M. [AWE Aldermaston, Reading RG7 4PR (United Kingdom); Glendinning, S. G.; Sorce, C. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)

2010-05-15

326

Asymmetrically-Driven Implosions  

NASA Astrophysics Data System (ADS)

Techniques to achieve uniform, near-spherical symmetry of radiation drive on a capsule in a laser-heated hohlraum have received detailed attention in the context of ICF. However, much less attention has been paid to the understanding of the hohlraum physics in cases where the radiation drive departs significantly from spherical symmetry. In recent work at the OMEGA laser, AWE has carried out a series of experiments to study the implosion dynamics of a capsule irradiated by a deliberately asymmetric X-ray drive. The experimental data provide a sensitive test of radiation transport in which drive symmetry is modulated by the use of variable albedo layers and asymmetric laser-beam timing. Data from foam-ball and thin-shell capsule experiments are presented, together with modelling using consecutively linked Lagrangian and Eulerian, as well as single-step ALE, calculational schemes. The thin-shell capsules exhibit much stronger sensitivity to asymmetry than foam balls resulting in the formation of a well-defined polar jet. These data are shown to challenge computational modelling in this highly asymmetric, strongly convergent regime.

Vaughan, Kelly

2009-11-01

327

Asymmetric droplet interface bilayers.  

PubMed

In cell membranes, the lipid compositions of the inner and outer leaflets differ. Therefore, a robust model system that enables single-channel electrical recording with asymmetric bilayers would be very useful. We and others recently developed the droplet interface bilayer (DIB), which is formed by connecting lipid monolayer-encased aqueous droplets submerged in an oil-lipid mixture. Here, we incorporate lipid vesicles of different compositions into aqueous droplets and immerse them in an oil bath to form asymmetric DIBs (a-DIBs). Both alpha-helical and beta-barrel membrane proteins insert readily into a-DIBs, and their activity can be measured by single-channel electrical recording. We show that the gating behavior of outer membrane protein G (OmpG) from Escherichia coli differs depending on the side of insertion in an asymmetric DIB with a positively charged leaflet opposing a negatively charged leaflet. The a-DIB system provides a general platform for studying the effects of bilayer leaflet composition on the behavior of ion channels and pores. PMID:18407631

Hwang, William L; Chen, Min; Cronin, Bríd; Holden, Matthew A; Bayley, Hagan

2008-05-01

328

JWST Absolute Flux Calibration Plan  

NASA Astrophysics Data System (ADS)

The plan for the absolute flux calibration of all JWST instruments will be described. The science goal is to be able to predict the absolute flux of stars for all JWST instruments to sub-1% accuracy. A set of A0V, solar, and hot stars have been picked as a preliminary calibration sample. This calibration sample has been chosen specifically to provide at least 5 stars of each type in all the observing modes of each JWST instrument. In addition to existing Hubble and Spitzer observations, ground-based high-resolution spectroscopy will be obtained to provide strong, independent constraints on the best fitting model stellar atmospheres for each star.

Gordon, Karl D.; Bohlin, R.

2013-01-01

329

Absolute classification with unsupervised clustering  

NASA Technical Reports Server (NTRS)

An absolute classification algorithm is proposed in which the class definition through training samples or otherwise is required only for a particular class of interest. The absolute classification is considered as a problem of unsupervised clustering when one cluster is known initially. The definitions and statistics of the other classes are automatically developed through the weighted unsupervised clustering procedure, which is developed to keep the cluster corresponding to the class of interest from losing its identity as the class of interest. Once all the classes are developed, a conventional relative classifier such as the maximum-likelihood classifier is used in the classification.

Jeon, Byeungwoo; Landgrebe, D. A.

1992-01-01

330

Synthesis and Properties of Asymmetric Polymers.  

National Technical Information Service (NTIS)

Polyamides were prepared by the interfacial condensation procedure using an optically active diacid chloride with the appropriate diamine and by aminolysis of 2,4-dinitrophenyl or succinimido diesters of the optically active acid with the required diamine...

C. G. Overberger

1972-01-01

331

Dehydrogenases and transaminases in asymmetric synthesis  

Microsoft Academic Search

Improved stereoselectivity in dehydrogenase-mediated reductions has been achieved by rationally designed gene overexpression and knockouts in Saccharomyces cerevisiae cells and by isolating and characterizing novel dehydrogenases from other organisms. Transaminases have been used to prepare unnatural amines and amino acids in good yields, particularly when the equilibria are shifted by selective product removal.

Jon D Stewart

2001-01-01

332

Asymmetric synthesis of tetrahydroquinolines through supramolecular organocatalysis.  

PubMed

Functionalized chiral tetrahydroquinolines were synthesized through supramolecular organocatalysis using quinidine-NH-thiourea /l-phenylalanine followed by reductive amination from the simple substrates. PMID:24840771

Ramachary, Dhevalapally B; Shruthi, Kodambahalli S

2014-06-01

333

Absolute pitch: Music and beyond  

Microsoft Academic Search

“Perfect pitch,” known in the scientific literature as “absolute pitch” (AP), is a rare phenomenon that has fascinated musicians and scientists alike for over a century. There has been a great deal of conflict in the literature between advocates of the two main theories on the etiology of AP: some believe that AP is learned early in life through intensive

David A. Ross; John C. Gore; Lawrence E. Marks

2005-01-01

334

Absolute transition probabilities of phosphorus.  

NASA Technical Reports Server (NTRS)

Use of a gas-driven shock tube to measure the absolute strengths of 21 P I lines and 126 P II lines (from 3300 to 6900 A). Accuracy for prominent, isolated neutral and ionic lines is estimated to be 28 to 40% and 18 to 30%, respectively. The data and the corresponding theoretical predictions are examined for conformity with the sum rules.-

Miller, M. H.; Roig, R. A.; Bengtson, R. D.

1971-01-01

335

Absolute focus lock for microscopes  

NASA Technical Reports Server (NTRS)

Mechanism absolutely immobilizes microscope stage at a preset focus, preserving focus indefinitely. The lock is a second-class lever consisting of a straight body having a fulcrum with a cylindrical bearing surface at one end and a thumbscrew at the other end.

Cone, C. D., Jr.; Loop, R. W.; Tongier, M., Jr.

1970-01-01

336

Absolute Pitch and Planum Temporale  

Microsoft Academic Search

An increased leftward asymmetry of the planum temporale (PT) in absolute-pitch (AP) musicians has been previously reported, with speculation that early exposure to music influences the degree of PT asymmetry. To test this hypothesis and to determine whether a larger left PT or a smaller right PT actually accounts for the increased overall PT asymmetry in AP musicians, anatomical magnetic

Julian Paul Keenan; Ven Thangaraj; Andrea R. Halpern; Gottfried Schlaug

2001-01-01

337

Asymmetric catalysis: science and opportunities (Nobel lecture).  

PubMed

Asymmetric catalysis, in its infancy in the 1960s, has dramatically changed the procedures of chemical synthesis, and resulted in an impressive progression to a level that technically approximates or sometimes even exceeds that of natural biological processes. The recent exceptional advances in this area attest to a range of conceptual breakthroughs in chemical sciences in general, and to the practical benefits of organic synthesis, not only in laboratories but also in industry. The growth of this core technology has given rise to enormous economic potential in the manufacture of pharmaceuticals, animal health products, agrochemicals, fungicides, pheromones, flavors, and fragrances. Practical asymmetric catalysis is of growing importance to a sustainable modern society, in which environmental protection is of increasing concern. This subject is an essential component of molecular science and technology in the 21st century. Most importantly, recent progress has spurred various interdisciplinary research efforts directed toward the creation of molecularly engineered novel functions. The origin and progress of my research in this field are discussed. PMID:19746595

Noyori, Ryoji

2002-06-17

338

Three dimensional asymmetric model of the magnetopause  

NASA Astrophysics Data System (ADS)

We identify 542 magnetopause crossings from Interball, Geotail, Wind, Cluster, LANL and GOES for normal and extreme solar wind conditions. The magnetopause crossings cover the global region in the near-Earth space, and corresponding upstream solar wind parameters, including the density ratio of helium to hydrogen, are identified from ACE or Wind with 5-min resolution by shifting time to maximize the correlation between the clock angle of the IMF and that of magnetosheath field. Based on these data, a three dimensional asymmetric model of the magnetopause is developed for GSM coordinates by Levenberg Marquardt (LM) method for nonlinear parameter fitting. This model can describe the possible structures of the magnetopause shape (MPS), including east-west asymmetry in the YGSM direction, south-north asymmetry in the ZGSM direction, asymmetry between the east-west direction and the southnorth direction, and indentations in the vicinity of the north and south cusps. In order to give the appropriate initial values for the fitting coefficients and get the reasonable relationship between the interplanetary solar wind parameters and the MPS, we separate the fitting regions into three parts by the divided-fitting method: the sub solar region, the low-middle latitude region and the high latitude region. According to the decrease in the standard deviation from fitting, the asymmetric model of MPS is optionally parameterized by solar wind dynamic pressure (DP), interplanetary magnetic field (BZ), and dipole tilt angle (?). Considering saturation effects of IMF BZ and DP on the MPS, we use multinomial functions first and then select the saturated functions to replace them. The preliminary result in the investigation show that the standard deviation and the absolute deviation of this model are decreased to 0.908RE and 0.665RE, and that this asymmetric model is appropriate to predict the MPS for both normal and extreme solar wind conditions.

Lin, Ruilin

339

Absolute-Gravity Workshop planned  

NASA Astrophysics Data System (ADS)

The new FG5 absolute gravimeter has a design goal accuracy of 1 ?Gal and represents the most recent of a series of gravimeters inspired by advances in gravimeter design by J. Faller at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colo. A 2-day workshop will be held in Boulder from March 22 to 23 to discuss current and future applications of absolute gravity (g). Details of the workshop appear at the end of this article. The instrument is based on the principle of interferometrically measuring the time and position of a weight falling in a vacuum, using a stabilized laser and an atomic clock [cf. Cook, 1967; Faller, 1963; Hammond, 1970; Zumberge, 1981; Niebauer, 1986].

Bilham, Roger; Sasagawa, Glenn

340

Absolute light output of scintillators  

Microsoft Academic Search

The absolute light outputs of BGO, CsI(Tl) and some new Ce-doped crystals have been measured to an accuracy of about ±5% using calibrated XP2020Q photomultipliers and standard S3590.03 and S2740.03 photodiodes. The use of small crystals, 9 mm in diameter and 1 mm thick, reduces the corrections for imperfections in the light collection process and in the photoelectron collection by

M. Moszynski; M. Kapusta; M. Mayhugh; D. Wolski; S. O. Flyckt

1997-01-01

341

Absolute Radiometric Calibration of SERTS  

NASA Astrophysics Data System (ADS)

The Solar EUV Rocket Telescope and Spectrograph (SERTS) obtains imaged high-resolution spectra of individual solar features, providing information about the Sun's corona and upper transition region. We have recently carried out a complete end-to-end calibration of the instrument to determine its absolute radiometric response over the full bandpass of 300 -- 365 Angstroms. The measurements were done at the Rutherford-Appleton Laboratory (RAL) in England, using the same vacuum facility and EUV radiation source used in the pre-flight calibration of the Coronal Diagnostic Spectrometer (CDS) experiment now flying aboard the SOHO spacecraft. For our SERTS project, the unique radiation source provided by the Physikalisch-Technische Bundesanstalt (PTB) in Germany was re-calibrated to an absolute accuracy of 7% (1sigma ) at 12 wavelengths covering our bandpass directly against the Berlin electron storage ring BESSY, which is a primary radiometric source standard. Scans of the SERTS aperture over a range of pitch and yaw angles were made to determine the instrument's absolute spectral sensitivity to <= 25%, considering all sources of error. These results will be matched against prior calibrations which relied on combining measurements of individual optical components, and on comparisons with theoretically predicted `insensitive' line ratios. The recent measurements at RAL also give information about the uniformity of illumination across the collimated source beam, as well as about polarization characteristics of both the instrument and radiation source, which may prove helpful in correctly interpreting the original CDS calibration data. We hope to repeat such calibration measurements and to provide future SERTS flights annually, at least throughout the duration of the SOHO mission. Coordinated observing programs would then allow these updated absolute calibrations to be transferred on a regular basis to several of the instruments onboard SOHO, including CDS, EIT, and CELIAS.

Thomas, R. J.; Condor, C. E.; Haas, J. P.; Linard, D. L., II; Swartz, M.; Kent, B. J.; Hollandt, J.

1997-12-01

342

Improved technique for measuring full-field absolute phases in a common-path heterodyne interferometer.  

PubMed

When an asymmetric triangle voltage signal is applied to drive an electro-optic modulator, interference signals with two groups of periodic sinusoidal segments with different frequencies are obtained. An improved method is proposed to fit these two groups of segments, and their associated phases can be determined. The absolute phase can be obtained by subtracting the initial phase from the phases of these two groups. This technique is applied to all pixels, and the full-field absolute phase measurements can be achieved. The validity of this method is demonstrated. PMID:20820216

Chen, Yen-Liang; Su, Der-Chin

2010-09-01

343

Preparation of asymmetric porous materials  

DOEpatents

A method for preparing an asymmetric porous material by depositing a porous material film on a flexible substrate, and applying an anisotropic stress to the porous media on the flexible substrate, where the anisotropic stress results from a stress such as an applied mechanical force, a thermal gradient, and an applied voltage, to form an asymmetric porous material.

Coker, Eric N. (Albuquerque, NM)

2012-08-07

344

Asymmetrically warped spacetimes  

SciTech Connect

We investigate spacetimes in which the speed of light along flat 4D sections varies over the extra dimensions due to different warp factors for the space and the time coordinates ('asymmetrically warped' spacetimes). The main property of such spaces is that while the induced metric is flat, implying Lorentz invariant particle physics on a brane, bulk gravitational effects will cause apparent violations of Lorentz invariance and of causality from the brane observer's point of view. An important experimentally verifiable consequence of this is that gravitational waves may travel with a speed different from the speed of light on the brane, and possibly even faster. We find the most general spacetimes of this sort, which are given by certain types of black hole spacetimes characterized by the m a s and the charge of the black hole. We show how to satisfy the junction conditions and analyze the properties of these space-times.

Csaki, C. (Csaba)

2001-01-01

345

Asymmetric Higgsino dark matter.  

PubMed

In the supersymmetric framework, prior to the electroweak phase transition, the existence of a baryon asymmetry implies the existence of a Higgsino asymmetry. We investigate whether the Higgsino could be a viable asymmetric dark matter candidate. We find that this is indeed possible. Thus, supersymmetry can provide the observed dark matter abundance and, furthermore, relate it with the baryon asymmetry, in which case the puzzle of why the baryonic and dark matter mass densities are similar would be explained. To accomplish this task, two conditions are required. First, the gauginos, squarks, and sleptons must all be very heavy, such that the only electroweak-scale superpartners are the Higgsinos. With this spectrum, supersymmetry does not solve the fine-tuning problem. Second, the temperature of the electroweak phase transition must be low, in the (1-10) GeV range. This condition requires an extension of the minimal supersymmetric standard model. PMID:23006163

Blum, Kfir; Efrati, Aielet; Grossman, Yuval; Nir, Yosef; Riotto, Antonio

2012-08-01

346

Synthesis, optical resolution, and configurational assignment of novel axially chiral quateraryls.  

