Mechanism of hydrolysis of isopropenyl glucopyranosides.: Spectroscopic evidence concerning the greater reactivity of the α anomer

The mechanism of hydrolysis of isopropenyl alpha- and beta-glucopyranosides (1 and 2, respectively) has been studied by temperature-dependent reaction kinetics, stereochemical analysis of products by H-1 NMR, solvent O-18- and H-2-labeling experiments, kinetic solvent deuterium isotope effects, and a-values for general acid catalysis. Compounds 1 and 2 are the first vinyl acetals to be shown to undergo hydrolysis exclusively by cleavage of the vinyl ether and not the acetal C-O bond. While both glucopyranosides undergo irreversible, rate-limiting C-protonation, 1 hydrolyzes approximately four times faster than 2 and has an enthalpy of activation that is 5.8 kcal mol(-1) lower than that of 2, suggesting that 1 is kinetically more basic than 2. Spectroscopic evidence indicates that conjugation of the glycosidic oxygen with the alkenyl double bond is greater in 1 than in 2, probably because of steric or conformational constraints.