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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 15, Number 12
BKCSDE 15(12)
December 20, 1994 

Asmmetric Inducing Effect of Substituents in Chiral Oxazaborolidines on Enantioselective Borane Reduction of Ketones
Byung Tae Cho*, Mi Hae Ryu
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 Me→n-Bu(-Bn)→ neopentyl→i-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.
1080 - 1084
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