PubMed

A one-pot general synthesis of highly functionalized quateraryls through carbanion-induced, base-catalyzed ring transformation of 5,6-diaryl-2H-pyran-2-ones and core-substituted phenylacetones is delineated. These conversions were found to give diversely functionalized benzenes bearing peripheral aryl rings, some of which possess inherent atropisomerism. Exemplarily for one of the quateraryls, the optical resolution of the respective atropo-enantiomers by HPLC on a chiral phase and the assignment of their absolute axial configurations succeeded by LC-CD coupling in combination with semiempirical CNDO/S and TDDFT CD calculations. This synthetic approach offers-in a transition metal-free environment-high flexibility in the construction of quateraryls with the desired conformational freedom along the molecular axis, which may help in exploring and developing new potential ligands for asymmetric synthesis. PMID:17784773

Goel, Atul; Singh, Fateh V; Kumar, Vijay; Reichert, Matthias; Gulder, Tobias A M; Bringmann, Gerhard

2007-09-28

347

Mean-absolute deviation portfolio optimization for mortgage-backed securities  

Microsoft Academic Search

We develop an integrated simulation\\/optimization model for managing portfolios of mortgage-backed securities. The mortgage portfolio problem is viewed in the same spirit of models used for the management of portfolios of equities. That is, it trades off rates of return with a suitable measure of risk. In this respect we employ amean-absolute deviation model which is consistent with the asymmetric

Stavros A. Zenios; Pan Kang

1993-01-01

348

Familial aggregation of absolute pitch.  

PubMed

Absolute pitch (AP) is a behavioral trait that is defined as the ability to identify the pitch of tones in the absence of a reference pitch. AP is an ideal phenotype for investigation of gene and environment interactions in the development of complex human behaviors. Individuals who score exceptionally well on formalized auditory tests of pitch perception are designated as "AP-1." As described in this report, auditory testing of siblings of AP-1 probands and of a control sample indicates that AP-1 aggregates in families. The implications of this finding for the mapping of loci for AP-1 predisposition are discussed. PMID:10924408

Baharloo, S; Service, S K; Risch, N; Gitschier, J; Freimer, N B

2000-09-01

349

Absolute D hadronic branching fractions  

NASA Astrophysics Data System (ADS)

Using 281 pb-1 of e^+e^- collisions recorded at the ?(3770) resonance with the CLEO-c detector at CESR, we determine absolute hadronic branching fractions of charged and neutral D mesons using a double tag technique. Among measurements for three D^0 and six D^+ modes, we measure reference branching fractions B(D^0 ->K^- +circ) and B(D^+ ->K^- +circ+circ). Using a determination of the integrated luminosity, we also extract the cross sections ?(e^+e^- ->D^0 D^0) and ?(e^+e^- ->D^+D^-).

Shi, Xin

2007-04-01

350

Catalytic asymmetric nucleophilic openings of 3-substituted oxetanes.  

PubMed

Asymmetric ring-opening of 3-substituted oxetanes provides rapid access to highly functionalized chiral building blocks. However, progress in this field is limited. Recently we developed a new catalytic system based on chiral Brønsted acids for this type of reaction and demonstrated the synthesis of a range of useful molecules under mild and operationally simple conditions. In this perspective, we describe the challenges, progress, and potential future efforts on this topic. PMID:24968137

Wang, Zhaobin; Chen, Zhilong; Sun, Jianwei

2014-07-23

351

Chlorine, an atom economical auxiliary for asymmetric aldol reactions.  

PubMed

An auxiliary strategy has been developed for asymmetric reactions of aldehydes in which the auxiliary itself is not chiral, but a single chlorine atom introduced via organocatalytic ?-chlorination. The stereodirecting influence of the chlorine atom is then exploited prior to its removal by radical reduction. This strategy is demonstrated in the synthesis of several aldols (92-99% ee) and the natural products (+)-dihydroyashabushiketol and (+)-solistatin. PMID:23370443

Halperin, Shira D; Britton, Robert

2013-02-13

352

Pd-Catalyzed Asymmetric ?-Hydride Elimination En Route to Chiral Allenes  

PubMed Central

We wish to report our preliminary results on the discovery and development of a catalytic, asymmetric ?-hydride elimination from vinyl Pd(II)-complexes derived from enol triflates to access chiral allenes. To achieve this, we developed a class of chiral phosphite ligands that demonstrate high enantioselectivity, allow access of either allene enantiomer, and are readily synthesized. The methodology is demonstrated on over 20 substrates and application to the formal asymmetric total synthesis of the natural product, (+)-epibatidine, is also provided.

Crouch, Ian T.; Neff, Robynne K.; Frantz, Doug E.

2013-01-01

353

[Asymmetric syntheses of azacycloalkanes by one-pot [1+2+n] cyclization].  

PubMed

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by C- and N-alkylation of the resulting lithium 3-aminoenolate with ?, ?-dihaloalkane, which can be regarded as a [1+2+n] cyclization. The usefulness of the developed methodology was clearly demonstrated by the short-step asymmetric syntheses of azirinomycin ester, nemonapride, and kopsinine. PMID:23995805

Harada, Shingo

2013-01-01

354

Pd-catalyzed asymmetric ?-hydride elimination en route to chiral allenes.  

PubMed

We wish to report our preliminary results on the discovery and development of a catalytic, asymmetric ?-hydride elimination from vinyl Pd(II)-complexes derived from enol triflates to access chiral allenes. To achieve this, we developed a class of chiral phosphite ligands that demonstrate high enantioselectivity, allow access of either allene enantiomer, and are readily synthesized. The methodology is demonstrated on over 20 substrates, and application to the formal asymmetric total synthesis of the natural product, (+)-epibatidine, is also provided. PMID:23488914

Crouch, Ian T; Neff, Robynne K; Frantz, Doug E

2013-04-01

355

Transformations of asymmetric dimethylhydrazine in soils  

NASA Astrophysics Data System (ADS)

High-performance liquid chromatography and mass spectrometry were used to find that the decomposition of asymmetric dimethylhydrazine (I) in soils occurred with the formation of dimethylamine, formaldehyde dimethylhydrazone, methylhydrazine, trimethylhydrazine, N-nitrosodimethylamine, 1-methyl-1,2,4-triazole, formic acid dimethylhydrazide, 1,5,5-trimethylformazane, 1-methyl-1,6-dihydro-1,2,4,5-tetrazine, N,N-dimethylaminoguanidine, and several other products. High-reliability structure identification was achieved using independent methods, including gas chromatography-mass spectrometry, 1H and 13C NMR, and UV spectroscopy, and by measuring retention times and spectral characteristics after the counter-synthesis of the suggested structures. The products of the decomposition of I potentially capable of forming initial I and characterized by high migration mobility in soils were identified.

Rodin, I. A.; Moskvin, D. N.; Smolenkov, A. D.; Shpigun, O. A.

2008-06-01

356

Asymmetric galaxy correlation functions  

NASA Astrophysics Data System (ADS)

We study the two-point cross-correlation function between two populations of galaxies: for instance, a bright population and a faint population. We show that this cross-correlation is asymmetric under the exchange of the line-of-sight coordinate of the galaxies, i.e. that the correlation is different if the bright galaxy is in front of, or behind, the faint galaxy. We give an intuitive, quasi-Newtonian derivation of all the effects that contribute to such an asymmetry in large-scale structure: gravitational redshift, Doppler shift, lensing, light-cone, evolution and Alcock-Paczynski effects; interestingly, the gravitational redshift term is exactly canceled by some of the others, assuming geodesic motion. Most of these effects are captured by previous calculations of general relativistic corrections to the observed galaxy density fluctuation; the asymmetry arises from terms that are suppressed by the ratio (H /k)—H is the Hubble constant and k is the wave number—which are more readily observable than the terms suppressed by (H /k)2. Some of the contributions to the asymmetry, however, arise from terms that are generally considered "Newtonian"—the lensing and evolution—and thus represent a contaminant in the search for general relativistic corrections. We propose methods to disentangle these different contributions. A simple method reduces the contamination to a level of ?10% for redshifts z ?1. We also clarify the relation to recent work on measuring gravitational redshifts by stacking clusters.

Bonvin, Camille; Hui, Lam; Gaztañaga, Enrique

2014-04-01

357

Error analysis of a new asymmetric Abel-inversion method  

SciTech Connect

Propagation of the various errors in a new asymmetric Abel-inversion method is investigated. In order to provide a meaningful error analysis of the inversion process, both the effects of geometrical factors and measuremental errors are studied. The dominant source of uncertainty in the determination of local electron density is due to the spline fitting of the interferometric measurements with uneven spacings. The propagation of these errors is highly spatially localized and heavily damped in this inversion method. The absolute range of error in the local electron density is {plus minus}1.5 {times} 10{sup 12} cm{sup {minus}3}. 4 refs., 3 figs.

Park, Hyeon K.

1990-06-01

358

A vector asymmetrical NNV equation  

NASA Astrophysics Data System (ADS)

A vector asymmetrical Nizhnik-Novikov-Veselov (NNV) equation is proposed based on its bilinear form. Soliton solutions expressed by Pfaffians are obtained. Bilinear Bäcklund transformation and the corresponding Lax pair for the vector ANNV equation are derived.

Yu, Guo-Fu; Tam, Hon-Wah

2008-08-01

359

Submillimeter spectroscopy for chemical analysis with absolute specificity.  

PubMed

A sensor based on rotational signatures in the submillimeter (SMM) region is described. This sensor uses frequency synthesis techniques in the region around 10 GHz, with nonlinear diode frequency multiplication to 210-270 GHz. This provides not only a nearly ideal instrument function, but also frequency control and agility that significantly enhance the performance of the spectrometer as a sensor. The SMM frequencies provide significantly stronger absorptions and broader spectroscopic coverage than lower-frequency microwave systems. Among the characteristics of the sensor are absolute specificity, low atmospheric clutter, good sensitivity, and near-term paths to systems that are both compact and very inexpensive. PMID:20479799

Medvedev, Ivan R; Neese, Christopher F; Plummer, Grant M; De Lucia, Frank C

2010-05-15

360

Synthesis and biodistribution of new oxo and nitrido 99mTc complexes with asymmetrical potentially dianionic or trianionic tetradentate SNNO ligands derived from methyl-2-aminocyclopentene-1-dithiocarboxylic acid.  

PubMed

In this work, 10 new asymmetrical tetradentate SNNO ligands were prepared by reaction of the amine function of methyl 2-[(beta-aminoethyl)amino]cyclopentene-1-dithiocarboxylate with various bifunctional substituents bearing hydroxyl/ketone and hydroxyl/aldehyde functional groups and with diethyl oxalate. 99mTc labeling efficiency was optimized by adjusting temperature and pH conditions. Seven nitrido and two oxo 99mTc complexes were isolated. Six of them proved to be stable near physiological conditions. Biodistribution studies in the rat showed a significant heart uptake for four of them and strong kidney and liver uptake for the other two. PMID:9466364

Belhadj-Tahar, H; Ouhayoun, E; Cros, G; Darbieu, M H; Tafani, J A; Fabre, J; Esquerre, J P; Coulais, Y

1998-01-01

361

Synthesis, characterization and antimicrobial activity of cobalt(II), nickel(II)and copper(II) complexes with new asymmetrical Schiff base ligands derived from 7-formyanil-substituted diamine-sulphoxine and acetylacetone  

Microsoft Academic Search

Asymmetric 7-formyanil-substituted-imino-4-(4-methyl-2-butanone)-8-hydroxyquinoline-5-sulphonic acid (Schiff bases), react with CoII, NiII and CuII ions to give 1:2, 1:1 and 2:1 complexes as established by conductometric titrations in 1:1 DMF:H2O. The complexes were investigated by elemental analyses, molecular weight determinations, molar conductance, magnetic moments, thermal analysis, i.r., u.v.–vis. and e.s.r. spectra. The complexes have an octahedral crystal structure and general formula [ML·(OH2)2], where

Wageih G. Hanna; Mona M. Moawad

2001-01-01

362

A Stereoselective Synthesis of (+)-Herboxidiene/GEX1A  

PubMed Central

A stereoselective synthesis of (+)-herboxidiene is described. The convergent synthesis utilized a Suzuki cross-coupling reaction to assemble the key segments. The synthesis of the functionalized tetrahydropyran ring utilized an Achmatowicz reaction as the key step. The synthesis of the C10-C19 segment was accomplished using Brown's crotylboration, asymmetric alkylation, and a stereoselective allylic chlorination reaction.

Ghosh, Arun K.; Li, Jianfeng

2010-01-01

363

A stereoselective synthesis of (+)-herboxidiene/GEX1A.  

PubMed

A stereoselective synthesis of (+)-herboxidiene is described. The convergent synthesis utilized a Suzuki cross-coupling reaction to assemble the key segments. The synthesis of the functionalized tetrahydropyran ring utilized an Achmatowicz reaction as the key step. The synthesis of the C10-C19 segment was accomplished using Brown's crotylboration, asymmetric alkylation, and a stereoselective allylic chlorination reactions. PMID:21126066

Ghosh, Arun K; Li, Jianfeng

2011-01-01

364

Enzymatic desymmetrization of 2-amino-2-methyl-1,3-propanediol: asymmetric synthesis of ( S)- N-Boc- N, O-isopropylidene-?-methylserinal and (4 R)-methyl-4-[2-(thiophen-2-yl)ethyl]oxazolidin-2-one  

Microsoft Academic Search

We report herein, the novel enzymatic desymmetrization of 2-tert-butoxycarbonylamino-2-methyl-1,3-propanediol 1. This method makes it possible to prepare (S)-N-Boc-N,O-isopropylidene-?-methylserinal 3, which is a chiral building block for the synthesis of a variety of ?-substituted alanine derivatives. Moreover, optically active (4R)-methyl-4-[2-(thiophen-2-yl)ethyl]oxazolidin-2-one 4, one of the key intermediates in the synthesis of a novel immunosuppressant, has been prepared by this methodology.

Takashi Tsuji; Yukiko Iio; Toshiyasu Takemoto; Takahide Nishi

2005-01-01

365

ASYMMETRIC SOLAR WIND ELECTRON DISTRIBUTIONS  

SciTech Connect

The present paper provides a possible explanation for the solar wind electron velocity distribution functions possessing asymmetric energetic tails. By numerically solving the electrostatic weak turbulence equations that involve nonlinear interactions among electrons, Langmuir waves, and ion-sound waves, it is shown that different ratios of ion-to-electron temperatures lead to the generation of varying degrees of asymmetric tails. The present finding may be applicable to observations in the solar wind near 1 AU and in other regions of the heliosphere and interplanetary space.

Yoon, Peter H.; Kim, Sunjung; Lee, Junggi; Lee, Junhyun; Park, Jongsun; Park, Kyungsun; Seough, Jungjoon [School of Space Research, Kyung Hee University, Yongin-Si, Gyeonggi-Do 446-701 (Korea, Republic of); Hong, Jinhy [Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701 (Korea, Republic of)

2012-08-20

366

Absolute interferometric test of Fresnel zone plates  

Microsoft Academic Search

An absolute interferometric test of two-level binary Fresnel zone plates (FZPs) is presented. Five measurements with a wavefront testing interferometer are required to fully separate interferometer errors from those of the FZP. The method provides both errors, pattern errors and surface figure errors of the zone plate absolutely. The test method is suitable for zone mirrors and zone lenses. Test

S. Reichelt; R. Freimann; H. J. Tiziani

2001-01-01

367

Absolute instability of the Gaussian wake profile  

NASA Technical Reports Server (NTRS)

Linear parallel-flow stability theory has been used to investigate the effect of viscosity on the local absolute instability of a family of wake profiles with a Gaussian velocity distribution. The type of local instability, i.e., convective or absolute, is determined by the location of a branch-point singularity with zero group velocity of the complex dispersion relation for the instability waves. The effects of viscosity were found to be weak for values of the wake Reynolds number, based on the center-line velocity defect and the wake half-width, larger than about 400. Absolute instability occurs only for sufficiently large values of the center-line wake defect. The critical value of this parameter increases with decreasing wake Reynolds number, thereby indicating a shrinking region of absolute instability with decreasing wake Reynolds number. If backflow is not allowed, absolute instability does not occur for wake Reynolds numbers smaller than about 38.

Hultgren, Lennart S.; Aggarwal, Arun K.

1987-01-01

368

Asymmetric perturbations of signalling oligomers.  

