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Good Enantioselectivity (good + enantioselectivity)
Selected AbstractsCellulose dimethylphenylcarbamate-immobilized zirconia for chiral separation in reversed-phase CECELECTROPHORESIS, Issue 22 2009Jurim Gwon Abstract Cellulose dimethylphenylcarbamate (CDMPC)-immobilized zirconia (CDMPCZ) was used as a chiral stationary phase for enantioseparation of a set of nine racemic compounds in reversed-phase CEC. Influences of the type and composition of organic modifier and the applied voltage on enantioseparation were examined. Separation data on CDMPCZ were also compared with those on CDMPC-immobilized silica (CDMPCS). Enantiomers of the analytes investigated are well separated in ACN/phosphate buffer mobile phases. Better enantioselectivity and resolution were obtained with ACN than MeOH as the organic modifier. Retention was longer but better enantioselectivity and resolution were obtained on CDMPCZ than CDMPCS. [source] Regioselective and Enantioselective Hydroformylation of DialkylacrylamidesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2010Abstract Dimethylacrylamide can be hydroformylated with very high chemo- and regioselectivity. Asymmetric hydroformylation of this substrate is possible, provided steps are taken to minimise racemisation of the aldehyde products, and this work demonstrates the effect of various conditions and variables on racemisation. Using the Landis diazaphospholane ligands up to 68% ee can be realised under very mild conditions. Other dialkylacrylamides were also hydroformylated under mild conditions giving similar or better enantioselectivities, including the Weinreb amide of acrylic acid (71% ee), and the asymmetric hydroformylation of diethylacrylamide producing the chiral aldehyde with up to 82% ee. [source] Highly Efficient Asymmetric Michael Reaction of Aldehydes to Nitroalkenes with Diphenylperhydroindolinol Silyl Ethers as OrganocatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Ren-Shi Luo Abstract New dihydroindole and perhydroindole derivatives were synthesized in good yields and evaluated as chiral catalysts in the asymmetric Michael reaction of aldehydes to nitroalkenes. (2S,3aS,7aS)-Diphenylperhydroindolinol silyl ether 10 facilitated the reaction of a wide range of aldehyde and nitroalkene substrates, providing Michael adducts in nearly optically pure form (99% ee), good yields and high diastereoselectivities (syn/anti up to 99:1). These results show for the first time that perhydroindole derivatives can also be highly efficient organocatalysts for the asymmetric Michael reaction, exhibiting comparable or even better enantioselectivities than proline derivatives. [source] Cellulose dimethylphenylcarbamate-immobilized zirconia for chiral separation in reversed-phase CECELECTROPHORESIS, Issue 22 2009Jurim Gwon Abstract Cellulose dimethylphenylcarbamate (CDMPC)-immobilized zirconia (CDMPCZ) was used as a chiral stationary phase for enantioseparation of a set of nine racemic compounds in reversed-phase CEC. Influences of the type and composition of organic modifier and the applied voltage on enantioseparation were examined. Separation data on CDMPCZ were also compared with those on CDMPC-immobilized silica (CDMPCS). Enantiomers of the analytes investigated are well separated in ACN/phosphate buffer mobile phases. Better enantioselectivity and resolution were obtained with ACN than MeOH as the organic modifier. Retention was longer but better enantioselectivity and resolution were obtained on CDMPCZ than CDMPCS. [source] Development of Diphenylamine-Linked Bis(imidazoline) Ligands and Their Application in Asymmetric Friedel,Crafts Alkylation of Indole Derivatives with NitroalkenesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7 2010Han Liu Abstract The new diphenylamine-linked bis(imidazoline) ligands were prepared through Kelly-You's imidazoline formation procedure mediated by Hendrickson's reagent in good yields. The novel ligands were tested in the asymmetric Friedel,Crafts alkylation of indole derivatives with nitroalkenes. In most cases, good yields (up to 97%) and excellent enantioselectivities (up to 98%) can be achieved. The optimized bis(imidazoline) ligand with trans -diphenyl substitution on the imidazoline ring gave better enantioselectivity than the corresponding bis(oxazoline) ligand. [source] Catalytic