Studies on the asymmetric reduction of β-oximino methyl ether boronates: reagent control, double diastereocontrol and transmitted remote asymmetric induction

Abstract

High asymmetric induction (94% ee) could be obtained in the reduction of the achiral E-oxime ether boronate 5 with a homochiral oxazaborolidine 13–BH₃–THF complex. Application of this homochiral reducing agent system to non-aromatic oxime ethers 21 produced low to moderate asymmetric induction. Application of the same homochiral reducing agent system to reduction of homochiral boronate E-oximes 3 and 4 produced extreme double diastereoselection effects, i.e. 8 and 95% ee respectively, which demonstrated that remote asymmetry was being ‘transmitted’ by a suitable choice of a ‘partner’ molecule from the boronate moiety to the oxime during reduction. However, attempts to obtain direct remote asymmetric induction in the reduction of homochiral β-oximino boronate methyl ethers 3 and 4 to the corresponding amines produced zero, with for example BH₃–THF complex, up to 28% ee with an Et₃N–BH₃–THF mixture (after oxidative boronate ester cleavage).