PubMed

This review focuses on rapid and reversible noncovalent interactions for symmetric oligomers of signalling proteins. Symmetry mismatch, transient symmetry breaking and asymmetric perturbations via chemical (ligand binding) and physical (electric or mechanic) effects can initiate the signalling events. Advanced biophysical methods can reveal not only structural symmetries of stable membrane-bound signalling proteins but also asymmetric functional transition states. Relevant techniques amenable to distinguish between symmetric and asymmetric architectures are discussed including those with the capability of capturing low-populated transient conformational states. Typical examples of signalling proteins are overviewed for symmetry breaking in dimers (GPCRs, growth factor receptors, transcription factors); trimers (acid-sensing ion channels); tetramers (voltage-gated cation channels, ionotropic glutamate receptor, CNG and CHN channels); pentameric ligand-gated and mechanosensitive channels; higher order oligomers (gap junction channel, chaperonins, proteasome, virus capsid); as well as primary and secondary transporters. In conclusion, asymmetric perturbations seem to play important functional roles in a broad range of communicating networks. PMID:24650570

Maksay, Gábor; T?ke, Orsolya

2014-05-01

369

Complete replication of poliovirus in vitro: preinitiation RNA replication complexes require soluble cellular factors for the synthesis of VPg-linked RNA.  

PubMed Central

Translation of poliovirion RNA in HeLa S10 extracts resulted in the formation of RNA replication complexes which catalyzed the asymmetric replication of poliovirus RNA. Synthesis of poliovirus RNA was detected in unfractionated HeLa S10 translation reactions and in RNA replication complexes isolated from HeLa S10 translation reactions by pulse-labeling with [32P]CTP. The RNA replication complexes formed in vitro contained replicative-intermediate RNA and were enriched in viral protein 3CD and the membrane-associated viral proteins 2C, 2BC, and 3AB. Genome-length poliovirus RNA covalently linked to VPg was synthesized in large amounts by the replication complexes. RNA replication was highly asymmetric, with predominantly positive-polarity RNA products. Both anti-VPg antibody and guanidine HCl inhibited RNA replication and virus formation in the HeLa S10 translation reactions without affecting viral protein synthesis. The inhibition of RNA synthesis by guanidine was reversible. The reversible nature of guanidine inhibition was used to demonstrate the formation of preinitiation RNA replication complexes in reaction mixes containing 2 mM guanidine HCl. Preinitiation complexes sedimented upon centrifugation at 15,000 x g and initiated RNA replication upon their resuspension in reaction mixes lacking guanidine. Initiation of RNA synthesis by preinitiation complexes did not require active protein synthesis or the addition of soluble viral proteins. Initiation of RNA synthesis by preinitiation complexes, however, was absolutely dependent on soluble HeLa cytoplasmic factors. Preinitiation complexes also catalyzed the formation of infectious virus in reaction mixes containing exogenously added capsid proteins. The titer of infectious virus produced in such trans-encapsidation reactions reached 4 x 10(7) PFU/ml. The HeLa S10 translation-RNA replication reactions represent an efficient in vitro system for authentic poliovirus replication, including protein synthesis, polyprotein processing, RNA replication, and virus assembly.

Barton, D J; Black, E P; Flanegan, J B

1995-01-01

370

Perfluoroalkanesulfonamide organocatalysts for asymmetric conjugate additions of branched aldehydes to vinyl sulfones.  

PubMed

Asymmetric conjugate additions of branched aldehydes to vinyl sulfones promoted by sulfonamide organocatalyst 6 or 7 have been developed, allowing facile synthesis of the corresponding adducts with all-carbon quaternary stereocenters in excellent yields with up to 95% ee. PMID:24287989

Nakashima, Kosuke; Murahashi, Miho; Yuasa, Hiroki; Ina, Mariko; Tada, Norihiro; Itoh, Akichika; Hirashima, Shin-ichi; Koseki, Yuji; Miura, Tsuyoshi

2013-01-01

371

Organocatalytic asymmetric decarboxylative cyanomethylation of isatins using l-proline derived bifunctional thiourea.  

PubMed

First asymmetric decarboxylative cyanomethylation of isatins is reported herewith using bifunctional thiourea derived from l-proline in good yields and enantioselectivities. This strategy enables the construction of various 3-cyanomethylene substituted 3-hydroxyoxindoles in enantioselective manner. Enantioselective synthesis of CPC-1 alkaloid has been accomplished in fewer steps. PMID:24824863

Pratap Reddy Gajulapalli, V; Vinayagam, Poopathy; Kesavan, Venkitasamy

2014-05-28

372

Studies of stereocontrolled allylation reactions for the total synthesis of phorboxazole A  

PubMed Central

A highly convergent total synthesis of the potent anticancer agent (+)-phorboxazole A (1) is accomplished. Four components (3–6) are assembled with considerations for control of absolute and relative stereochemistry. Iterative asymmetric allylation methodology addresses key stereochemical features in the preparation of the 2,6-cis- and 2,6-trans-tetrahydropyranyl rings of the C3–C19 component (3). The stereocontrolled asymmetric allylation process is also used for development of the C28–C41 fragment (4). Novel Barbier coupling reactions of ?-iodomethyl oxazoles and related thiazoles are described with samarium iodide. The convergent assembly of components 4 and 5 features formation of the fully substituted C22–C26 pyran by intramolecular capture of an allyl cation intermediate with high facial selectivity, and further efforts lead to E-C19/C20 olefination. The synthesis culminates with use of a modified Julia olefination for attachment of the C42–C46 segment and subsequent late-stage macrocyclization by installation of the (Z)-C2/C3 ?,?-unsaturated lactone.

Williams, David R.; Kiryanov, Andre A.; Emde, Ulrich; Clark, Michael P.; Berliner, Martin A.; Reeves, Jonathan T.

2004-01-01

373

Asymmetric Organocatalysis at the Service of Medicinal Chemistry  

PubMed Central

The application of the most representative and up-to-date examples of homogeneous asymmetric organocatalysis to the synthesis of molecules of interest in medicinal chemistry is reported. The use of different types of organocatalysts operative via noncovalent and covalent interactions is critically reviewed and the possibility of running some of these reactions on large or industrial scale is described. A comparison between the organo- and metal-catalysed methodologies is offered in several cases, thus highlighting the merits and drawbacks of these two complementary approaches to the obtainment of very popular on market drugs or of related key scaffolds.

2014-01-01

374

Recent progress in asymmetric fluorination and trifluoromethylation reactions.  

PubMed

All domains of society are impacted by fluorine chemistry. In particular, fluorine plays a key role in medicinal, pharmaceutical and agrochemical sciences in which the synthesis and evaluation of molecules featuring one or more fluorine atoms is nowadays routine in every new discovery and development program. Since the beginning of this century, enantiopure fluorinated molecules are at the forefront of innovation of the tremendous achievements in fluorine chemistry. This review introduces the reader to the recent progress in asymmetric installation of one fluorine and a trifluoromethyl group via nucleophilic, electrophilic and radical diastereo- and enantioselective reactions. PMID:24484424

Bizet, Vincent; Besset, Tatiana; Ma, Jun-An; Cahard, Dominique

2014-01-01

375

Asymmetric vinylogous Mannich reactions: a versatile approach to functionalized heterocycles.  

PubMed

Asymmetric vinylogous Mannich reaction (VMR) of 2-(tert-butyldimethylsilyloxy)furan (TBSOF, 1) with (R(S))- or (S(S))-t-BS-imines (3) furnished 5-aminoalkylbutenolides 7a-k in 75-87% yields with anti/syn ratios ranging from 75:25 to 97:3. Butenolides 7a-f,k were readily converted into substituted lactones 8 and 5 and 6-substituted 5-hydroxypiperidin-2-ones 11a-g, which are, in turn, key intermediates for the synthesis of many bioactive compounds. PMID:21854016

Ruan, Shu-Tang; Luo, Jie-Min; Du, Yu; Huang, Pei-Qiang

2011-09-16

376

New 'side-off' coordination asymmetric homobinuclear Ni(II) and heterobinuclear Ni(II)Zn(II) complexes as models for hydrolysis of p-nitrophenylphosphate: synthesis, characterization and electrochemical studies.  

PubMed

A series of new phenol based coordination asymmetric side-off homobinuclear Ni(II) and heterobinuclear Ni(II)Zn(II) complexes have been synthesized and characterized by elemental analysis and spectral analysis. The electronic spectra of all the complexes show "Red shift" in LMCT band, for the ligand H(2)L(2) having electron donating C(2)H(5) groups as N-substituents in the free side arms compared to ligand H(2)L(1) having relatively lower electron donating CH(3) groups. Electrochemical studies of the complexes reveal that all the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an "anodic shift" in the first reduction potential of the complexes of the ligand H(2)L(1) when compared to that of the complexes of the ligand H(2)L(2) due to the presence of higher electron donating C(2)H(5) N-substituents in the later case than the relatively lower electron donating N-substituents in the former case. Hydrolysis of p-nitrophenylphosphate using homobinuclear Ni(II) and heterobinuclear Ni(II)-Zn(II) complexes as catalysts showed that the homobinuclear complexes have higher rate constant values than that of the corresponding heterobinuclear complexes. A comparison of the spectral, electrochemical and catalytic behavior of the complexes of the ligands is discussed on the basis of the electron donating nature of the N-substituents at the free side arms. PMID:23314389

Shanmuga Bharathi, K; Sreedaran, S; Kalilur Rahiman, A; Narayanan, Vengidusamy

2013-03-15

377

New 'side-off' coordination asymmetric homobinuclear Ni(II) and heterobinuclear Ni(II)sbnd Zn(II) complexes as models for hydrolysis of p-nitrophenylphosphate: Synthesis, characterization and electrochemical studies  

NASA Astrophysics Data System (ADS)

A series of new phenol based coordination asymmetric side-off homobinuclear Ni(II) and heterobinuclear Ni(II)sbnd Zn(II) complexes have been synthesized and characterized by elemental analysis and spectral analysis. The electronic spectra of all the complexes show "Red shift" in LMCT band, for the ligand H2L2 having electron donating C2H5 groups as N-substituents in the free side arms compared to ligand H2L1 having relatively lower electron donating CH3 groups. Electrochemical studies of the complexes reveal that all the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an "anodic shift" in the first reduction potential of the complexes of the ligand H2L1 when compared to that of the complexes of the ligand H2L2 due to the presence of higher electron donating C2H5 N-substituents in the later case than the relatively lower electron donating N-substituents in the former case. Hydrolysis of p-nitrophenylphosphate using homobinuclear Ni(II) and heterobinuclear Ni(II)sbnd Zn(II) complexes as catalysts showed that the homobinuclear complexes have higher rate constant values than that of the corresponding heterobinuclear complexes. A comparison of the spectral, electrochemical and catalytic behavior of the complexes of the ligands is discussed on the basis of the electron donating nature of the N-substituents at the free side arms.

Shanmuga Bharathi, K.; Sreedaran, S.; Kalilur Rahiman, A.; Narayanan, Vengidusamy

2013-03-01

378

Y2MoSe3O12 and Y2MoTe3O12: Solid-state synthesis, structure determination, and characterization of two new quaternary mixed metal oxides containing asymmetric coordination environment  

NASA Astrophysics Data System (ADS)

Two new quaternary yttrium molybdenum selenium/tellurium oxides, Y2MoSe3O12 and Y2MoTe3O12 have been prepared by standard solid-state reactions using Y2O3, MoO3, and SeO2 (or TeO2) as reagents. Single-crystal X-ray diffraction was used to determine the crystal structures of the reported materials. Although both of the materials contain second-order Jahn-Teller (SOJT) distortive cations and are stoichiometrically similar, they reveal different structural features: while Y2MoSe3O12 shows a three-dimensional framework consisting of YO8, MoO6, and SeO3 groups, Y2MoTe3O12 exhibits a layered structure composed of YO8, MoO4, TeO3, and TeO4 polyhedra. With the Mo6+ cations in Y2MoSe3O12, a C3-type intraoctahedral distortion toward a face is observed, in which the direction of the out-of-center distortion for Mo6+ is away from the oxide ligand linked to a Se4+ cation. The Se4+ and Te4+ cations in both materials are in asymmetric coordination environment attributed to the lone pairs. Elemental analyses, infrared spectroscopy, thermal analyses, intraoctahedral distortions, and dipole moment calculations for the compounds are also presented.

Bang, Seong-eun; Pan, Zhi; Kim, Yeong Hun; Lee, Dong Woo; Ok, Kang Min

2013-12-01

379

The Simplicity Argument and Absolute Morality  

ERIC Educational Resources Information Center

In this paper the author has maintained that there is a similarity of thought to be found in the writings of Cudworth, Emerson, and Husserl in his investigation of an absolute system of morality. (Author/RK)

Mijuskovic, Ben

1975-01-01

380

The Parabola Test for Absolute Stability.  

National Technical Information Service (NTIS)

In applying the Popov stability test, a certain straight line is drawn; the Popov locus must lie on one side of this line. Thus geometric considerations alone indicate that the interesting sectors of absolute stability for conditionally stable systems can...

A. R. Bergen M. A. Sapiro

1967-01-01

381

A New Gimmick for Assigning Absolute Configuration.  

ERIC Educational Resources Information Center

A five-step procedure is provided to help students in making the assignment absolute configuration less bothersome. Examples for both single (2-butanol) and multi-chiral carbon (3-chloro-2-butanol) molecules are included. (JN)

Ayorinde, F. O.

1983-01-01

382

Magnifying absolute instruments for optically homogeneous regions  

SciTech Connect

We propose a class of magnifying absolute optical instruments with a positive isotropic refractive index. They create magnified stigmatic images, either virtual or real, of optically homogeneous three-dimensional spatial regions within geometrical optics.

Tyc, Tomas [Institute of Theoretical Physics and Astrophysics, Masaryk University, Kotlarska 2, CZ-61 137 Brno (Czech Republic)

2011-09-15

383

A note on exponential absolute stability.  

NASA Technical Reports Server (NTRS)

A new sufficient condition is formulated for the Lur'e type nonlinear continuous system to be exponentially absolutely stable. The condition relaxes the assumptions on the nonlinear characteristic by modifying the requirements on the linear part of the system.

Siljak, D. D.; Sun, C. K.

1972-01-01

384

A Method of Measuring Low Absolute Pressures.  

National Technical Information Service (NTIS)

The method of measuring low absolute pressures by using a single-capillary compression manometer differs in that, to increase accuracy and exclude random errors, mercury flows to the measuring capillary entrance and gradually rises in the capillary withou...

A. V. Eryukhin

1964-01-01

385

Novel optoelectronic systems for use in absolute position sensing applications  

NASA Astrophysics Data System (ADS)

This thesis describes the development of two novel optoelectronic systems which have been designed for use in absolute position sensing applications. Chapter 1 provides an introduction to the work and reviews the wide variety of optical techniques which are presently available for position measurement. The second chapter provides a synopsis of a new type of optical position sensor designed for medium range sensing applications. This novel system uses optical feedback to a semiconductor laser to measure position. Unlike other proposed feedback sensing systems, this arrangement monitors the frequencies of RF beat modes generated within the laser diode to produce a position measurement capable of micron scale accuracy. The system operates using low optical powers and may be configured to operate as a non-contact, contact or fibre-addressed sensor. Chapter 4 examines the stability of the sensor. In particular, it assesses how the sensor is affected by variations in key operating parameters, such as bias current and feedback strength. Meanwhile, chapter 5 describes a number of signal processing systems which may be used to interface with the RF signal generated by the optical system to provide a real-time output of a suitable form. In addition to examining the experimental operation of the sensor, a thorough theoretical analysis of the operation of the system is presented in chapters 3 and 6. Both steady-state and dynamic approaches are used to analyse the behaviour of the optical system. These allow accurate predictions of the sensor performance to be generated, both for general operation and for stability analysis. Finally chapter 7 describes a separate novel optical position sensor, which provides absolute position measurement with accuracies of 0.004% of measurement range. This sensor utilises reflection from a novel dual track grating, which consists of a pseudo-random binary sequence written in parallel with a periodic asymmetric pattern.

