Regio- and Stereocontrolled Formation of Chiral Epoxy Oxazolidines via Bromocarbamation of <i>N</i>-Boc Alkenyl Oxazolidines. Application to Asymmetric Synthesis

Abstract

Treatment of alpha-alkenyl N-Boc oxazolidines with N-bromosuccinimide leads to epoxy oxazolidines via a bromocyclocarbamation reaction which is completely stereoselective. Action of sodium azide on these epoxides, followed by a few functional group manipulations, eventually affords chiral beta-amino alcohols which are intermediates for the enantioselective synthesis of bioactive products: the anti side chain of taxol and a hydroxyethylamine isostere. Both the bromocarbamation cyclization and the nucleophilic cleavage of epoxides are totally regioselective. AM1 calculations suggest that this selectivity is controlled by the positive charge distribution at the electrophilic centers.