Asymmetric Cycloaddition of Carbon Dioxide and Propylene Oxide Using Novel Chiral Polymers of BINOL-Salen- Cobalt(III) SaltsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009Peng Yan Abstract Four new chiral polymers of BINOL-Salen-cobalt(III) salt complexes have been designed, synthesized and applied to the direct fabrication of chiral propylene carbonate from racemic propylene oxide. The (R/S)-polymer catalyst 2 and (S/R)-polymer catalyst 4 exhibit better enantioselectivity than the (R/R)-polymer catalyst 1 and the (S/S)-polymer catalyst 3 and have been recovered and reused more than ten times without loss of activity and enantioselectivity. [source] Catalytic Asymmetric Aldol Reactions of Enolizable Carbon Pronucleophiles with Formaldehyde and Ethyl GlyoxylateADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4-5 2007Isao Fukuchi Abstract We herein describe novel catalytic asymmetric aldol reactions of acidic carbon pronucleophiles including ,-keto esters and specific ketones with reactive electrophiles such as formaldehyde and ethyl glyoxylate. In the presence of a catalytic amount of chiral Pd(II)-BINAP complexes, the hydroxymethylation of ,-keto esters with paraformaldehyde or formalin was examined, and the corresponding adducts were obtained in good yields with good to high enantioselectivity (ca. 86,% ee). In some cases, the similar Pt(II) complex also worked well to give the products with better enantioselectivity. Furthermore, these complexes were found to promote the aldol reactions of less acidic ketones, and ethyl glyoxylate underwent reaction with 3-coumaranone and ,-tetralone, affording the aldol products with a significant level of enantioselectivity of up to 83,% ee. [source] The First General, Highly Enantioselective Lewis Base Organo- catalyzed Hydrosilylation of Benzoxazinones and QuinoxalinonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Zhou-Yang Xue Abstract The first general, highly enantioselective hydrosilylation of benzoxazinones and quinoxalinones has been developed. The chiral Lewis base organocatalysts that are readily accessible from (1S,2R)-ephedrine and (1R,2S)-ephedrine promoted the title reaction to afford various chiral dihydrobenzoxazinones and dihydroquinoxalinones with good yields as well as good enantioselectivities. [source] Copper-Catalyzed Enantioselective [2+2],Cycloadditions of 2-Nitrosopyridine with KetenesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2010Indranil Chatterjee Abstract Copper(I)-catalyzed [2+2],cycloadditions of various ketenes with 2-nitrosopyridine to afford synthetically highly valuable 1,2-oxazetidine-3-ones are shown to occur with good enantioselectivities. The thermal uncatalyzed process furnishes the unstable regioisomeric oxazetidinone. Density function theory (DFT) calculations give evidence that the reaction occurs via a concerted [2+2],cycloaddtion pathway. [source] Enantioselective Synthesis of Chiral ,-Aryloxy Alcohols by Asymmetric Hydrogenation of ,-Aryloxy Aldehydes via Dynamic Kinetic ResolutionADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2009Zhang-Tao Zhou Abstract A catalytic enantioselective hydrogenation of racemic ,-aryloxy aldehydes via dynamic kinetic resolution has been developed by using (diamine)(spirodiphosphine)ruthenium(II) chloride [RuCl2(SDPs)(diamine)] catalysts. Employing this new reaction system a variety of optically active ,-aryloxy primary alcohols were synthesized in high yields and moderate to good enantioselectivities. [source] Chiral iridium(I) bis(NHC) complexes as catalysts for asymmetric transfer hydrogenationAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 7 2010Claus Diez Abstract The common use of NHC complexes in transition-metal mediated C,C coupling and metathesis reactions in recent decades has established N -heterocyclic carbenes as a new class of ligand for catalysis. The field of asymmetric catalysis with complexes bearing NHC-containing chiral ligands is dominated by mixed carbene/oxazoline or carbene/phosphane chelating ligands. In contrast, applications of complexes with chiral, chelating bis(NHC) ligands are rare. In the present work new chiral iridium(I) bis(NHC) complexes and their application in the asymmetric transfer hydrogenation of ketones are described. A series of chiral bis(azolium) salts have been prepared following a