Griffiths, Roger Phillip

386

A Cryogenic Radiometer for Absolute Radiometric Measurements  

Microsoft Academic Search

The adoption in 1979 of a new definition of the candela, which permitted a detector-based approach to the realization of the unit, has emphasized the importance of high-accuracy absolute radiation detectors. This paper describes a new electrical-substitution absolute radiometer operating at 5 K, based on a standard commercial helium cryostat, which has been developed at NPL for optical radiant-power measurements.

J E Martin; N P Fox; P J Key

1985-01-01

387

The absolute magnitudes of Type IA supernovae  

Microsoft Academic Search

Absolute magnitudes in the B, V, and I bands are derived for nine well-observed Type Ia supernovae, using host galaxy distances estimated via the surface brightness fluctuations or Tully-Fisher methods. These data indicate that there is a significant intrinsic dispersion in the absolute magnitudes at maximum light of Type Ia supernovae, amounting to +\\/- 0.8 mag in B, +\\/- 0.6

M. M. Phillips

1993-01-01

388

Asymmetric desymmetrization of 2,2',6,6'-tetrahydroxybiphenyl through annulation with enantiomerically pure bis(mesylate)  

PubMed

[formula: see text] 2,2',6,6'-Tetrahydroxybiphenyl undergoes a facile annulation reaction with bis(mesylate) derived from (S)-1,2-propanediol in the presence of Cs2CO3 to give the corresponding asymmetric desymmetrization product of S axial chirality with exclusive diastereoselectivity. The desymmetrization product can be utilized as a versatile chiral building block in asymmetric synthesis of axially chiral 6,6'-disubstituted 2,2'-biphenyldiols. PMID:10810737

Harada; Tuyet; Oku

2000-05-01

389

Mechanochemical organic synthesis.  

PubMed

Recently, mechanical milling using a mixer mill or planetary mill has been fruitfully utilized in organic synthesis under solvent-free conditions. This review article provides a comprehensive overview of various solvent-free mechanochemical organic reactions, including metal-mediated or -catalyzed reactions, condensation reactions, nucleophilic additions, cascade reactions, Diels-Alder reactions, oxidations, reductions, halogenation/aminohalogenation, etc. The ball milling technique has also been applied to the synthesis of calixarenes, rotaxanes and cage compounds, asymmetric synthesis as well as the transformation of biologically active compounds. PMID:23660585

Wang, Guan-Wu

2013-09-21

390

Measurement of the absolute branching fraction of D0-->K-pi+.  

PubMed

We measure the absolute branching fraction for D(0)-->K(-)pi(+) using partial reconstruction of B(0)-->D(*+)Xl(-)nu(l) decays, in which only the charged lepton and the pion from the decay D(*+)-->D(0)pi(+) are used. Based on a data sample of 230 x 10(6) BB pairs collected at the Upsilon(4S) resonance with the BABAR detector at the PEP-II asymmetric-energy B factory at SLAC, we obtain B(D(0)-->K(-)pi(+)) = (4.007+/-0.037+/-0.072)%, where the first uncertainty is statistical and the second is systematic. PMID:18352359

Aubert, B; Bona, M; Boutigny, D; Karyotakis, Y; Lees, J P; Poireau, V; Prudent, X; Tisserand, V; Zghiche, A; Garra Tico, J; Grauges, E; Lopez, L; Palano, A; Eigen, G; Ofte, I; Stugu, B; Sun, L; Abrams, G S; Battaglia, M; Brown, D N; Button-Shafer, J; Cahn, R N; Groysman, Y; Jacobsen, R G; Kadyk, J A; Kerth, L T; Kolomensky, Yu G; Kukartsev, G; Lopes Pegna, D; Lynch, G; Mir, L M; Orimoto, T J; Pripstein, M; Roe, N A; Ronan, M T; Tackmann, K; Wenzel, W A; Del Amo Sanchez, P; Hawkes, C M; Watson, A T; Held, T; Koch, H; Lewandowski, B; Pelizaeus, M; Schroeder, T; Steinke, M; Cottingham, W N; Walker, D; Asgeirsson, D J; Cuhadar-Donszelmann, T; Fulsom, B G; Hearty, C; Knecht, N S; Mattison, T S; McKenna, J A; Khan, A; Saleem, M; Teodorescu, L; Blinov, V E; Bukin, A D; Druzhinin, V P; Golubev, V B; Onuchin, A P; Serednyakov, S I; Skovpen, Yu I; Solodov, E P; Todyshev, K Yu; Bondioli, M; Curry, S; Eschrich, I; Kirkby, D; Lankford, A J; Lund, P; Mandelkern, M; Martin, E C; Stoker, D P; Abachi, S; Buchanan, C; Foulkes, S D; Gary, J W; Liu, F; Long, O; Shen, B C; Zhang, L; Paar, H P; Rahatlou, S; Sharma, V; Berryhill, J W; Campagnari, C; Cunha, A; Dahmes, B; Hong, T M; Kovalskyi, D; Richman, J D; Beck, T W; Eisner, A M; Flacco, C J; Heusch, C A; Kroseberg, J; Lockman, W S; Schalk, T; Schumm, B A; Seiden, A; Williams, D C; Wilson, M G; Winstrom, L O; Chen, E; Cheng, C H; Dvoretskii, A; Fang, F; Hitlin, D G; Narsky, I; Piatenko, T; Porter, F C; Mancinelli, G; Meadows, B T; Mishra, K; Sokoloff, M D; Blanc, F; Bloom, P C; Chen, S; Ford, W T; Hirschauer, J F; Kreisel, A; Nagel, M; Nauenberg, U; Olivas, A; Smith, J G; Ulmer, K A; Wagner, S R; Zhang, J; Gabareen, A M; Soffer, A; Toki, W H; Wilson, R J; Winklmeier, F; Zeng, Q; Altenburg, D D; Feltresi, E; Hauke, A; Jasper, H; Merkel, J; Petzold, A; Spaan, B; Wacker, K; Brandt, T; Klose, V; Lacker, H M; Mader, W F; Nogowski, R; Schubert, J; Schubert, K R; Schwierz, R; Sundermann, J E; Volk, A; Bernard, D; Bonneaud, G R; Latour, E; Lombardo, V; Thiebaux, Ch; Verderi, M; Clark, P J; Gradl, W; Muheim, F; Playfer, S; Robertson, A I; Xie, Y; Andreotti, M; Bettoni, D; Bozzi, C; Calabrese, R; Cecchi, A; Cibinetto, G; Franchini, P; Luppi, E; Negrini, M; Petrella, A; Piemontese, L; Prencipe, E; Santoro, V; Anulli, F; Baldini-Ferroli, R; Calcaterra, A; de Sangro, R; Finocchiaro, G; Pacetti, S; Patteri, P; Peruzzi, I M; Piccolo, M; Rama, M; Zallo, A; Buzzo, A; Contri, R; Lo Vetere, M; Macri, M M; Monge, M R; Passaggio, S; Patrignani, C; Robutti, E; Santroni, A; Tosi, S; Chaisanguanthum, K S; Morii, M; Wu, J; Dubitzky, R S; Marks, J; Schenk, S; Uwer, U; Bard, D J; Dauncey, P D; Flack, R L; Nash, J A; Nikolich, M B; Panduro Vazquez, W; Behera, P K; Chai, X; Charles, M J; Mallik, U; Meyer, N T; Ziegler, V; Cochran, J; Crawley, H B; Dong, L; Eyges, V; Meyer, W T; Prell, S; Rosenberg, E I; Rubin, A E; Gritsan, A V; Guo, Z J; Lae, C K; Denig, A G; Fritsch, M; Schott, G; Arnaud, N; Béquilleux, J; Davier, M; Grosdidier, G; Höcker, A; Lepeltier, V; Le Diberder, F; Lutz, A M; Pruvot, S; Rodier, S; Roudeau, P; Schune, M H; Serrano, J; Sordini, V; Stocchi, A; Wang, W F; Wormser, G; Lange, D J; Wright, D M; Chavez, C A; Forster, I J; Fry, J R; Gabathuler, E; Gamet, R; Hutchcroft, D E; Payne, D J; Schofield, K C; Touramanis, C; Bevan, A J; George, K A; Di Lodovico, F; Menges, W; Sacco, R; Cowan, G; Flaecher, H U; Hopkins, D A; Jackson, P S; McMahon, T R; Salvatore, F; Wren, A C; Brown, D N; Davis, C L; Allison, J; Barlow, N R; Barlow, R J; Chia, Y M; Edgar, C L; Lafferty, G D; West, T J; Yi, J I; Anderson, J; Chen, C; Jawahery, A; Roberts, D A; Simi, G; Tuggle, J M; Blaylock, G; Dallapiccola, C; Hertzbach, S S; Li, X; Moore, T B; Salvati, E; Saremi, S; Cowan, R; Fisher, P H; Sciolla, G; Sekula, S J; Spitznagel, M; Taylor, F; Yamamoto, R K; McLachlin, S E; Patel, P M; Robertson, S H; Lazzaro, A; Palombo, F; Bauer, J M; Cremaldi, L; Eschenburg, V; Godang, R; Kroeger, R; Sanders, D A; Summers, D J; Zhao, H W; Brunet, S; Côté, D; Simard, M; Taras, P; Viaud, F B; Nicholson, H; De Nardo, G; Fabozzi, F; Lista, L; Monorchio, D; Sciacca, C; Baak, M A; Raven, G; Snoek, H L; Jessop, C P; Losecco, J M; Benelli, G; Corwin, L A; Gan, K K; Honscheid, K; Hufnagel, D; Kagan, H; Kass, R; Morris, J P; Rahimi, A M; Regensburger, J J; Ter-Antonyan, R; Wong, Q K; Blount, N L; Brau, J; Frey, R; Igonkina, O; Kolb, J A; Lu, M; Rahmat, R; Sinev, N B; Strom, D; Strube, J; Torrence, E; Gagliardi, N; Gaz, A; Margoni, M; Morandin, M; Pompili, A; Posocco, M; Rotondo, M; Simonetto, F; Stroili, R; Voci, C; Ben-Haim, E; Briand, H; Chauveau, J; David, P; Del Buono, L; de la Vaissière, Ch; Hamon, O; Hartfiel, B L; Leruste, Ph; Malclès, J; Ocariz, J; Perez, A; Gladney, L; Biasini, M; Covarelli, R; Manoni, E; Angelini, C; Batignani, G; Bettarini, S; Calderini, G; Carpinelli, M; Cenci, R; Cervelli, A; Forti, F

2008-02-01

391

First total synthesis of (+)-indicanone.  

PubMed

The first total synthesis of the guaiane-type sesquiterpene, (+)-indicanone (1), isolated from the root of Wikstroemia indica, was accomplished based on the rhodium(I)-catalyzed Pauson-Khand-type reaction of the allenyne derivative, which was derived from (+)-limonene. This total synthesis unambiguously confirmed the complete structure of (+)-indicanone involving its absolute stereochemistry. PMID:22588646

Hayashi, Yujiro; Ogawa, Kumiko; Inagaki, Fuyuhiko; Mukai, Chisato

2012-06-28

392

Spin-Asymmetric Josephson Effect  

NASA Astrophysics Data System (ADS)

We propose that with ultracold Fermi gases one can realize a spin-asymmetric Josephson effect in which the two spin components of a Cooper pair are driven asymmetrically—corresponding to driving a Josephson junction of two superconductors with different voltages V? and V? for spin up and down electrons, respectively. We predict that the spin up and down components oscillate at the same frequency but with different amplitudes. Furthermore our results reveal that the standard interpretation of the Josephson supercurrent in terms of coherent bosonic pair tunneling is insufficient. We provide an intuitive interpretation of the Josephson supercurrent as interference in Rabi oscillations of pairs and single particles, the latter causing the asymmetry.

Heikkinen, M. O. J.; Massel, F.; Kajala, J.; Leskinen, M. J.; Paraoanu, G. S.; Törmä, P.

2010-11-01

393

Spin-asymmetric Josephson effect.  

PubMed

We propose that with ultracold Fermi gases one can realize a spin-asymmetric Josephson effect in which the two spin components of a Cooper pair are driven asymmetrically--corresponding to driving a Josephson junction of two superconductors with different voltages V(?) and V(?) for spin up and down electrons, respectively. We predict that the spin up and down components oscillate at the same frequency but with different amplitudes. Furthermore our results reveal that the standard interpretation of the Josephson supercurrent in terms of coherent bosonic pair tunneling is insufficient. We provide an intuitive interpretation of the Josephson supercurrent as interference in Rabi oscillations of pairs and single particles, the latter causing the asymmetry. PMID:21231393

Heikkinen, M O J; Massel, F; Kajala, J; Leskinen, M J; Paraoanu, G S; Törmä, P

2010-11-26

394

Asymmetric e+e- Colliders  

NASA Astrophysics Data System (ADS)

In these lecture notes, I will focus on some of the interesting details of B-meson physics at an Asymmetric collider which are relevant for the study of CP-violating asymmetries. As an instructive example, I will describe the basic experimental tools used in the measurement of sin 2?. In addition, I will describe the use of a blind analysis technique in the measurement of CP asymmetries.

Roodman, Aaron

2001-09-01

395

Asymmetric information and macroeconomic dynamics  

NASA Astrophysics Data System (ADS)

We show how macroeconomic dynamics can be derived from asymmetric information. As an illustration of the utility of this approach we derive the equilibrium density, non-equilibrium densities and the equation of motion for the response to a demand shock for productivity in a simple economy. Novel consequences of this approach include a natural incorporation of time dependence into macroeconomics and a common information-theoretic basis for economics and other fields seeking to link micro-dynamics and macro-observables.

Hawkins, Raymond J.; Aoki, Masanao; Roy Frieden, B.

2010-09-01

396

Secrecy Types for Asymmetric Communication  

Microsoft Academic Search

We develop a typed process calculus for security protocols inwhich types convey secrecy properties. We focus on asymmetric communicationprimitives, especially on public-key encryption. These presentspecial difficulties, partly because they rely on related capabilities (e.g.,"public" and "private" keys) with different levels of secrecy and scopes.1 IntroductionA secret is something you tell to one person at a time, according to a populardefinition.

Martín Abadi; Bruno Blanchet

2001-01-01

397

Dividend Policy under Asymmetric Information  

Microsoft Academic Search

ABSTRACT We extend the standard finance model of the firm's dividend\\/investment\\/financing decisions by allowing the firm's managers,to know,more than outside investors about the true state of the firm's current earnings. The extension endogenizes the dividend (and financing) announcement,effects amply documented,in recent research. But once trading of shares is admitted to the model along with asymmetric information, the familiar Fisherian criterion

Merton H. Miller; Kevin Rock

1985-01-01

398

Stereodivergent synthesis of chiral fullerenes by [3 + 2] cycloadditions to C??.  

PubMed

A wide range of new dipoles and catalysts have been used in 1,3-dipolar cycloadditions of N-metalated azomethine ylides onto C60 yielding a full stereodivergent synthesis of pyrrolidino[60]fullerenes with complete diastereoselectivities and very high enantioselectivities. The use of less-explored chiral ?-iminoamides as starting 1,3-dipoles leads to an interesting double asymmetric induction resulting in a matching/mismatching effect depending upon the absolute configuration of the stereocenter in the starting ?-iminoamide. An enantioselective process was also found in the retrocycloaddition reaction as revealed by mass spectrometry analysis on quasi-enantiomeric pyrrolidino[60]fullerenes. Theoretical DFT calculations are in very good agreement with the experimental data. On the basis of this agreement, a plausible reaction mechanism is proposed. PMID:24359021

Maroto, Enrique E; Filippone, Salvatore; Suárez, Margarita; Martínez-Álvarez, Roberto; de Cózar, Abel; Cossío, Fernando P; Martín, Nazario

2014-01-15

399

Stereodivergent Synthesis of Chiral Fullerenes by [3 + 2] Cycloadditions to C60  

PubMed Central

A wide range of new dipoles and catalysts have been used in 1,3-dipolar cycloadditions of N-metalated azomethine ylides onto C60 yielding a full stereodivergent synthesis of pyrrolidino[60]fullerenes with complete diastereoselectivities and very high enantioselectivities. The use of less-explored chiral ?-iminoamides as starting 1,3-dipoles leads to an interesting double asymmetric induction resulting in a matching/mismatching effect depending upon the absolute configuration of the stereocenter in the starting ?-iminoamide. An enantioselective process was also found in the retrocycloaddition reaction as revealed by mass spectrometry analysis on quasi-enantiomeric pyrrolidino[60]fullerenes. Theoretical DFT calculations are in very good agreement with the experimental data. On the basis of this agreement, a plausible reaction mechanism is proposed.

