The Role of Sugar Substituents in Glycoside Hydrolysis

A series of monosubstituted deoxy and deoxyfluoro 2,4-dinitrophenyl (DNP) β-d-glycopyranosides was synthesized and used to probe the mechanism of spontaneous β-glycoside hydrolysis. Their relative rates of hydrolysis followed the order 2-deoxy > 4-deoxy > 3-deoxy ≈ 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 σI value for the sugar ring substituent were linear (r = 0.95 to 0.999, ρI = −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 substi...