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Asymmetric Inducing Effect Of Substituents In Chiral Oxazaborolidines On Enantioselective Borane Reduction Of Ketones.
Published 1994 · Chemistry
Asymmetric inducing effects of substituents attached at nitrogen, the 5-position and boron in oxazaborolidine rings on asymmetric borane reduction of ketones were investigated. Thus, the effect of N-substituents examined with the oxazaborolidines prepared from (lR,2S)-N-alkyl norephedrine derivatives showed the remarkable decrease of enantioselectivities of the product alcohols by the variation of the steric size of alkyl groups on nitrogen from Men-Bu(Bn) neopentyli-Pr, such as 83 ee with 5b, 22 ee with 5c, 23 ee with 5f, 16 ee with 5e, and 3 ee with 5d for the reduction of acetophenone. The presence of diphenyl groups at the 5-position enhanced the enantioselectivities dramatically. The effect of B-alkyl substituents in the oxazaborolidines derived from (lR,2S)-ephedrine showed that the enantioselectivities of product alcohols decreased gradually when the substituents were changed from hydrogen to steric bulky groups such as methyl, n-butyl, thexyl and phenyl.