2013-01-01

400

Structure determination and total synthesis of (+)-16-hydroxy-16,22-dihydroapparicine.  

PubMed

Herein, we describe the first asymmetric total synthesis and determination of the relative and absolute stereochemistry of naturally occurring 16-hydroxy-16,22-dihydroapparicine. The key steps include 1)?a novel phosphinimine-mediated cascade reaction to construct the unique 1-azabicyclo[4.2.2]decane core, including a pseudo-aminal-type moiety; 2)?a highly stereospecific 1,2-addition of 2-acylindole or a methylketone through a Felkin-Anh transition state for the construction of a tetrasubstituted carbon center; and 3)?an intramolecular chirality-transferring Michael reaction of the ketoester, with neighboring-group participation, to introduce a chiral center at C15 in the target molecule. In addition, we evaluated the antimalarial activity of synthetic (+)-(15S,16R)-16-hydroxy-16,22-dihydroapparicine and its intermediate against chloroquine-resistant Plasmodium falciparum (K1 strain) parasites. PMID:23788004

Hirose, Tomoyasu; Noguchi, Yoshihiko; Furuya, Yujiro; Ishiyama, Aki; Iwatsuki, Masato; Otoguro, Kazuhiko; Omura, Satoshi; Sunazuka, Toshiaki

2013-08-01

401

Asymmetric Dimethylarginine, Endothelial Dysfunction and Renal Disease  

PubMed Central

l-Arginine (Arg) is oxidized to l-citrulline and nitric oxide (NO) by the action of endothelial nitric oxide synthase (NOS). In contrast, protein-incorporated Arg residues can be methylated with subsequent proteolysis giving rise to methylarginine compounds, such as asymmetric dimethylarginine (ADMA) that competes with Arg for binding to NOS. Most ADMA is degraded by dimethylarginine dimethyaminohydrolase (DDAH), distributed widely throughout the body and regulates ADMA levels and, therefore, NO synthesis. In recent years, several studies have suggested that increased ADMA levels are a marker of atherosclerotic change, and can be used to assess cardiovascular risk, consistent with ADMA being predominantly absorbed by endothelial cells. NO is an important messenger molecule involved in numerous biological processes, and its activity is essential to understand both pathogenic and therapeutic mechanisms in kidney disease and renal transplantation. NO production is reduced in renal patients because of their elevated ADMA levels with associated reduced DDAH activity. These factors contribute to endothelial dysfunction, oxidative stress and the progression of renal damage, but there are treatments that may effectively reduce ADMA levels in patients with kidney disease. Available data on ADMA levels in controls and renal patients, both in adults and children, also are summarized in this review.

Aldamiz-Echevarria, Luis; Andrade, Fernando

2012-01-01

402

Deformation of an asymmetric film  

NASA Astrophysics Data System (ADS)

Recent experiments have investigated shape changes of polymer films induced by asymmetric swelling by a chemical vapor. Inspired by recent work on the shaping of elastic sheets by non-Euclidean metrics [1,2], we represent the effect of chemical vapors by a change in the target metric tensor. In this problem, unlike Refs. [1,2], the target metric is asymmetric between the two sides of the film. Changing this metric induces a curvature of the film, which may be Gaussian curvature into a sphere or mean curvature into a cylinder. We calculate the elastic energy for each of these shapes, and show that the sphere is favored for films smaller than a critical size, which depends on the film thickness, while the cylinder is favored for larger films. We compare the formalism for asymmetric films with previous theoretical work on symmetric films, and compare the predictions with experimental results. [4pt] [1] Y. Klein, E. Efrati, and E. Sharon, Science 315, 1116 (2007).[0pt] [2] E. Efrati, E. Sharon, and R. Kupferman, J. Mech. Phys. Solids 57, 762 (2009).

Geng, Jun; Selinger, Jonathan

2010-03-01

403

Nickel/bis(oxazoline)-catalyzed asymmetric Kumada reactions of alkyl electrophiles: cross-couplings of racemic alpha-bromoketones.  

PubMed

The first asymmetric Kumada reactions of alkyl electrophiles are described, specifically, cross-couplings of racemic alpha-bromoketones with aryl Grignard reagents. Several features of this investigation are of interest. First, the couplings proceed at remarkably low temperature (-40 or -60 degrees C), which enables the asymmetric synthesis of racemization-prone alpha-arylketones. Second, dialkyl ketones undergo enantioselective coupling in good ee and yield. Third, readily available bis(oxazolines) are shown for the first time to be effective ligands for cross-couplings of alkyl electrophiles, thereby opening the door to new opportunities in asymmetric catalysis. PMID:20050651

Lou, Sha; Fu, Gregory C

2010-02-01

404

Nickel/Bis(oxazoline)-Catalyzed Asymmetric Kumada Reactions of Alkyl Electrophiles: Cross-Couplings of Racemic ?-Bromoketones  

PubMed Central

The first asymmetric Kumada reactions of alkyl electrophiles are described, specifically, cross-couplings of racemic ?-bromoketones with aryl Grignard reagents. Several features of this investigation are of interest. First, the couplings proceed at remarkably low temperature (?40 °C or ?60 °C), which enables the asymmetric synthesis of racemization-prone ?-arylketones. Second, dialkyl ketones undergo enantioselective coupling in good ee and yield. Third, readily available bis(oxazolines) are shown for the first time to be effective ligands for cross-couplings of alkyl electrophiles, thereby opening the door to new opportunities in asymmetric catalysis.

Lou, Sha; Fu, Gregory C.

2010-01-01

405

Rational development of iron catalysts for asymmetric transfer hydrogenation.  

PubMed

The asymmetric reduction of ketones and imines by transfer of hydrogen from isopropanol as the solvent catalyzed by metal complexes is a very useful method for preparing valuable enantioenriched alcohols and amines. Described here is the development of three generations of progressively more active iron catalysts for this transformation. Key features of this process of discovery involved the realization that one carbonyl ligand was needed (as in hydrogenases), the synthesis of modular ligands templated by iron, the elucidation of the mechanisms of catalyst activation and action, as well as the rational synthesis of precursors that lead directly and easily to the species in the catalytic cycle. The discovery that iron, an abundant element that is essential to life, can form catalysts of these hydrogenation reactions is a contribution to green chemistry. PMID:24763699

Sues, Peter E; Demmans, Karl Z; Morris, Robert H

2014-06-01

406

20 CFR 404.1205 - Absolute coverage groups.  

Code of Federal Regulations, 2013 CFR

...2013-04-01 false Absolute coverage groups. 404.1205 Section 404.1205 ...Employees of State and Local Governments What Groups of Employees May Be Covered § 404.1205 Absolute coverage groups. (a) General. An absolute...

2013-04-01

407

Organocatalytic sequential one-pot double cascade asymmetric synthesis of Wieland-Miescher ketone analogues from a Knoevenagel/hydrogenation/Robinson annulation sequence: scope and applications of organocatalytic biomimetic reductions.  

PubMed

A practical and novel organocatalytic chemo- and enantioselective process for the cascade synthesis of highly substituted 2-alkyl-cyclohexane-1,3-diones and Wieland-Miescher (W-M) ketone analogs is presented via reductive alkylation as a key step. First time, we developed the one-step alkylation of dimedone and 1,3-cyclohexanedione with aldehydes and Hantzsch ester through an organocatalytic reductive alkylation strategy. Direct combination of l-proline-catalyzed cascade Knoevenagel/hydrogenation and cascade Robinson annulation of CH acids (dimedone and 1,3-cyclohexanedione), aldehydes, Hantzsch ester, and methyl vinyl ketone furnished the highly functionalized W-M ketone analogues in good to high yields and with excellent enantioselectivities. Many of the reductive alkylation products show a direct application in pharmaceutical chemistry. PMID:17552564

Ramachary, Dhevalapally B; Kishor, Mamillapalli

2007-07-01

408

Asymmetric aminolytic kinetic resolution of racemic epoxides using recyclable chiral polymeric Co(III)-salen complexes: a protocol for total utilization of racemic epoxide in the synthesis of (R)-Naftopidil and (S)-Propranolol.  

PubMed

Chiral polymeric Co(III) salen complexes with chiral ((R)/(S)-BINOL, diethyl tartrate) and achiral (piperazine and trigol) linkers with varying stereogenic centers were synthesized for the first time and used as catalysts for aminolytic kinetic resolution (AKR) of a variety of terminal epoxides and glycidyl ethers to get enantio-pure epoxides (ee, 99%) and N-protected ?-amino alcohols (ee, 99%) with quantitative yield in 16 h at RT under optimized reaction conditions. This protocol was also used for the synthesis of two enantiomerically pure drug molecules (R)-Naftopidil (?1-blocker) and (S)-Propranolol (?-blocker) as a key step via AKR of single racemic naphthylglycidyl ether with Boc-protected isoproylamine with 100% epoxide utilization at 1 g level. The catalyst 1 was successfully recycled for a number of times. PMID:23899243

Kumar, Manish; Kureshy, Rukhsana I; Shah, Arpan K; Das, Anjan; Khan, Noor-ul H; Abdi, Sayed H R; Bajaj, Hari C

2013-09-20

409

Total synthesis of (-)-dihydrosporothriolide utilizing an indium-mediated reformatsky-claisen rearrangement.  

PubMed

The asymmetric synthesis of (-)-dihydrosporothriolide (1), a biologically active bis-?-butyrolactone, is described, that proceeds through a d-proline-catalyzed asymmetric aminooxylation, indium-mediated Reformatsky-Claisen rearrangement of an ?,?-dibromoacetate derivative, and diastereoselective dihydroxylation. The route requires no protective group manipulation and allows the concise seven-step synthesis of 1 from n-octanal. PMID:24877583

Ishihara, Jun; Tsuru, Hiroaki; Hatakeyama, Susumi

2014-06-20

410

New absolute magnitude calibrations for detached binaries  

NASA Astrophysics Data System (ADS)

Lutz-Kelker bias corrected absolute magnitude calibrations for the detached binary systems with main-sequence components are presented. The absolute magnitudes of the calibrator stars were derived at intrinsic colours of Johnson-Cousins and 2MASS (Two Micron All Sky Survey) photometric systems. As for the calibrator stars, 44 detached binaries were selected from the Hipparcos catalogue, which have relative observed parallax errors smaller than 15% (??/??0.15). The calibration equations which provide the corrected absolute magnitude for optical and near-infrared pass bands are valid for wide ranges of colours and absolute magnitudes: -0.18<(B-V)0<0.91, -1.6absolute magnitude calibrations of this study can be used as a convenient statistical tool to estimate the true distances of detached binaries out of Hipparcos' distance limit.

Bilir, S.; Ak, T.; Soydugan, E.; Soydugan, F.; Yaz, E.; Filiz Ak, N.; Eker, Z.; Demircan, O.; Helvaci, M.

2008-10-01

411

Absolute magnitude calibration for red clump stars  

NASA Astrophysics Data System (ADS)

We combined the ( K s , J- K s ) data in Laney et al. (Mon. Not. R. Astron. Soc. 419:1637, 2012) with the V apparent magnitudes and trigonometric parallaxes taken from the Hipparcos catalogue and used them to fit the M_{Ks} absolute magnitude to a linear polynomial in terms of V- K s colour. The mean and standard deviation of the absolute magnitude residuals, -0.001 and 0.195 mag, respectively, estimated for 224 red clump stars in Laney et al. (2012) are (absolutely) smaller than the corresponding ones estimated by the procedure which adopts a mean M_{Ks}=-1.613 {mag} absolute magnitude for all red clump stars, -0.053 and 0.218 mag, respectively. The statistics estimated by applying the linear equation to the data of 282 red clump stars in Alves (Astrophys. J. 539:732, 2000) are larger, ? M_{Ks}=0.209 and ?=0.524 mag, which can be explained by a different absolute magnitude trend, i.e. condensation along a horizontal distribution.

Karaali, S.; Bilir, S.; Yaz Gökçe, E.

2013-07-01

412

Efficient syntheses of korupensamines A, B and michellamine B by asymmetric Suzuki-Miyaura coupling reactions.  

PubMed

Efficient asymmetric Suzuki-Miyaura coupling reactions are employed for the first time in total syntheses of chiral biaryl natural products korupensamine A and B in combination with an effective diastereoselective hydrogenation, allowing ultimately a concise and stereoselective synthesis of michellamine B. Chiral monophosphorus ligands L1-3 are effective for the syntheses of a series of functionalized chiral biaryls by asymmetric Suzuki-Miyaura coupling reactions in excellent yields and enantioselectivities (up to 99% ee). The presence of a polar-? interaction between the highly polarized BOP group and the extended ? system of arylboronic acid coupling partner is believed to be important for the high enantioselectivity. PMID:24147559

Xu, Guangqing; Fu, Wenzhen; Liu, Guodu; Senanayake, Chris H; Tang, Wenjun

2014-01-15

413

Absolute isotopic abundances of TI in meteorites  

NASA Astrophysics Data System (ADS)

The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using 46Ti/48Ti. Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. The authors provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components.

Niederer, F. R.; Papanastassiou, D. A.; Wasserburg, G. J.

1985-03-01

414

Absolute Photoacoustic Thermometry in Deep Tissue  

PubMed Central

Photoacoustic (PA) thermography is a promising tool for temperature measurement in deep tissue. Here, we propose an absolute temperature measurement method based on the dual temperature dependences of the Grüneisen parameter and the speed of sound in tissue. By taking ratiometric measurements at two adjacent temperatures, we can eliminate the factors that are temperature irrelevant but difficult to correct for in deep tissue. To validate our method, absolute temperatures of blood-filled tubes embedded ~9 mm deep in chicken tissue were measured in a biologically relevant range from 28 °C to 46 °C. The temperature measurement accuracy was ~0.6 °C. The results suggest that our method can be potentially used for absolute temperature monitoring in deep tissue during thermotherapy.

Tai, Stephen; Zhou, Yong; Wang, Lihong V.

2013-01-01

415

Absolute photoacoustic thermometry in deep tissue.  

PubMed

Photoacoustic thermography is a promising tool for temperature measurement in deep tissue. Here we propose an absolute temperature measurement method based on the dual temperature dependences of the Grüneisen parameter and the speed of sound in tissue. By taking ratiometric measurements at two adjacent temperatures, we can eliminate the factors that are temperature irrelevant but difficult to correct for in deep tissue. To validate our method, absolute temperatures of blood-filled tubes embedded ~9 mm deep in chicken tissue were measured in a biologically relevant range from 28°C to 46°C. The temperature measurement accuracy was ~0.6°C. The results suggest that our method can be potentially used for absolute temperature monitoring in deep tissue during thermotherapy. PMID:24322224

Yao, Junjie; Ke, Haixin; Tai, Stephen; Zhou, Yong; Wang, Lihong V

2013-12-15

416

Absolute Zero: Community Education Outreach Guide  

NSDL National Science Digital Library

This guide provides curricular resources for study of the history and science of the quest for ever colder temperature. Designed for teachers and informal educators of middle school students. this guide offers hands-on demonstrations, questions to encourage student participation, suggestions for class activities, and ways to encourage students to continue studying the science. Topics include low-temperature physics and the impact of technologies such as air conditioning, refrigeration and liquefied gases. This material is related to a two-part public broadcasting special, Absolute Zero, produced by Meridian Productions and Windfall Films. Absolute Zero is underwritten by the National Science Foundation and the Alfred P. Sloan Foundation and is based largely on Tom Shachtmanâs acclaimed book, Absolute Zero and the Conquest of Cold.

