Synthesis of a series of phenylacetic acid 1-β-O-acyl glucosides and comparison of their acyl migration and hydrolysis kinetics with the corresponding acyl glucuronides

Abstract

We report the synthesis of the 1-β-O-acyl glucoside conjugates of phenylacetic acid (PAA), R- and S-α-methyl-PAA and α,α′-dimethyl-PAA, and measurement of their transacylation and hydrolysis reactivity by NMR methods. These are analogues of acyl glucuronides, the transacylation kinetics of which could be important in adverse drug effects. One aim of this work was to investigate whether, as previously postulated, the free carboxylate group of the acyl glucuronides plays a part in the mechanism of the internal acyl migration. In addition, such acyl glucosides are known to be endogenous biochemicals in their own right and investigation of their acyl migration propensities is novel. Our previously described selective acylation procedure has proved highly successful for 1-β-O-acyl glucuronide synthesis and when subsequently applied to 6-O-trityl glucose, it gave good yields and excellent anomeric selectivity. Mild acidolysis of the O-trityl intermediates gave the desired acyl glucosides in excellent yield with essentially complete β-selectivity. Measurement of the acyl glucoside transacylation kinetics by ¹H NMR spectroscopy, based simply on the disappearance of the 1-β-isomer in aqueous buffer at pH 7.4, showed marked differences depending on the degree of methyl substitution. Further kinetic modelling of the isomerisation and hydrolysis reactions of the acyl glucosides showed considerable differences in kinetics for the various isomeric reactions. Reactions involving the –CH₂OH group, presumably via a 6-membered ring ortho-ester intermediate, are facile and the α-glucoside anomers are significantly more reactive than their β-counterparts. By comparison with degradation rates for the corresponding acyl glucuronides, it can be inferred that substitution of the carboxylate by –CH₂OH in the acyl glucosides has a significant effect on acyl migration for those compounds, especially for rapidly transacylating molecules, and that thus the charged glucuronide carboxylate is a factor in the kinetics.