PERIODATE OXIDATION OF METHYL GLYCOPYRANOSIDES: RATE COEFFICIENTS AND RELATIVE STABILITIES OF INTERMEDIATE HEMIACETALS

Article abstract of DOI:10.1016/S0008-6215(00)85230-1

For oxidation in aqueous sodium metaperiodate at 20 deg C, rate coefficients describing the decay of intact glycoside (k_G) and the consumption of periodate (k_P) were measured for the methyl α- and β-glycopyranosides of D-glucose, D-galactose, D-mannose, D-xylose, and D-(or L-)arabinose, and for methyl α-L-rhamnopyranoside.For glycosides containing a cis-1,2-diol group, k_G was four to sixteen times large than for those containing only trans-1,2-diol groups.For anomeric pairs, (k_G)_α/(k_G)_β varied from 0.6 to 1.7.The kinetics showed that the singly oxidised intermediates existed partly in an unreactive form, which was inferred to be a cyclic hemiacetal.For hexopyranosides, but not pentopyranosides, the stabilities of the hemiacetals, as indicated by the limiting values of k_P, were much greater with α than with β anomers.From a consideration of possible structures and conformations of the hemiacetals, it was inferred that this result was a manifestation of the anomeric effect.