2008-09-18

417

Absolute calibration in vivo measurement systems  

SciTech Connect

Lawrence Livermore National Laboratory (LLNL) is currently investigating a new method for obtaining absolute calibration factors for radiation measurement systems used to measure internally deposited radionuclides in vivo. Absolute calibration of in vivo measurement systems will eliminate the need to generate a series of human surrogate structures (i.e., phantoms) for calibrating in vivo measurement systems. The absolute calibration of in vivo measurement systems utilizes magnetic resonance imaging (MRI) to define physiological structure, size, and composition. The MRI image provides a digitized representation of the physiological structure, which allows for any mathematical distribution of radionuclides within the body. Using Monte Carlo transport codes, the emission spectrum from the body is predicted. The in vivo measurement equipment is calibrated using the Monte Carlo code and adjusting for the intrinsic properties of the detection system. The calibration factors are verified using measurements of existing phantoms and previously obtained measurements of human volunteers. 8 refs.

Kruchten, D.A.; Hickman, D.P.

1991-02-01

418

Quantitative standards for absolute linguistic universals.  

PubMed

Absolute linguistic universals are often justified by cross-linguistic analysis: If all observed languages exhibit a property, the property is taken to be a likely universal, perhaps specified in the cognitive or linguistic systems of language learners and users. In many cases, these patterns are then taken to motivate linguistic theory. Here, we show that cross-linguistic analysis will very rarely be able to statistically justify absolute, inviolable patterns in language. We formalize two statistical methods-frequentist and Bayesian-and show that in both it is possible to find strict linguistic universals, but that the numbers of independent languages necessary to do so is generally unachievable. This suggests that methods other than typological statistics are necessary to establish absolute properties of human language, and thus that many of the purported universals in linguistics have not received sufficient empirical justification. PMID:24117660

Piantadosi, Steven T; Gibson, Edward

2014-05-01

419

Absolute Continuity in Noncommutative Measure Theory  

NASA Astrophysics Data System (ADS)

Recent results on absolute continuity of Banach space valued operators and convergence theorems on operator algebras are deepened and summarized. It is shown that absolute continuity of an operator T on a von Neumann algebra M with respect to a positive normal functional ? on M is not implied by the fact that the null projections of ? are the null projections of T. However, it is proved that the implication above is true whenever M is finite or T is weak*-continuous. Further it is shown that the absolute value preserves the Vitali-Hahn-Saks property if, and only if, the underlying algebra is finite. This result improves classical results on weak compactness of sets of noncommutative measures.

Hamhalter, Jan

2010-12-01

420

A concise, biomimetic total synthesis of (+)-davanone.  

PubMed

A concise, biomimetic synthesis of the antifungal and antispasmodic natural product (+)-davanone is described. The key stereoselective reactions are a Sharpless asymmetric epoxidation, a thiazolium-catalyzed esterification, and a palladium-mediated cyclization. All carbons are derived from isoprene units and no protecting groups are used, permitting an atom- and redox-economical synthesis. PMID:19366191

Morrison, Karen C; Litz, Jonathan P; Scherpelz, Kathryn P; Dossa, Paul D; Vosburg, David A

2009-05-21

421

Enantioselective and diastereoselective hydroxylation of bufuralol. Absolute configuration of the 7-(1-hydroxyethyl)-2-[1-hydroxy-2-(tert-butylamino)ethyl]benzofurans, the benzylic hydroxylation metabolites.  

PubMed

Asymmetric synthesis of the diastereomeric 7-(1-hydroxyethyl)-2-[1-hydroxy-2-(tert-butylamino)ethyl]benzofurans (2), the benzylic hydroxylation metabolites of bufuralol (1), is described, and the absolute configurations of these diastereomers are assigned. 1"-Oxobufuralol (3) was reduced with a complex of (2S)-(-)-2-amino-3-methyl-1,1-diphenylbutan-1-ol and borane, yielding 2, which had a 95:5 ratio of the possible 1"R and 1"S isomers as determined by HPLC. Separation of the resulting diastereomers was facilitated by derivatization with the enantiomers of 1-phenethyl isocyanate (PEIC). The absolute configurations 1'S,1"R and 1'R,1"R were assigned to the diastereomers formed in excess, 2c and 2b, on the basis of the known stereochemistry of reduction of closely related alkyl phenyl ketones to R alcohols by using this chiral borane reagent. The circular dichroism spectra of the four isomeric benzylic alcohols were in agreement with these assignments. In the presence of the rat liver microsomal fraction, benzylic hydroxylation of bufuralol was significantly product stereoselective favoring formation of diastereomers with the 1"R absolute stereochemistry at the new chiral center in products from (1'R)-1 by a ratio of 4.5:1 [(1'R,1"R)-2:(1'R,1"S)-2] and by nearly 8:1 [(1'S,1"R)-2:(1'S,1"S)-2] from (1'S)-1. (1'R)-Bufuralol was more rapidly hydroxylated than was (1'S)-1, by about 3-fold. In the presence of human liver microsomes, (1'R)-bufuralol was also more rapidly hydroxylated than was (1'S)-1, by ca. 2.5-fold. However, product stereoselectivity from the 1'R enantiomer was reversed from that observed in the rat liver microsomal oxidation, with more (1"S)-carbinol being formed than 1"R isomer by nearly 4-fold. From (1'S)-1, about equal amounts of the two possible hydroxybufuralol diastereomers were formed. The results from the human liver microsomal studies are consistent with observed enantioselectivity of hydroxylation of bufuralol in vivo in humans. PMID:1681107

Weerawarna, S A; Geisshüsler, S M; Murthy, S S; Nelson, W L

1991-10-01

422

Diffusion on asymmetric fractal networks  

NASA Astrophysics Data System (ADS)

We derive a renormalization method to calculate the spectral dimension d¯ of deterministic self-similar networks with arbitrary base units and branching constants. The generality of the method allows the affect of a multitude of microstructural details to be quantitatively investigated. In addition to providing models for physical networks, the results allow precise tests of theories of diffusive transport. For example, the properties of a class of nonrecurrent trees (d¯>2) with asymmetric elements and branching violate the Alexander-Orbach scaling law.

Haynes, Christophe P.; Roberts, Anthony P.

2010-12-01

423

Microemulsions in Asymmetric Polymer Blends  

NASA Astrophysics Data System (ADS)

Microemulsions and lamellar phases have been observed in previous experiments wherein block copolymers are added to blends of immiscible homopolymers. To our knowledge, all of the previous studies are restricted to homopolymers of nearly identical chain lengths with critical volume fractions in the vicinity of 0.5 (symmetric systems). The present study concerns the formation of microemulsions and lamellar phases phases in blends of immiscible polymers with substantial differences in chain lengths and critical volume fractions far removed from 0.5 (asymmetric systems). The characteristics of the block copolymers that enable the creation of these phases will be discussed in the presentation.

Nedoma, Alisyn; Robertson, Megan; Balsara, Nitash

2009-03-01

424

Absolute Distance Measurement with the MSTAR Sensor  

NASA Technical Reports Server (NTRS)

The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. The sensor uses a single laser in conjunction with fast phase modulators and low frequency detectors. We describe the design of the system - the principle of operation, the metrology source, beamlaunching optics, and signal processing - and show results for target distances up to 1 meter. We then demonstrate how the system can be scaled to kilometer-scale distances.

Lay, Oliver P.; Dubovitsky, Serge; Peters, Robert; Burger, Johan; Ahn, Seh-Won; Steier, William H.; Fetterman, Harrold R.; Chang, Yian

2003-01-01

425

Optimal asymmetric phase-covariant quantum cloning  

NASA Astrophysics Data System (ADS)

We propose a relatively simple method to derive the optimal asymmetric approximate quantum cloning. The results obtained agree with the previous contributions. Utilizing the method we also obtain the optimal asymmetric 1?3 phase-covariant quantum cloning and 1?2 and 1?3 economical ones, which work without ancilla, in 2-dimension. Maybe the method can be effective to derive the optimal asymmetric approximate N?M quantum cloning in d-dimension.

Zhang, Wen-Hai; Yu, Long-Bao; Ye, Liu

2006-08-01

426

Asymmetric rolling of metal powders and granules  

Microsoft Academic Search

The paper examines asymmetric rolling of aluminum powder and granules on driving rolls of different diameters at the same\\u000a angular velocity. The ratio of roll diameters is 1.1 to 1.4. It is established that asymmetric rolling of powder material\\u000a allows removing drawbacks peculiar to traditional asymmetric rolling. These drawbacks are roll camber and destruction caused\\u000a by nonuniform distribution of contact

G. Ya. Kalutskii; K. A. Gogaev; V. S. Voropaev; V. V. Nepomnyashchii

2007-01-01

427

Enantioselective total synthesis of macrolide (+)-neopeltolide.  

PubMed

The asymmetric total synthesis of the anti-proliferative macrolide (+)-neopeltolide has been completed. The stereochemically defined trisubstituted tetrahydropyran ring was constructed via a catalytic hetero-Diels-Alder reaction creating two new chiral centers in a highly diastereoselective manner. The other key features of this synthesis included Brown's asymmetric allylation to install the requisite C-11 and C-13 stereocenters. The synthesis of the oxazole side chain consisted of a hydrozirconation of an alkynyl stannane to establish the Z stereochemistry, followed by a palladium catalyzed cross coupling to introduce the desired Z olefin in the oxazole side chain. PMID:24121457

Ghosh, Arun K; Shurrush, Khriesto A; Dawson, Zachary L

2013-11-28

428

Symmetric mortality and asymmetric suicide cycles.  

PubMed

In this investigation, tests were performed to determine whether mortality cycles are asymmetric. Results from an asymmetry test of U.S. time-series data from 1951 to 2005 provide no evidence that all-cause mortality or mortality caused by disease causes asymmetric cycles. However, the rate of fatalities from suicide exhibits the pattern of an asymmetric cycle. The evidence for asymmetric suicide cycles is statistically significant for men and working-age groups but not for women and non-working-age groups. PMID:20378221

Wu, Wen-Chieh; Cheng, Hui-Pei

2010-06-01

429

Asymmetric Activity Waves in Synaptic Cortical Systems  

NASA Astrophysics Data System (ADS)

This study explores asymmetric activity wave propagation in cortical systems, both experimentally and mathematically. Experimentally, we find that activity waves passing through the same neural tissue but in different directions exhibit different speeds and spatial profiles and evoke different firing patterns in individual neurons. Mathematically, we extend our previously published results on the existence of traveling pulse solutions to account for our experimental observations. Specifically, our analysis suggests that asymmetric connectivity is sufficient to account for asymmetric propagation. Moreover, it predicts that unidirectional propagation can occur in cases of extreme asymmetry. Finally, we experimentally validate our analytic findings by examining wave propagation in a cortical region where asymmetric coupling is known to exist.

Pinto, David J.; Troy, William; Kneezel, Timothy

2009-01-01

430

The asymmetric Goos-Hänchen effect  

NASA Astrophysics Data System (ADS)

We show under which conditions optical Gaussian beams, propagating throughout an homogeneous dielectric right angle prism, present an asymmetric Goos-Hänchen (GH) effect. This asymmetric behavior is seen for incidence at critical angles and happens in the propagation direction of the outgoing beam. The asymmetric GH effect can also be seen as an amplification of the standard GH shift. Due to the fact that it only depends on the ratio between the wavelength and the minimal waist size of the incoming Gaussian beam, it can also be used to determine one of these parameters. Multiple-peak interference is an additional phenomenon seen in the presence of such asymmetric effects.

Araujo, Manoel P.; Carvalho, Silvânia A.; De Leo, Stefano

2014-01-01

431

Asymmetrical membranes and surface tension.  

PubMed Central

The (31)P-nuclear magnetic resonance chemical shift of phosphatidic acid in a membrane is sensitive to the lipid head group packing and can report qualitatively on membrane lateral compression near the aqueous interface. We have used high-resolution (31)P-nuclear magnetic resonance to evaluate the lateral compression on each side of asymmetrical lipid vesicles. When monooleoylphosphatidylcholine was added to the external monolayer of sonicated vesicles containing dioleoylphosphatidylcholine and dioleoylphosphatidic acid, the variation of (31)P chemical shift of phosphatidic acid indicated a lateral compression in the external monolayer. Simultaneously, a slight dilation was observed in the inner monolayer. In large unilamellar vesicles on the other hand the lateral pressure increased in both monolayers after asymmetrical insertion of monooleoylphosphatidylcholine. This can be explained by assuming that when monooleoylphosphatidylcholine is added to large unilamellar vesicles, the membrane bends until the strain is the same in both monolayers. In the case of sonicated vesicles, a change of curvature is not possible, and therefore differential packing in the two layers remains. We infer that a variation of lipid asymmetry by generating a lateral strain in the membrane can be a physiological way of modulating the conformation of membrane proteins.

Traikia, Mounir; Warschawski, Dror E; Lambert, Olivier; Rigaud, Jean-Louis; Devaux, Philippe F

2002-01-01

432

Active matter on asymmetric substrates  

NASA Astrophysics Data System (ADS)

For collections of particles in a thermal bath interacting with an asymmetric substrate, it is possible for a ratchet effect to occur where the particles undergo a net dc motion in response to an ac forcing. Ratchet effects have been demonstrated in a variety of systems including colloids as well as magnetic vortices in type-II superconductors. Here we examine the case of active matter or self-driven particles interacting with asymmetric substrates. Active matter systems include self-motile colloidal particles undergoing catalysis, swimming bacteria, artificial swimmers, crawling cells, and motor proteins. We show that a ratchet effect can arise in this type of system even in the absence of ac forcing. The directed motion occurs for certain particle-substrate interaction rules and its magnitude depends on the amount of time the particles spend swimming in one direction before turning and swimming in a new direction. For strictly Brownian particles there is no ratchet effect. If the particles reflect off the barriers or scatter from the barriers according to Snell's law there is no ratchet effect; however, if the particles can align with the barriers or move along the barriers, directed motion arises. We also find that under certain motion rules, particles accumulate along the walls of the container in agreement with experiment. We also examine pattern formation for synchronized particle motion. We discuss possible applications of this system for self-assembly, extracting work, and sorting as well as future directions such as considering collective interactions and flocking models.

Olson Reichhardt, C. J.; Drocco, J.; Mai, T.; Wan, M. B.; Reichhardt, C.

2011-09-01

433

Absolute rate theories of epigenetic stability  

Microsoft Academic Search

Spontaneous switching events in most characterized genetic switches are rare, resulting in extremely stable epigenetic properties. We show how simple arguments lead to theories of the rate of such events much like the absolute rate theory of chemical reactions corrected by a transmission factor. Both the probability of the rare cellular states that allow epigenetic escape and the transmission factor

Aleksandra M. Walczak; José N. Onuchic; Peter G. Wolynes

2005-01-01

434

Absolute Measurement of exp 152 Eu.  

National Technical Information Service (NTIS)

A new method of the absolute measurement for exp 152 Eu was established based on the 4 pi beta - gamma spectroscopic anti-coincidence method. It is a coincidence counting method consisting of a 4 pi beta -counter and a Ge(Li) gamma -ray detector, in which...

