Novel oxidations of methyl glycopyranosides by periodic acid in dimethyl sulfoxide

Abstract

The following methyl glycopyranosides reduced only 1 mole of oxidant when oxidized by periodic acid in dimethyl sulfoxide: methyl β-l-arabinopyranoside, methyl α-d-lyxopyranoside, methyl β-d-ribopyranoside, methyl β-d-xylopyranoside, methyl α-d-galactopyranoside, methyl α-d-glucopyranoside, methyl α-d-mannopyranoside, and methyl α-l-rhamnopyranoside. A polysaccharide, Dextran 500 (an α, 1 → 6 linked glucan), also reduced only 1 mole of oxidant per anhydrohexose unit when oxidized under the same conditions. Selective cleavage of the C2—C3 bond was observed in the oxidations of the lyxoside, riboside, xyloside, and rhamnoside. The C3—C4 bond was oxidized specifically in the arabinoside and galactoside. Nonselective oxidation occurred in the glucoside, mannoside, and dextran. Rapid cyclization of the initial oxidation products to give an intramolecular hemiacetal is the probable explanation of the limited oxidations in dimethyl sulfoxide. The product of such a cyclization was isolated as a crystalline acetate from the oxidation of methyl β-l-arabinopyranoside, and its structure was established by nuclear magnetic resonance spectroscopy. Most of the stereoselective oxidations could be explained by assuming that vicinal glycol groups in an axial–equatorial orientation were oxidized faster than those in an equatorial–equatorial arrangement.