Bakers yeast-mediated transformations of alpha-keto epoxides

Abstract

Alpha beta-Epoxy ketones on treatment with baker's yeast yield different types of products depending on their substitution. Small groups such as H or Me attached at the epoxy end protect that end from attack. Thus, 1-acyl epoxides with H, methyl or propyl as the 2-epoxy substituent give solely the epoxy alcohol product with moderate stereoselectivity (13-64% d.e.). With a 2-phenyl substituent the sole product is the 1.2.3-triol as a single racemic diastereoisomer derived by a reduction/hydrolysis sequence involving a syn ring-opening of the epoxide. More than one enzyme is probably involved in both of these transformations which tend to favour S-reduction. The detailed mechanism of product formation in both processes has been undertaken and the formation of the triols has been shown by O-18 labelling studies to involve asymmetric reduction of the ketone and a double inversion during epoxide ring-opening involving a carbocation intermediate.