H. Baba S. Baba S. Ichikawa T. Sekine I. Ishikawa

1981-01-01

435

Absolute versus relative time in process algebras  

Microsoft Academic Search

Timed process algebras are useful tools for the specification and verification of real time systems. We study the relationships between two of these algebras, cIpa ('Closed Interval Process Algebra') and TCCS ('Temporal CCS') which deal with temporal aspects of concurrent systems by following very different interpretations: durational actions versus durationless actions, absolute time versus relative time, timed functional behavior versus

Flavio Corradini

1997-01-01

436

Absolute versus Relative Time in Process Algebras  

Microsoft Academic Search

Timed process algebras are useful tools for the specification and verification of real-time systems. We study the relationships between two of these algebras, cIpa (closed interval process Algebra) and TCCS (temporal CCS), which deal with temporal aspects of concurrent systems by following very different interpretations: durational actions versus durationless actions, absolute time versus relative time, timed functional behavior versus time

Flavio Corradini

2000-01-01

437

Teaching Absolute Value Inequalities to Mature Students  

ERIC Educational Resources Information Center

This paper gives an account of a teaching experiment on absolute value inequalities, whose aim was to identify characteristics of an approach that would realize the potential of the topic to develop theoretical thinking in students enrolled in prerequisite mathematics courses at a large, urban North American university. The potential is…

Sierpinska, Anna; Bobos, Georgeana; Pruncut, Andreea

2011-01-01

438

Absolute Points for Multiple Assignment Problems  

ERIC Educational Resources Information Center

An algorithm is presented to solve multiple assignment problems in which a cost is incurred only when an assignment is made at a given cell. The proposed method recursively searches for single/group absolute points to identify cells that must be loaded in any optimal solution. Unlike other methods, the first solution is the optimal solution. The…

Adlakha, V.; Kowalski, K.

2006-01-01

439

Precision absolute positional measurement of laser beams.  

PubMed

We describe an instrument which, coupled with a suitable coordinate measuring machine, facilitates the absolute measurement within the machine frame of the propagation direction of a millimeter-scale laser beam to an accuracy of around ±4 ?m in position and ±20 ?rad in angle. PMID:23669658

Fitzsimons, Ewan D; Bogenstahl, Johanna; Hough, James; Killow, Christian J; Perreur-Lloyd, Michael; Robertson, David I; Ward, Henry

2013-04-20

440

Resonance absolute quantum reflection at selected energies.  

PubMed

We present one-dimensional local potentials with an absolute reflection at a given energy value which can be above barriers. The corresponding energy dependence of a reflection coefficient exhibits resonance behavior. The inversion technique provides the potentials with specified widths of reflection resonances, their number and positions. The multichannel systems (exact models) with a complete reflection are also given. PMID:11690195

Chabanov, V M; Zakhariev, B N

2001-10-15

441

Absolutely Uniform Illumination of Laser Fusion Pellets.  

National Technical Information Service (NTIS)

Absolutely uniform illumination of spherical laser fusion pellets is possible when the energy deposition from a single laser beam is given by a simple cos 3 theta distribution. Conditions can be derived for which the laser beam targeting angles allow this...

A. J. Schmitt

1984-01-01

442

An Absolute Electrometer for the Physics Laboratory  

ERIC Educational Resources Information Center

A low-cost, easy-to-use absolute electrometer is presented: two thin metallic plates and an electronic balance, usually available in a laboratory, are used. We report on the very good performance of the device that allows precise measurements of the force acting between two charged plates. (Contains 5 footnotes, 2 tables, and 6 figures.)

Straulino, S.; Cartacci, A.

2009-01-01

443

Enantioselective Total Syntheses of (-)-Taiwaniaquinone H and (-)-Taiwaniaquinol B by Iridium-Catalyzed Borylation and Palladium-Catalyzed Asymmetric ?-Arylation  

PubMed Central

We report a concise, enantioselective total synthesis of (?)-taiwaniaquinone H and the first enantioselective total synthesis of (?)-taiwaniaquinol B by a route that includes enantioselective, palladium-catalyzed ?-arylation of a ketone with an aryl bromide that was generated by sterically controlled halogenation via iridium-catalyzed C–H borylation. This asymmetric ?-arylation creates the benzylic, quaternary stereogenic center present in the taiwaniaquinoids. The synthesis was completed efficiently by developing a Lewis acid-promoted cascade to construct the [6,5,6] tricyclic core of an intermediate common to the synthesis of a number of taiwaniaquinoids. Through the preparation of these compounds, we demonstrate the utility of constructing benzylic, quaternary, stereogenic centers, even those lacking a carbonyl group in the ?-position, by asymmetric ?-arylation.

Liao, Xuebin; Stanley, Levi M.; Hartwig, John F.

2011-01-01

444

A Facile Stereoselective Total Synthesis of (R)-Rugulactone  

PubMed Central

An efficient and novel synthesis of (R)-rugulactone has been achieved employing Sharpless asymmetric epoxidation of allyl alcohols followed by selective hydride reduction of epoxy alcohols and olefin cross metathesis reactions.

Reddy, B. Narasimha; Singh, R. P.

2014-01-01

445

Defect-free ultrahigh flux asymmetric membranes  

DOEpatents

Defect-free, ultrahigh flux integrally-skinned asymmetric membranes having extremely thin surface layers (<0.2 .mu.m) comprised of glassy polymers are disclosed. The membranes are formed by casting an appropriate drope followed by forced convective evaporation of solvent to obtain a dry phase separated asymmetrical structure. The structure is then washed in a precipitation liquid and dried.

Pinnau, Ingo (Austin, TX) [Austin, TX; Koros, William J. (Austin, TX) [Austin, TX

1990-01-01

446

Asymmetric Price Adjustment and Economic Fluctuations  

Microsoft Academic Search

This paper considers a possible explanation for asymmetric adjustment of nominal prices. We present a menu-cost model in which positive trend inflation causes firms' relative prices to decline automatically between price adjustments. In this environment, shocks that raise firms' desired prices trigger larger price responses than shocks that lower desired prices. We use this model of asymmetric adjustment to address

Laurence Ball; N. Gregory Mankiw

1994-01-01

447

Search with learning: understanding asymmetric price adjustments  

Microsoft Academic Search

In many retail markets, prices rise faster than they fall. We develop a model of search with learning to explain this phenomenon of asymmetric price adjustments. By extending our static game analysis to the dynamic setting, we demonstrate that asymmetric price adjustments arise naturally. When a positive cost shock occurs, all the searchers immediately learn the true state; the search

Huanxing Yang; Lixin Ye

2008-01-01

448

Asymmetric laplace laws and modeling financial data  

Microsoft Academic Search

Asymmetric Laplace laws form a subclass of geometric stable distributions, the limiting laws in the random summation scheme with a geometric number of terms. Among geometric stable laws, they play a role analogous to that of normal distribution among stable Paretian laws. However, with steeper peaks and heavier tails than normal distribution, asymmetric Laplace laws reflect properties of empirical financial

K. Podgórski

2001-01-01

449

Gauge symmetries in heterotic asymmetric orbifolds  

NASA Astrophysics Data System (ADS)

We study heterotic asymmetric orbifold models. By utilizing the lattice engineering technique, we classify (22,6)-dimensional Narain lattices with right-moving non-Abelian group factors which can be starting points for Z3 asymmetric orbifold construction. We also calculate gauge symmetry breaking patterns.

Beye, Florian; Kobayashi, Tatsuo; Kuwakino, Shogo

2013-10-01

450

Asymmetric cancer cell division regulated by AKT  

PubMed Central

Human tumors often contain slowly proliferating cancer cells that resist treatment, but we do not know precisely how these cells arise. We show that rapidly proliferating cancer cells can divide asymmetrically to produce slowly proliferating “G0-like” progeny that are enriched following chemotherapy in breast cancer patients. Asymmetric cancer cell division results from asymmetric suppression of AKT/PKB kinase signaling in one daughter cell during telophase of mitosis. Moreover, inhibition of AKT signaling with small-molecule drugs can induce asymmetric cancer cell division and the production of slow proliferators. Cancer cells therefore appear to continuously flux between symmetric and asymmetric division depending on the precise state of their AKT signaling network. This model may have significant implications for understanding how tumors grow, evade treatment, and recur.

Dey-Guha, Ipsita; Wolfer, Anita; Yeh, Albert C.; G. Albeck, John; Darp, Revati; Leon, Eduardo; Wulfkuhle, Julia; Petricoin, Emanuel F.; Wittner, Ben S.; Ramaswamy, Sridhar

2011-01-01

451

Asymmetric cancer cell division regulated by AKT.  

PubMed

Human tumors often contain slowly proliferating cancer cells that resist treatment, but we do not know precisely how these cells arise. We show that rapidly proliferating cancer cells can divide asymmetrically to produce slowly proliferating "G0-like" progeny that are enriched following chemotherapy in breast cancer patients. Asymmetric cancer cell division results from asymmetric suppression of AKT/PKB kinase signaling in one daughter cell during telophase of mitosis. Moreover, inhibition of AKT signaling with small-molecule drugs can induce asymmetric cancer cell division and the production of slow proliferators. Cancer cells therefore appear to continuously flux between symmetric and asymmetric division depending on the precise state of their AKT signaling network. This model may have significant implications for understanding how tumors grow, evade treatment, and recur. PMID:21757645

Dey-Guha, Ipsita; Wolfer, Anita; Yeh, Albert C; G Albeck, John; Darp, Revati; Leon, Eduardo; Wulfkuhle, Julia; Petricoin, Emanuel F; Wittner, Ben S; Ramaswamy, Sridhar

2011-08-01

452

BASL Controls Asymmetric Cell Division in Arabidopsis  

PubMed Central

SUMMARY Development in multicellular organisms requires the organized generation of differences. A universal mechanism for creating such differences is asymmetric cell division. In plants, as in animals, asymmetric divisions are correlated with the production of cellular diversity and pattern; however, structural constraints imposed by plant cell walls and the absence of homologs of known animal or fungal cell polarity regulators necessitates that plants utilize new molecules and mechanisms to create asymmetries. Here, we identify BASL, a novel regulator of asymmetric divisions in Arabidopsis. In asymmetrically dividing stomatal-lineage cells, BASL accumulates in a polarized crescent at the cell periphery before division, and then localizes differentially to the nucleus and a peripheral crescent in self-renewing cells and their sisters after division. BASL presence at the cell periphery is critical for its function, and we propose that BASL represents a plant-specific solution to the challenge of asymmetric cell division.

Dong, Juan; MacAlister, Cora A.; Bergmann, Dominique C.

2014-01-01

453

Asymmetrically Coupled Directed Percolation Systems  

NASA Astrophysics Data System (ADS)

We introduce a dynamical model of coupled directed percolation systems with two particle species. The two species A and B are coupled asymmetrically in that A particles branch B particles, whereas B particles prey on A particles. This model may describe epidemic spreading controlled by reactive immunization agents. We study nonequilibrium phase transitions with attention focused on the multicritical point where both species undergo the absorbing phase transition simultaneously. In one dimension, we find that the inhibitory coupling from B to A is irrelevant and the model belongs to the unidirectionally coupled directed percolation class. On the contrary, a mean-field analysis predicts that the inhibitory coupling is relevant and a new universality appears with a variable dynamic exponent. Numerical simulations on small-world networks confirm our predictions.

Noh, Jae Dong; Park, Hyunggyu

2005-04-01

454

Superfluidity in asymmetric nuclear matter  

SciTech Connect

The onset of superfluidity in isospin-asymmetric nuclear matter is investigated within the BCS theory. A neutron-proton superfluid state in the channel {sup 3}S{sub 1}-{sup 3}D{sub 1} comes about from the interplay between thermal excitations and separation {delta}{mu} of the two Fermi surfaces. The superfluid state disappears above the threshold value of the density-asymmetry parameter {alpha}=(n{sub n}{minus}n{sub p})/n{approx_equal}0.35. For large enough shift between the two Fermi surfaces {delta}{mu}=(1)/(2)({mu}{sub n}{minus}{mu}{sub p}) the transition to the normal state becomes a first-order transition and a second gap solution develops. This solution, however, corresponds to a metastable superfluid state which is unstable with respect to the transition to the normal state. {copyright} {ital 1997} {ital The American Physical Society}

Sedrakian, A.; Alm, T. [Max-Planck-Gesellschaft, AG Theoretische Vielteilchenphysik, Universitaet Rostock, D-18051 Rostock (Germany)] [Max-Planck-Gesellschaft, AG Theoretische Vielteilchenphysik, Universitaet Rostock, D-18051 Rostock (Germany); Lombardo, U. [Dipartimento di Fisica and INFN, 57 Corso Italia, I-95129 Catania (Italy)] [Dipartimento di Fisica and INFN, 57 Corso Italia, I-95129 Catania (Italy)

1997-02-01

455

Total synthesis of (+)-kopsihainanine A.  

PubMed

Total synthesis of (+)-kopsihainanine A was accomplished on the basis of (i) Stoltz's enantioselective decarboxylative asymmetric allylation and (ii) the proposed biogenetic pathway from the related alkaloid, kopsihainanine B. In addition, HPLC analysis of the synthetic (+)-kopsihainanine A confirmed its ee to be 99% with [?] = 25.35. PMID:24756124

Mizutani, Masaya; Yasuda, Shigeo; Mukai, Chisato

2014-06-01

456

Stereoselective synthesis of the macrolactone core of (+)-neopeltolide.  

PubMed

A stereoselective synthesis of the macrolactone core of the potent anticancer agent neopeltolide is disclosed. The key steps of the synthesis include asymmetric allylation using Krische' protocol, conjugate reduction using MacMillan's methodology, and an asymmetric hetero-Diels-Alder reaction using Jacobsen's catalyst. Substrate controlled diastereoselective 1,3-anti reduction of a keto alcohol, Luche reduction followed by Ireland-Claisen rearrangement, oxymercuration, and reductive lithiation are other key steps. PMID:22515229

Raghavan, Sadagopan; Samanta, Pradip Kumar

2012-05-01

457

Absolute and relative dosimetry for ELIMED  

SciTech Connect

The definition of detectors, methods and procedures for the absolute and relative dosimetry of laser-driven proton beams is a crucial step toward the clinical use of this new kind of beams. Hence, one of the ELIMED task, will be the definition of procedures aiming to obtain an absolute dose measure at the end of the transport beamline with an accuracy as close as possible to the one required for clinical applications (i.e. of the order of 5% or less). Relative dosimetry procedures must be established, as well: they are necessary in order to determine and verify the beam dose distributions and to monitor the beam fluence and the energetic spectra during irradiations. Radiochromic films, CR39, Faraday Cup, Secondary Emission Monitor (SEM) and transmission ionization chamber will be considered, designed and studied in order to perform a fully dosimetric characterization of the ELIMED proton beam.

