An Apparent Stereochemical Effect in MnO2 Oxidation of Some Allylic Alcohols

Abstract

3,4-Di-O-acetyl-D-xylal undergoes acid-catalyzed reaction with ethanol to give the anomeric mixture of ethyl 2,3-dideoxy-4-O-acetyl-D-glycero-pent-2-enopyranosides which can be separated after deacetylation. The α-D anomer 1 is readily oxidized by manganese dioxide to the α,β-unsaturated ketone but the β-D anomer 2 is inert. Thorough investigation of the 100 and 220 MHz n.m.r. spectra of 1 and 2, their acetates, and dinitrobenzoates indicates that each exists conformationally pure in the half-chair form having the benefit of the anomeric effect. Hence in the oxidizable anomer 1 the C-4 allylic hydroxyl group is pseudo-equatorial; in the unoxidizable one it is pseudoaxial. Hydrogen bonding does not appear to be the reason for failure of 2 to oxidize.The ethoxy groups of 1 and 2 are found to be ABX3 system with unusually large separation for the AB protons.