Mechanism of the reductive cleavage reaction of permethylated methyl D-glycopyranosides

The mechanism of the reductive cleavage reaction of permethylated methyl D- glycopyranosides was investigated by measuring the rate of reaction. Glycos ides employed were of alpha-Glc, beta-Glc, alpha-Man, alpha-Gal, and beta-G al. Seven silanes were used to explore the reactivities of the reducing age nts as well as to examine the stereoelectronic effects of the agents. Trime thylsilyl trifluoromethanesulfonate was employed as catalyst. In general, t he rates of beta anomers were about twice as fast as those of the alpha ano mers. The rates of anomerization were about five to ten times lower than th ose of reduction. A cyclic oxonium ion has been proposed as a sole intermed iate for the reductive cleavage of the alpha-glycoside linkage, but the att ack of the reducing agent on both cyclic and acyclic forms as well as on th e substrate-Lewis acid complex seems to be involved for the beta anomer. (C ) 1999 Elsevier Science Ltd. All rights reserved.