The role of sugar substituents in glycoside hydrolysis

A series of monosubstituted deoxy and deoxyfluoro 2,4-dinitropbenyl (DNP) beta-D-glycapyranosides was synthesized and used to probe the mechanism of spontaneous beta-glycoside hydrolysis. Their relative rates of hydrolysis followed the order 2-deoxy > 4-deoxy > 3-deoxy approximate to 6-deoxy > parent > 6-deoxy-6-fluoro > 3-deoxy-3-fluoro > 4-deoxy-4-fluoro > 2-deoxy-2-fluoro. Hammett correlations of the pH-independent hydrolysis rates of each of the 6-, 4-, 3-, and 2-position substituted glycosides with the sigma(1) value for the sugar ring substituent were linear (r = 0.95 to 0.999, rho(I) = -2.2 to -10.7), consistent with hydrolysis rates being largely dictated by field effects on an electron-deficient transition state. The relative rates of hydrolysis of the DNP glucosides can be rationalized on the basis of the stabilities of the oxocarbenium ion-like transition states, as predicted by the Kirkwood-Westheimer model. The primary determinant of the rate of hydrolysis within a series appears to be the field effect of the ring substituent on O5, the principal center of charge development at the transition state. Differences in the rates of hydrolysis between different series of hexopyranosides may not arise solely from field effects and likely also reflect differences in steric factors or solvation.