synthetic pathway, starting from L -valinol and the modular buildup allows the structural variation of the ligand precursors. The iridium complexes were formed via a one-pot transmetallation procedure. The prepared complexes were applied as catalysts in the asymmetric transfer hydrogenation of various prochiral ketones, affording the corresponding chiral alcohols in high yields and moderate to good enantioselectivities of up to 68%. The enantioselectivities of the catalysts were strongly affected by the various, terminal N -substituents of the chelating bis(NHC) ligands. The results presented in this work indicate the potential of bis-carbenes as stereodirecting ligands for asymmetric catalysis and are offering a base for further developments. Copyright © 2010 John Wiley & Sons, Ltd. [source] Asymmetric 1,4-conjugation addition of diethylzinc to cyclic enones catalyzed by a self-assembled copper complexAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 12 2009Shaohua Gou Abstract A new self-assembled catalyst system based on copper complex was developed for the 1,4-conjugation addition of diethyl zinc to cyclic enones. The self-assembled catalyst was readily prepared from (R)-[1,1,]binaphthalenyl-2,2,-diamine (1a), (L)-tryptophan (3f) and copper(I) chloride (CuCl). Several enones were found to be suitable substrates in the presence of the self-assembled copper catalyst [10 mol% (R)-1a, 10 mol% (L)-3f and 10 mol% CuCl] under optimized conditions. The desired products were afforded with high isolated yields (up to 98%) and moderate to good enantioselectivities (up to 80% ee) in mild conditions (at 0 °C). Copyright © 2009 John Wiley & Sons, Ltd. [source] Preparation of novel axially chiral NHC,Pd(II) complexes and their application in oxidative kinetic resolution of secondary alcoholsAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 5 2009Shi-Jia Liu Abstract Novel axially chiral N -heterocyclic carbene (NHC) Pd(II) complexes were prepared from optically active 1,1,-binaphthalenyl-2,2,-diamine (BINAM) and H8 -BINAM and their crystal structures were unambiguously determined by X-ray diffraction. These chiral N -heterocyclic carbene (NHC) Pd(II) complexes were applied in the oxidative kinetic resolution of secondary alcohols using molecular oxygen as a terminal oxidant or under aerobic conditions, affording the corresponding sec -alcohols in good yields with moderate to good enantioselectivities. Copyright © 2009 John Wiley & Sons, Ltd. [source] The Holy Grail of Organocatalysis: Intermolecular ,-Alkylation of AldehydesCHEMCATCHEM, Issue 4 2009Andrea-Nekane Alba This is the cup of a chemist: Organocatalytic intermolecular ,-alkylation has been accomplished. The groups of Melchiorre and Cozzi have recently reported the realization of this long-time goal in organocatalysis, by the crucial use of stabilized carbocations. Alkylated products are obtained in good yields and with moderate to good enantioselectivities. [source] Asymmetric 1,4-Addition of Oxazolones to Nitroalkenes by Bifunctional Cinchona Alkaloid Thiourea Organocatalysts: Synthesis of ,,,-Disubstituted ,-Amino AcidsCHEMISTRY - A EUROPEAN JOURNAL, Issue 35 2008José Alemán Dr. Abstract An easy and simple synthetic approach to optically active ,,,-quaternary ,-amino acids using asymmetric organocatalysis is presented. The addition of oxazolones to nitroalkenes catalyzed by thiourea cinchona derivatives provides the corresponding ,,,-quaternary ,-amino acid derivatives with good yields, excellent diastereoselectivities (up to 98,% dr), and from moderate to good enantioselectivities (up to 92,% ee). The reaction can be performed on a large scale. The optically active oxazolone,nitroalkene addition products can be opened in a one-pot reaction to the corresponding ester,amide derivatives. Additional transformations are also presented, such as the synthesis of amino esters, amino acids, and transformation into 3,4-disubstituted pyrrolidin-2-ones. [source] Catalytic Asymmetric Bromination and Chlorination of ,-KetoestersCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2004Mauro Marigo Abstract The first general catalytic asymmetric bromination and chlorination of ,-ketoesters has been developed. The reactions proceed for both acyclic and cyclic ,-ketoesters catalyzed by chiral bisoxazolinecopper(II) complexes giving the corresponding optically active ,-bromo- and ,-chloro-,-ketoesters in high yields and moderate to good enantioselectivities. For the optically active chlorinated products the isolated yields are in the range of 88,99,% and the enantiomeric excesses up to 77,% ee, while the optically active brominated adducts are formed in 70,99,% isolated yield and up to 82,% ee. Based on the absolute configuration of the optically active products, the face selectivity for the catalytic enantioselective halogenation is discussed based on a bidentate coordination of the ,-ketoester to the chiral catalyst and a X-ray structure of chiral ,,,-diketoesterenolatebisoxazolinecopper(II) complex. [source] Readily available sulfamide,amine alcohols for enantioselective phenylacetylene addition to aldehydes in the absence of Ti(OiPr)4CHIRALITY, Issue 5 2005Jincheng Mao Abstract Ephedrine-derived sulfamide,amine alcohol 3 was found to be an effective catalyst for the asymmetric phenylacetylene addition to aldehydes at room temperature without using Ti(OiPr)4 and Zn(OTf)2. It afforded the propargylic alcohols in high yields (up to 99%) and good enantioselectivities (up to 84% ee), which were much higher than that based on N -methylephedrine under the same reaction conditions. Its weakly coordinative sulfonamide moiety of the ligand plays an important role for further acceleration and stereocontrol in the alkynylation. Chirality 17:245,249, 2005. © 2005 Wiley-Liss, Inc. [source] Potent New Heterogeneous Asymmetric CatalystsHELVETICA CHIMICA ACTA, Issue 5 2003Jacques Rouzaud A set of new, air-stable, RhI -based heterogeneous asymmetric hydrogenation catalysts have been synthesised, characterised, and tested. Individual members of this new family all exhibit good enantioselectivity. [source] Silver Acetate/TF-BiphamPhos-Catalyzed endo -Selective Enantioselective 1,3-Dipolar Cycloaddition of Azomethine Ylides with Vinyl Phenyl SulfoneADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 18 2009Gang Liang Abstract The first catalytic endo -selective 1,3-dipolar cycloaddition of azomethine ylides and vinyl phenyl sulfone has been developed successfully. The highly efficient silver acetate (AgOAc)/TF-BiphamPhos catalytic system exhibited high reactivity, excellent diastereoselectivity (>98:2), good enantioselectivity (67,92% ee) and broad substrate scope under mild conditions. [source] Design and Application of 2,2-Dibromodimedone as Organic Brominating Reagent for Asymmetric Bromination of 1,3-Dicarbonyl Compounds and Ketones Catalysed by Chiral Amino AcidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2009Papori Goswami Abstract A green and ecofriendly enantioselective ,-bromination of carbonyl and 1,3-dicarbonyl compounds is reported involving the synthesis of a novel organic brominating source. The organic brominating reagent can be recovered after each cycle, rebrominated and reused. The reaction is catalysed by chiral amino acids and completed within a short reaction time with good enantioselectivity and exclusive formation of only ,-monobrominated carbonyl compounds. [source] Enantioselective Trifunctional Organocatalysts for Rate- Enhanced Aza-Morita,Baylis,Hillman Reactions at Room TemperatureADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2009Jean-Marc Garnier Abstract A Brønsted acid-activated trifunctional organocatalyst, based on the BINAP scaffold, was used for the first time to catalyze aza-Morita-Baylis,Hillman reactions between N -tosylimines and methyl vinyl ketone with fast reaction rates and good enantioselectivity at room temperature. This trifunctional catalyst, containing a Lewis base, a Brønsted base, and a Brønsted acid, required acid activation to confer its enantioselectivity and rate improvement for both electron-rich and electron-deficient imine substrates. The role of the amino Lewis base of 1a was investigated and found to be the activity switch in response to an acid additive. The counterion of the acid additive was found to influence not only the excess ratio but also the sense of asymmetric induction. [source] Osmium-Catalyzed Olefin Dihydroxylation and Aminohydroxylation in the Second Catalytic CycleADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2006Peng Wu Abstract Two catalytic cycles operate in the osmium-catalyzed olefin dihydroxylation and aminohydroxylation. Slow hydrolysis of the Os(VI) monoglycolate (or