Formation of glycosides by epoxidation-ring closure of open-chain hydroxyenol ethers obtained from sugars

Z- and E-Hydroxyenol ethers, obtained from aldopentoses by a Horner–Wittig reaction, have been epoxidized using m-chloroperbenzoic acid or t-butyl hydroperoxide. A spontaneous intramolecular cyclization of the intermediate epoxides due to the free hydroxy group present in the starting enol ethers occurs, giving 1,2-trans-glycopyranosides from E-enol ethers. From Z-enol ethers, 1,2-cis-glycopyranosides are obtained, although in the case of complex aglycones the cyclization is not entirely stereospecific. The stereochemical course of both the epoxidation reaction and the pyranose ring formation from the intermediate epoxides is discussed.