Epoxidation with triphenylphosphine and diethyl azodicarboxylate. II. Methyl 4,6-O-benzylidene-D-aldohexopyranosides

Abstract

Epoxidation of the conformationally rigid diaxial diol methyl 4,6-O-benzylidene-α-D-altropyranoside with triphenylphosphine and diethyl azodicarboxylate proceeds in high yield under very mild conditions to give the corresponding 2,3-anhydro-D-mannoside. By contrast, the corresponding diequatorial diol, methyl 4,6-0-benzylidene-α-D-glucopyranoside, undergoes epoxidation only under forcing conditions and gives the D-allo isomer. Evidence is presented for the formation of a cyclic phosphorane intermediate in the latter case.