Asymmetric induction by β-cyclodextrins in NaBH4 reduction of ketones

Abstract

Asymmetric reduction of various prochiral ketones was achieved with sodium borohydride utilizing β-CD or its derivative, mono-6-deoxy-6-[N-(2-aminoethyl)]amino-β-CD (β-CD-en) as a chiral template. It was found that pre-equilibrium between ketone and β-CD derivative and low reaction temperature increase asymmetric induction. The extent of asymmetric induction and the absolute configuration of the resulting secondary alcohols are highly dependent upon the nature of the ketones and also β-CD derivatives. A mechanistic scheme is suggested to explain the dependency.