monoazaglycolate in aminohydroxylation) intermediate often results in the addition of another molecule of olefin thereby shunting the catalysis into the second catalytic cycle. As a result, both enantio- and chemoselectivity are reduced. A series of new chelating ligands were devised, which force the catalysis into the second cycle while maintaining enantiocontrol in the olefin addition step. Excellent catalytic turnover and moderate to good enantioselectivity were achieved. [source] Asymmetric Sulfonium Ylide Mediated Cyclopropanation: Stereocontrolled Synthesis of (+)-LY354740CHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2006Varinder K. Aggarwal Prof. Abstract The reaction of ester-stabilized sulfonium ylides with cyclopentenone to give (+)- 5 ((1S,5R,6S)-ethyl 2-oxobicyclo[3.1.0]hexane-6-carboxylate), an important precursor to the pharmacologically important compound (+)-LY354740, has been studied using chiral sulfides operating in both catalytic (sulfide, Cu(acac)2, ethyl diazoacetate, 60,°C) and stoichiometric modes (sulfonium salt, base, room temperature). It was found that the reaction conditions employed had a major influence over both diastereo- and enantioselectivity. Under catalytic conditions, good enantioselectivity with low diastereoselectivity was observed, but under stoichiometric conditions low enantioselectivity with high diastereoselectivity was observed. When the stoichiometric reactions were conducted at high dilution, diastereoselectivity was reduced. This indicated that base-mediated betaine equilibration was occurring (which is slow relative to ring closure at high dilution). Based on this model, conditions for achieving high enantioselectivity were established as follows: use of a preformed ylide, absence of base, hindered ester (to reduce ylide-mediated betaine equilibration), and low concentration. Under these conditions high enantioselectivity (95,% ee) was achieved, albeit with low diastereocontrol. Our model for selectivity has been applied to other sulfonium ylide mediated cyclopropanation reactions and successfully accounts for the diastereoselectivity observed in all such reported reactions to date. [source] A New Pyridine-2,6-bis(oxazoline) for Efficient and Flexible Lanthanide-Based Catalysts of Enantioselective Reactions with 3-Alkenoyl-2-oxazolidinonesCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2005Giovanni Desimoni Prof. Abstract A new pyridine-2,6-bis(oxazoline) (4) has been easily synthesised from the reaction of (1S,2S)-2-amino-1-phenylpropane-1,3-diol (1) and dimethyl pyridine-2,6-dicarboximidate (2), followed by TIPS (TIPS=triisopropylsilyl) protection of the 4,-CH2OH group. The catalysts derived from 4 and eight lanthanide(III) triflates have been tested over three reactions involving 3-acryloyl- and 3-crotonoyloxazolidinones (5,a,b): the Diels,Alder (DA) reaction with cyclopentadiene, the 1,3-dipolar cycloaddition with diphenyl nitrone and the Mukaiyama,Michael reaction with 2-trimethylsilyloxyfuran. Several reactions exhibit very good enantioselectivity (ee>90,%), and the opposite enantiomers can be easily obtained simply by changing the cation. This specific feature of the ligand can be appreciated in the DA reaction of 5,a, since the catalyst [ScIII(4)] gives the adduct (2,S)- 9,a with 99,% ee, whereas the catalyst [YIII(4)] gives the opposite enantiomer with 95,% ee. A rationale of the enantioselectivity is proposed on the basis of the NMR spectra of La-based complexes involving 4 and 5 as ligands. [source] PdII -Catalyzed Asymmetric Addition Reactions of 1,3-Dicarbonyl Compounds: Mannich-Type Reactions with N -Boc Imines and Three-Component AminomethylationCHEMISTRY - AN ASIAN JOURNAL, Issue 8-9 2008Yoshitaka Hamashima Dr. Abstract This paper describes catalytic asymmetric Mannich-type reactions of ,-ketoesters and malonates using chiral palladium complexes. Although readily enolizable, carbonyl compounds are attractive substrates, the use of such compounds as nucleophiles in Mannich-type reactions has not been extensively investigated. In the presence of chiral Pd aqua complexes (2.5,mol,%), ,-ketoesters reacted with various N -Boc imines (Boc=tert -butoxycarbonyl) to afford the desired ,-aminocarbonyl compounds in good yield with high to excellent stereoselectivity (up to 96:4 d.r., 95,99,%,ee in most cases). In these reactions, construction