Showing papers by "Xiaohua Liu published in 2009"
TL;DR: A series of amide-based bifunctional organocatalysts utilizing the combination of various Lewis base functional groups, such as guanidine, N-oxide and phosphane, with an amide moiety with unique reactivity and selectivity in asymmetric reactions are utilized.
187 citations
TL;DR: Both the guanidine group and the NH proton of the amide were important for the dual activation and the method allows the efficient synthesis of optically pure beta-amino acid esters.
Abstract: Zwei Aktivierungen sind besser als eine: Das chirale difunktionelle Guanidin 1, das ein Aminoamid-Ruckgrat enthalt, katalysiert die asymmetrische Michael-Addition unterschiedlichster Substrate unter milden Bedingungen und liefert die Produkte mit exzellenten Stereoselektivitaten. Die Methode ermoglicht auserdem die effiziente Synthese optisch reiner β-Aminosaureester. Die Guanidingruppe und das NH-Proton des Amids sind fur die doppelte Aktivierung zustandig.
156 citations
TL;DR: A facile and efficient enantioselective Strecker reaction of ketimines catalyzed by a chiral alkali-metal salt has been developed and trimethylsilyl cyanide was indicated to be the real reactive nucleophile despite the existence of PBAP and a possible working model was proposed to explain the origin of the asymmetric induction.
Abstract: A facile and efficient enantioselective Strecker reaction of ketimines catalyzed by a chiral alkali-metal salt has been developed. When 10 mol % BNPNa (BNP = 1,1'-binaphthyl-2,2'-diylphosphate) prepared in situ and 10 mol % para-tert-butyl-ortho-adamantylphenol (PBAP) were introduced into the reaction, up to 96% yield and up to 95% ee (ee = enantiomeric excess) were obtained. Both aliphatic and aromatic ketimines, especially sterically bulky cyclic ketimines derived from beta-acetonaphthone, alpha-indanone, and alpha-tetralone were found suitable for this reaction. On the basis of the experimental results and previous reports, trimethylsilyl cyanide (TMSCN) was indicated to be the real reactive nucleophile despite the existence of PBAP, and a possible working model was proposed to explain the origin of the asymmetric induction. The facile availability of 1,1'-binaphthyl-2,2'-diylphosphoric acid (BNPH) and the simplicity of the procedure are beneficial for practical applications.
90 citations
TL;DR: CNCOOEt was successfully employed for the first time in the asymmetric Strecker reaction of aldimines and ketimines, affording various alpha-amino nitriles with excellent yields and ee values (up to > 99% yield and >99% ee).
Abstract: Full investigation of cyanation of aldehydes, ketones, aldimines and ketimines with trimethylsilyl cyanide (TMSCN) or ethyl cyanoformate (CNCOOEt) as the cyanide source has been accomplished by employing an in situ generated catalyst from cinchona alkaloid, tetraisopropyl titanate [Ti(OiPr)(4)] and an achiral modified biphenol. With TMSCN as the cyanide source, good to excellent results have been achieved for the Strecker reaction of N-Ts (Ts=p-toluenesulfonyl) aldimines and ketimines (up to >99% yield and >99% ee) as well as for the cyanation of ketones (up to 99% yield and 98% ee). By using CNCOOEt as the alternative cyanide source, cyanation of aldehyde was accomplished and various enantioenriched cyanohydrin carbonates were prepared in up to 99% yield and 96% ee. Noteworthy, CNCOOEt was successfully employed for the first time in the asymmetric Strecker reaction of aldimines and ketimines, affording various alpha-amino nitriles with excellent yields and ee values (up to >99% yield and >99% ee). The merits of current protocol involved facile availability of ligand components, operational simplicity and mild reaction conditions, which made it convenient to prepare synthetically important chiral cyanohydrins and alpha-amino nitriles. Furthermore, control experiments and NMR analyses were performed to shed light on the catalyst structure. It is indicated that all the hydroxyl groups in cinchona alkaloid and biphenol complex with Ti(IV), forming the catalyst with the structure of (biphenoxide)Ti(OR*)(OiPr). The absolute configuration adopted by biphenol 4 m in the catalyst was identified as S configuration according to the evidence from control experiments and NMR analyses. Moreover, the roles of the protonic additive (iPrOH) and the tertiary amine in the cinchona alkaloid were studied in detail, and the real cyanide reagent in the catalytic cycle was found to be hydrogen cyanide (HCN). Finally, two plausible catalytic cycles were proposed to elucidate the reaction mechanisms.
