Bulletin of the Korean Chemical Society

Korean Chemical Society (대한화학회)
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Asymmetric Inducing Effect of Substituents in Chiral Oxazaborolidines on Enantioselective Borane Reduction of Ketones

  • Published : 1994.12.20
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Abstract

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${\leftrightarro}$n-Bu(${\simeq}$Bn)${\leftrightarro}$ neopentyl${\leftrightarro}$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.

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References

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Cited by

  1. ChemInform Abstract: Asymmetric Inducing Effect of Substituents in Chiral Oxazaborolidines on Enantioselective Borane Reduction of Ketones. vol.26, pp.32, 1994, https://doi.org/10.1002/chin.199532053