THE METHYLGLYCOSIDES OF THE NATURALLY OCCURRING HEXURONIC ACIDS

THE ACID HYDROLYSIS OF GLYCOSIDES: II. EFFECT OF SUBSTITUENTS AT C-5

First-order rate coefficients at three temperatures, and energies and entropies of activation, have been determined for the acid-catalyzed hydrolysis of methyl glucopyranosides containing various substituents at C-5 and for glycopyranosiduronic acids with different aglycones. Substitution at C-5 increased the stability towards acids of methyl α- and β-D-glucopyranosides, but there was no correlation between either the polarity or the size of the substituent and the rates of hydrolysis. The operation of either an inductive or a conformational effect alone was accordingly deemed unlikely.Methyl α- and β-D-glucopyranosiduronic acids and methyl α-D-galactopyranosiduronic acid were only slightly more stable towards acids than the glycoside analogs, while benzyl β-D-glucopyranosiduronic acid was three times more stable. The presence of a methyl ester group at the carboxyl function increased the stability of the glycuronide bond. Isopropyl, n-butyl, isobutyl, and neopentyl β-D-glucopyranosiduronic acids were hydrolyzed approximately twice and cyclohexy β-D-glucopyranosiduronic acid five times as fast as the corresponding glucosides. This appears to be the first time that glycuronides have been found to be hydrolyzed by dilute acid at a higher rate than their glycoside analogs.The energies and, especially, the entropies of activation were, throughout, lower for the glycuronides than for the glycosides. The difference in entropy suggests that the two classes of compounds are hydrolyzed by different mechanisms.