86 citations
TL;DR: In this article, a type of C2-symmetric secondary amine amide catalysts were developed for the asymmetric Michael addition of 4-hydroxycoumarin to α,β-unsaturated ketones.
60 citations
TL;DR: In this article, a highly enantioselective fluorescent sensor, containing benzylaminomethyl groups at 3,3′-position of 1,1′-bi-2-naphthol (BINOL), has been used to conduct the chiral recognition of α-amino acid derivatives.
38 citations
TL;DR: An efficient enantioselective Friedel-Crafts alkylation of indoles to ethyl glyoxylate catalyzed by chiral N,N'-dioxide-Sc(III) complex was developed, and the corresponding 3-indolyl(hydroxy)acetates compounds were afforded in good yields with high enantiOSElectivities.
Abstract: An efficient enantioselective Friedel−Crafts alkylation of indoles to ethyl glyoxylate catalyzed by chiral N,N′-dioxide−Sc(III) complex was developed. The corresponding 3-indolyl(hydroxy)acetates compounds were afforded in good yields with high enantioselectivities (up to 95% ee).
33 citations
TL;DR: In this paper, the NH-Proton des Amids and Guanidin 1, a chirale difunktionelle Aminoamid-Ruckgrat, katalysiert die asymmetrische Michael-Addition unterschiedlichster Substrate unter milden Bedingungen and liefert die Produkte with exzellenten Stereoselektivitaten.
Abstract: Zwei Aktivierungen sind besser als eine: Das chirale difunktionelle Guanidin 1, das ein Aminoamid-Ruckgrat enthalt, katalysiert die asymmetrische Michael-Addition unterschiedlichster Substrate unter milden Bedingungen und liefert die Produkte mit exzellenten Stereoselektivitaten. Die Methode ermoglicht auserdem die effiziente Synthese optisch reiner β-Aminosaureester. Die Guanidingruppe und das NH-Proton des Amids sind fur die doppelte Aktivierung zustandig.
9 citations
TL;DR: In this article, a bifunctional titanium catalyst system has been developed for the asymmetric direct-type aldol reaction of ethyl diazo-acetate with aldehydes, which produced the desired products in good yields with excellent enantioselectivities (up to 94% ee).
Abstract: A bifunctional titanium catalyst system has been developed for the asymmetric direct-type aldol reaction of ethyl diazo-acetate with aldehydes, which produced the desired products in good yields (up to 83%) with excellent enantioselectivities (up to 94% ee). A wide range of aromatic, heteroaromatic and aliphatic aldehydes were found to be suitable substrates in the presence of (S)-BINOL (5 mol%), cinchonine (5 mol%), Ti(Oi-Pr) 4 (5 mol%) and H 2 O (15 mol%). On the basis of the experimental results and previous reports, a possible working model has been proposed to explain the origin of the activation and asymmetric induction.
3 citations
TL;DR: A facile and efficient enantioselective Strecker reaction of ketimines catalyzed by a chiral alkali-metal salt has been developed in this article, where 10 mol % BNPNa (BNP = 1,1'-binaphthyl-2,2'-diylphosphate) prepared in situ and 10 mol percent para-tert-butyl-ortho-adamantylphenol (PBAP) were introduced into the reaction, up to 96% yield and up to 95% ee (ee = enanti
Abstract: A facile and efficient enantioselective Strecker reaction of ketimines catalyzed by a chiral alkali-metal salt has been developed. When 10 mol % BNPNa (BNP = 1,1'-binaphthyl-2,2'-diylphosphate) prepared in situ and 10 mol % para-tert-butyl-ortho-adamantylphenol (PBAP) were introduced into the reaction, up to 96% yield and up to 95% ee (ee = enantiomeric excess) were obtained. Both aliphatic and aromatic ketimines, especially sterically bulky cyclic ketimines derived from beta-acetonaphthone, alpha-indanone, and alpha-tetralone were found suitable for this reaction. On the basis of the experimental results and previous reports, trimethylsilyl cyanide (TMSCN) was indicated to be the real reactive nucleophile despite the existence of PBAP, and a possible working model was proposed to explain the origin of the asymmetric induction. The facile availability of 1,1'-binaphthyl-2,2'-diylphosphoric acid (BNPH) and the simplicity of the procedure are beneficial for practical applications.