Cirrone, G. A. P.; Schillaci, F.; Scuderi, V. [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and Institute of Physics Czech Academy of Science, ELI-Beamlines project, Na Slovance 2, Prague (Czech Republic)] [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and Institute of Physics Czech Academy of Science, ELI-Beamlines project, Na Slovance 2, Prague (Czech Republic); Cuttone, G.; Candiano, G.; Musumarra, A.; Pisciotta, P.; Romano, F. [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania (Italy)] [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania (Italy); Carpinelli, M. [INFN Sezione di Cagliari, c/o Dipartimento di Fisica, Università di Cagliari, Cagliari (Italy)] [INFN Sezione di Cagliari, c/o Dipartimento di Fisica, Università di Cagliari, Cagliari (Italy); Leonora, E.; Randazzo, N. [INFN-Sezione di Catania, Via Santa Sofia 64, Catania (Italy)] [INFN-Sezione di Catania, Via Santa Sofia 64, Catania (Italy); Presti, D. Lo [INFN-Sezione di Catania, Via Santa Sofia 64, Catania, Italy and Università di Catania, Dipartimento di Fisica e Astronomia, Via S. Sofia 64, Catania (Italy)] [INFN-Sezione di Catania, Via Santa Sofia 64, Catania, Italy and Università di Catania, Dipartimento di Fisica e Astronomia, Via S. Sofia 64, Catania (Italy); Raffaele, L. [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and INFN-Sezione di Catania, Via Santa Sofia 64, Catania (Italy)] [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and INFN-Sezione di Catania, Via Santa Sofia 64, Catania (Italy); Tramontana, A. [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and Università di Catania, Dipartimento di Fisica e Astronomia, Via S. Sofia 64, Catania (Italy)] [INFN, Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, Italy and Università di Catania, Dipartimento di Fisica e Astronomia, Via S. Sofia 64, Catania (Italy); Cirio, R.; Sacchi, R.; Monaco, V. [INFN, Sezione di Torino, Via P.Giuria, 1 10125 Torino, Italy and Università di Torino, Dipartimento di Fisica, Via P.Giuria, 1 10125 Torino (Italy)] [INFN, Sezione di Torino, Via P.Giuria, 1 10125 Torino, Italy and Università di Torino, Dipartimento di Fisica, Via P.Giuria, 1 10125 Torino (Italy); Marchetto, F.; Giordanengo, S. [INFN, Sezione di Torino, Via P.Giuria, 1 10125 Torino (Italy)] [INFN, Sezione di Torino, Via P.Giuria, 1 10125 Torino (Italy)

2013-07-26

458

Absolute effective areas of the HETGS  

NASA Astrophysics Data System (ADS)

XRCF measurements of the flight AXAF High Energy Transmission Grating Spectrometer throughput were used to determine absolute effective areas. The result are compared with component models of the HRMA, HETG and the ACIS-S. The comparison provides an independent view on HETG efficiencies as well as the detector efficiencies along the dispersion direction. Using the XRCF double crystal monochromator measurements in the range from 0.9 to 8.7 keV, the effective areas in the 1st order MEG were determined with an accuracy of better than 10 percent, in the 1st order HEG better than 15 percent throughout most of the energy range. This is within the goal set for the XRCF measurements to refine state of the art composite component model predictions, which in the future will allow us to draw conclusions on the in-flight HETGS absolute effective area.

Schulz, Norbert S.; Dewey, Daniel; Marshall, Herman L.

1998-11-01

459

Absolute plate motions by boundary velocity minimizations  

NASA Technical Reports Server (NTRS)

The main interaction of the earth's interior with the lithosphere is as a material source and sink. An absolute reference frame defined by minimizing the translational motion of tectonic plate boundaries differs by 0.6 cm/year from a frame defined by hot spot traces and by 0.4 cm/year from the frame defined by the most plausible model of drag forces on the plates. The rms absolute translational velocities are about 2 cm/year for ocean-ocean plate boundaries and 1.5 cm/year for ocean-continent plate boundaries. The close agreement between the source and sink and the drag-dependent definitions suggests that the lithosphere, as a stress guide, to some extent controls the locations of its sources and sinks.

Kaula, W. M.

1975-01-01

460

Absolute Proper Motions Outside the Plane (APOP)  

NASA Astrophysics Data System (ADS)

Most of the discovered exoplanets are close to our sun. Usually their host star is with large proper motions, which is an important parameter for exoplanet searching. The first version of absolute proper motions catalog achieved based on Digitized Sky Survey Schmidt plate where outside the galactic plane |b|>=27° is presented, resulting in a zero point error less than +/- 0.3 mas/yr, and the overall accuracy better than +/- 4.5 mas/yr for objects brighter than R F =18.5, and ranging from 4.5 to 9.0 mas/yr for objects with magnitude 18.5absolute proper motions related to the position, magnitude and color are practically all removed. The sky cover of this catalog is 22,525 degree 2, the mean density is 6444 objects/degree 2 and the magnitude limit is around R F =20.5.

Qi, Zhaoxiang; Yu, Yong; Smart, Richard L.; Lattanzi, Mario G.; Tang, Zhenghong; Bucciarelli, Beatrice; Vecchiato, Alberto; Spagna, Alessandro; McLean, Brian J.

2014-04-01

461

Absolute-magnitude Distributions of Supernovae  

NASA Astrophysics Data System (ADS)

The absolute-magnitude distributions of seven supernova (SN) types are presented. The data used here were primarily taken from the Asiago Supernova Catalogue, but were supplemented with additional data. We accounted for both foreground and host-galaxy extinction. A bootstrap method is used to correct the samples for Malmquist bias. Separately, we generate volume-limited samples, restricted to events within 100 Mpc. We find that the superluminous events (MB < -21) make up only about 0.1% of all SNe in the bias-corrected sample. The subluminous events (MB > -15) make up about 3%. The normal Ia distribution was the brightest with a mean absolute blue magnitude of -19.25. The IIP distribution was the dimmest at -16.75.

Richardson, Dean; Jenkins, Robert L., III; Wright, John; Maddox, Larry

2014-05-01

462

Dynamo action by turbulence in absolute equilibrium  

NASA Astrophysics Data System (ADS)

We consider the generation of a large-scale magnetic field by a turbulent flow driven by a small-scale helical forcing in a low magnetic Prandtl number fluid. We provide an estimate of the dynamo threshold that takes into account the presence of large-scale turbulent fluctuations by considering that the scales of the flow that mostly contribute to the dynamo process are roughly in absolute equilibrium. We show that turbulent flows in absolute equilibrium do generate dynamos and we compare their growth rates to their laminar counterparts. Finally, we show that the back reaction of the growing magnetic field modifies the statistical properties of turbulent flow by suppressing its kinetic helicity at large magnetic Reynolds number.

Gopalakrishnan Ganga Prasath, Srinivasa; Fauve, Stéphan; Brachet, Marc

2014-04-01

463

An absolute measure for a key currency  

NASA Astrophysics Data System (ADS)

It is generally considered that the US dollar and the euro are the key currencies in the world and in Europe, respectively. However, there is no absolute general measure for a key currency. Here, we investigate the 24-hour periodicity of foreign exchange markets using a recurrence plot, and define an absolute measure for a key currency based on the strength of the periodicity. Moreover, we analyze the time evolution of this measure. The results show that the credibility of the US dollar has not decreased significantly since the Lehman shock, when the Lehman Brothers bankrupted and influenced the economic markets, and has increased even relatively better than that of the euro and that of the Japanese yen.

Oya, Shunsuke; Aihara, Kazuyuki; Hirata, Yoshito

464

From Hubble's NGSL to Absolute Fluxes  

NASA Technical Reports Server (NTRS)

Hubble's Next Generation Spectral Library (NGSL) consists of R-l000 spectra of 374 stars of assorted temperature, gravity, and metallicity. Each spectrum covers the wavelength range, 0.18-1.00 microns. The library can be viewed and/or downloaded from the website, http://archive.stsci.edu/prepds/stisngsll. Stars in the NGSL are now being used as absolute flux standards at ground-based observatories. However, the uncertainty in the absolute flux is about 2%, which does not meet the requirements of dark-energy surveys. We are therefore developing an observing procedure that should yield fluxes with uncertainties less than 1 % and will take part in an HST proposal to observe up to 15 stars using this new procedure.

Heap, Sara R.; Lindler, Don

2012-01-01

465

Absolute orientation of molecules at interfaces.  

PubMed

A method to determine the absolute orientation of molecules at liquid interfaces by sum frequency generation (SFG) is reported. It is based on measurements of the orientations of two nonparallel vibrationally active chromophores in the molecule of interest combined with a rotation matrix formulation to obtain the absolute molecular orientation. We chose m-tolunitrile, a planar molecule adsorbed to the air/water interface, as a proof-of-method experiment. Quantitative analysis of different polarization sum frequency intensities facilitate unique peak assignments of the methyl and nitrile groups of m-tolunitrile. The SFG analysis of the measurement yields a nitrile group tilting at 53 degrees to the surface normal, and the C3 axis of the methyl group is almost upright at 23 degrees with respect to the surface normal. Using a rotation matrix formulation, we found that the angle between the surface plane and the m-tolunitrile molecular plane is 70 degrees. PMID:16471739

Rao, Yi; Comstock, Matthew; Eisenthal, Kenneth B

2006-02-01

466

Absolute Plate Motions by Boundary Velocity Minimizations  

Microsoft Academic Search

The main interaction of the interior with the lithosphere is as a material source and sink. An absolute reference frame defined by minimizing the translational motion of tectonic plate boundaries differs by 0.6 cm\\/yr from a frame defined by hot spo_t traces and by 0.4 cm\\/yr from the frame defined by the most plausible model of drag forces on the

William M. Kaula

1975-01-01

467

On absolute CM-periods, II  

Microsoft Academic Search

For a CM-fieldK, Shimura defined the period symbolpK by factorizing periods of abelian varieties with complex multiplication. We define the absolute period symbolgK using division values of the multiple gamma function and conjecture that pK coincides with gK up to the multiplication by algebraic numbers. Taking the action of Gal(Q Q) into account, we present a refined version of this

Hiroyuki Yoshida

1998-01-01

468

Absolute photoionization cross sections of atomic oxygen  

NASA Technical Reports Server (NTRS)

The absolute values of photoionization cross sections of atomic oxygen were measured from the ionization threshold to 120 A. An auto-ionizing resonance belonging to the 2S2P4(4P)3P(3Do, 3So) transition was observed at 479.43 A and another line at 389.97 A. The experimental data is in excellent agreement with rigorous close-coupling calculations that include electron correlations in both the initial and final states.

Samson, J. A. R.; Pareek, P. N.

1985-01-01

469

Absolute photoionization cross sections of atomic oxygen  

NASA Technical Reports Server (NTRS)

The absolute values of photoionization cross sections of atomic oxygen were measured from the ionization threshold to 120 A. An auto-ionizing resonance belonging to the 2S2P4(4P)3P(3Do, 3So) transition was observed at 479.43 A and another line at 389.97 A. The experimental data is in excellent agreement with rigorous close-coupling calculations that include electron correlations in both the initial and final states.

Samson, J. A. R.; Pareek, P. N.

1982-01-01

470

Absolute negative mobility in a vibrational motor  

NASA Astrophysics Data System (ADS)

An anomalous transport phenomenon termed absolute negative mobility (ANM) was observed in a vibrational motor, where an additional time-periodic signal filled the role usually played by noise in a Brownian motor. Within a tailored parameter regime, the ANM behavior is maximized at two regimes upon variation of the bias. The observed ANM still survives at a wide range of the driving strength and angular frequency of the additional signal.

Du, Luchun; Mei, Dongcheng

2012-01-01

471

Construction of the N1-C3 linkage stereogenic centers by catalytic asymmetric amination reaction of 3-bromooxindoles with indolines.  

PubMed

The catalytic asymmetric amination reaction of 3-bromooxindoles with indolines for the construction of the N1-C3 linkage stereogenic centers has been realized for the first time. Moreover, the racemic substrates (3-substituted indolines) were also applicable under the same chiral conditions. The newly developed method conveniently led to a formal synthesis of (+)-psychotrimine. PMID:24725065

Zhang, Hailong; Kang, Hong; Hong, Liang; Dong, Weiping; Li, Guolin; Zheng, Xin; Wang, Rui

2014-05-01

472

Serum concentrations of asymmetric (ADMA) and symmetric (SDMA) dimethylarginine in renal failure patients  

Microsoft Academic Search

Serum concentrations of asymmetric (ADMA) and symmetric (SDMA) dimethylarginine in renal failure patients.BackgroundNitric oxide (NO) synthesis is inhibited by the ADMA that accumulates in the plasma of patients with renal failure; however, the concentration of SDMA also is enhanced. Therefore, it has been hypothesized that ADMA and SDMA may contribute to hypertension in these patients.MethodsWe measured the concentrations of ADMA,

CHRISTIAN FLECK; ALEXANDER JANZ; FRANK SCHWEITZER; ELKE KARGE; MARKUS SCHWERTFEGER; GÖNTER STEIN

2001-01-01

473

Recent developments in the catalytic, asymmetric construction of pyrroloindolines bearing all-carbon quaternary stereocenters.  

PubMed

Pyrroloindoline alkaloids constitute a large family of natural products that has inspired the development of an impressive array of new reactions to prepare the key heterocyclic motif. This synopsis will address catalytic, asymmetric reactions developed to synthesize pyrroloindolines bearing C3a all-carbon quaternary stereocenters. The methods described herein include both transition-metal-catalyzed and organocatalyzed reactions that have been demonstrated to be suitable for the synthesis of the pyrroloindoline framework. PMID:24295135

Repka, Lindsay M; Reisman, Sarah E

2013-12-20

474

Kinetic simulation of capacitively coupled plasmas driven by trapezoidal asymmetric voltage pulses  

NASA Astrophysics Data System (ADS)

A kinetic Particle-In-Cell simulation with Monte Carlo Collisions was performed of a geometrically symmetric capacitively coupled, parallel-plate discharge in argon, driven by trapezoidal asymmetric voltage pulses with a period of 200 ns. The discharge was electrically asymmetric, making the ion energy distributions at the two electrodes different from one another. The fraction of the period (?), during which the voltage was kept at a constant (top-flat) positive value, was a critical control parameter. For the parameter range investigated, as ? increased, the mean ion energy on the grounded electrode increased and the ions became more directional, whereas the opposite was found for the ions striking the powered electrode. The absolute value of the DC self-bias voltage decreased as ? increased. Plasma instabilities, promoted by local double layers and electric field reversals during the time of the positive voltage excursion, were characterized by electron plasma waves launched from the sheath edge.

Diomede, Paola; Economou, Demetre J.

2014-06-01

475

Optimal asymmetric economical state-dependent cloners  

NASA Astrophysics Data System (ADS)

We present the optimal asymmetric economical 1-3 phase-covariant and real state cloning in two-dimensions. The fidelity distributions of copies of two state-dependent cloners are the same, and higher than that of the optimal asymmetric universal quantum cloning. Using a quantum network, we calculate the single-qubit rotation angles to implement two symmetric economical state-dependent cloners. We also propose an experimentally feasible scheme to implement the optimal asymmetric economical 1-3 phase-covariant cloner. The scheme can be realized with the success probability of 100%.

Zhang, Wen-Hai; Ye, Liu

2009-07-01

476

Asymmetric dark matter in braneworld cosmology  

NASA Astrophysics Data System (ADS)

We investigate the effect of a braneworld expansion era on the relic density of asymmetric dark matter. We find that the enhanced expansion rate in the early universe predicted by the Randall-Sundrum II (RSII) model leads to earlier particle freeze-out and an enhanced relic density. This effect has been observed previously by Okada and Seto (2004) for symmetric dark matter models and here we extend their results to the case of asymmetric dark matter. We also discuss the enhanced asymmetric annihilation rate in the braneworld scenario and its implications for indirect detection experiments.

Meehan, Michael T.; Whittingham, Ian B.

2014-06-01

477

Lift production through asymmetric flapping  

NASA Astrophysics Data System (ADS)

At present, there is a strong interest in developing Micro Air Vehicles (MAV) for applications like disaster management and aerial surveys. At these small length scales, the flight of insects and small birds suggests that unsteady aerodynamics of flapping wings can offer many advantages over fixed wing flight, such as hovering-flight, high maneuverability and high lift at large angles of attack. Various lift generating mechanims such as delayed stall, wake capture and wing rotation contribute towards our understanding of insect flight. We address the effect of asymmetric flapping of wings on lift production. By visualising the flow around a pair of rectangular wings flapping in a water tank and numerically computing the flow using a discrete vortex method, we demonstrate that net lift can be produced by introducing an asymmetry in the upstroke-to-downstroke velocity profile of the flapping wings. The competition between generation of upstroke and downstroke tip vortices appears to hold the key to understanding this lift generation mechanism.

Jalikop, Shreyas; Sreenivas, K. R.

2009-11-01

478

Reflection asymmetric shapes in nuclei  

SciTech Connect

Experimental data show that there is no even-even nucleus with a reflection asymmetric shape in its ground state. Maximum octupole- octupole correlations occur in nuclei in the mass 224 (N{approximately}134, Z{approximately}88) region. Parity doublets, which are the characteristic signature of octupole deformation, have been observed in several odd mass Ra, Ac and Pa nuclei