of highly crowded vicinal quaternary and tertiary carbon centers was achieved in one step. A chiral Pd enolate is considered to be the key intermediate. To elucidate the stereoselectivity observed in the reaction, possible transition-state models are discussed, taking into account steric and stereoelectronic effects. Furthermore, this catalytic system was applied to the Mannich-type reaction of malonates with N -Boc imine as well as one-pot classical aminomethylation of ,-ketoesters using benzylamine salt and formalin. The reactions proceeded smoothly, and the corresponding Mannich products were obtained in high yield with moderate to good enantioselectivity. [source] 3-Substituted BINOL Schiff bases and their reductive products for catalytic asymmetric addition of diethylzinc to aldehydesCHIRALITY, Issue 7 2008Bing Liu Abstract Three new chiral 3-substituted BINOL Schiff bases and their reductive 3-arylaminomethyl BINOL products have been synthesized and used for the asymmetric addition reaction of diethylzinc to aldehydes in the presence of titanium tetraisopropoxide. The reaction provided optically active secondary alcohols in high yield (86,100%) and good enantioselectivity (up to 92% ee). Chirality, 2008. © 2008 Wiley-Liss, Inc. [source] Preparation and enantioseparation characteristics of two chiral stationary phases based on mono(6A -azido-6A -deoxy)-perphenylcarbamoylated ,- and ,-cyclodextrinCHIRALITY, Issue 9 2004Xiang-Hua Lai Abstract Two chiral stationary phases, ph-,-CD and ph-,-CD, were prepared from mono(6A -azido-6A -deoxy)perphenylcarbamoylated ,- and ,-cyclodextrin immobilized onto silica gel via the Staudinger reaction. The chromatographic characteristics of these two chiral stationary phases were evaluated. The influence of different cyclodextrins (CDs) on the enantioselectivities was also investigated in this study. Compared to ph-,-CD, ph-,-CD exhibited quite good enantioselectivity toward the analytes with bulky molecular structures. It was found that the formation of inclusion complex might play a quite important role in the chiral recognition not only under reverse phases but also under normal phases. Chirality 16:592,597, 2004. © 2004 Wiley-Liss, Inc. [source] Novel chiral sulfur-containing ferrocenyl ligands for palladium-catalyzed asymmetric allylic substitutionCHIRALITY, Issue 1 2004Shuichi Nakamura Abstract New chiral ferrocenyl ligands having chiral sulfinyl and phosphinyl groups were prepared. The palladium-catalyzed allylic substitution reaction using these chiral ligands showed good enantioselectivity. The mechanism for the asymmetric reaction is proposed on the basis of the stereochemical outcome. Chirality 16:10,12, 2004. © 2003 Wiley-Liss, Inc. [source] Novel cinchona carbamate selectors with complementary enantioseparation characteristics for N-acylated amino acidsCHIRALITY, Issue S1 2003Karl Heinz Krawinkler Abstract The synthesis and chromatographic evaluation of the enantiomer separation capabilities of covalently immobilized calix[4]arene-cinchona carbamate hybrid type receptors derived from quinine (QN) and its corresponding C9-epimer (eQN) in different solvents are reported. The receptors display complementary enantiomer separation profiles in terms of elution order, chiral substrate specificity, and mobile phase characteristics, indicating the existence of two distinct chiral recognition mechanisms. The QN-derived receptor binds the (S)-enantiomers of N-acylated amino acids more strongly, shows preferential recognition of open-chained amino acids, and superior enantioselectivity in polar media such as methanol/acetic acid. In contrast, the eQN congener preferentially recognizes the corresponding (R)-enantiomers, displays good enantioselectivity (, up to 1.74) for cyclic amino acids, and enhanced stereodiscriminating properties in apolar mobile phases, e.g., chloroform/acetic acid. A comparison of the enantiomer separation profiles with those of the corresponding QN and eQN tert -butyl carbamate congeners indicates no significant level of cooperativity between the calix[4]arene module and the cinchona units in terms of overall chiral recognition, most probably as a consequence of residual conformational flexibility of the calixarene module and the carbamate linkage. Chirality 15:S17,S29, 2003. © 2003 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||