A stable and easily prepared catalyst for the enantioselective reduction of ketones. Applications to multistep syntheses

Chemistry and Pharmacology of Analgesic Indole Alkaloids from the Rubiaceous Plant, Mitragyna speciosa

“…Synthesis of the optically pure 19 was achieved by a combination of Corey asymmetric reduction and Sharpless asymmetric epoxidation as follows. By reducing enone 16 with a chiral oxazaborolidine catalyst, 16) an optically active alcohol (R)-(ϩ)-17 was obtained in 97% ee. Next, the allylic alcohol 17 (97% ee) was subjected to Sharpless asymmetric epoxidation under kinetic resolution conditions 17) to give the (Ϫ)-epoxide 18 in Ͼ99% ee.…”

“…The racemic acetate 38, which was prepared from commercially available 6-chloronicotinic acid, was subjected to enzymatic hydrolysis using Lipase SAM II under phosphatebuffered (pH 7.0) conditions to produce the secondary alcohol (ϩ)-40 (32% chemical yield, 100% ee) and the acetate (Ϫ)-38 (38% chemical yield, 100% ee). Alternatively, by reduction of the ketone derivative 39 using a chiral oxazaborolidine catalyst, 16) an optically active alcohol (ϩ)-40 (93% ee) was obtained in 80% yield, which was then esterified with (R)-O-methylmandelic acid in order to prepare the optically pure alcohol 40. The resulting diastereomeric mixture was separated by column chromatography and then the diastereomers were respectively hydrolyzed to give the enantiomerically pure alcohols 40.…”

Ligand Design for Catalytic Asymmetric Reduction

“…[117][118][119] This reaction was originally developed as a stoichiometric system consisting of diphenylvalinol and borane, 120 but was later extended to a useful catalytic method. 121 Because of the high efficiency of this reaction, many chiral oxazaborolidines have been synthesized from b-amino alcohols. [117][118][119] Among them the prolinol-derived oxazaborolidine is one of the most widely used catalysts.…”

CBS Oxazaborolidines - Versatile Catalysts for Asymmetric Synthesis

“…1,2 Recently, the substrate scope has been extended to include oxime ethers 3 and ketimines. 4 Accordingly, these reagents have been extensively applied in natural product synthesis, for instance in the recently published total syntheses of (-)-laulimalide, 5 brevetoxin-B, 6 (+)-tanikolide, 7 and bistramide A.…”

“…9b,10 More recently, the protonated oxazaborolidinium salts 2a and 2b were used as chiral Lewis acids 11 in catalytic enantioselective Diels-Alder reactions, 12 the cyanosilylation of aldehydes, 13 Michael additions, 14 and a vinylogous Mukaiyama aldol reaction. 15 Compound 1a is typically generated in situ by reaction of (S)-or (R)-2,2-diphenylhydroxymethylpyrrolidine with BH 3 ·THF, 2 while oxazaborolidines 1b,c may be prepared using the corresponding boronic acids. 1c, 16 In a similar fashion, reaction of the same chiral pyrrolidine substrate with o-tolylboroxine furnishes o-tolyl-CBS-oxazaborolidine, 16 which, when protonated with triflic acid or triflimide, affords the oxazaborolidinium salts 2a and 2b, respectively.…”