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. Glycosides employed were of α-Glc, β-Glc, α-Man, α-Gal, and β-Gal. Seven silanes were used to explore the reactivities of the reducing agents as well as to examine the stereoelectronic effects of the agents. Trimethylsilyl trifluoromethanesulfonate was employed as catalyst. In general, the rates of β anomers were about twice as fast as those of the α anomers. The rates of anomerization were about five to ten times lower than those of reduction. A cyclic oxonium ion has been proposed as a sole intermediate for the reductive cleavage of the α-glycoside linkage, but the attack of the reducing agent on both cyclic and acyclic forms as well as on the substrate–Lewis acid complex seems to be involved for the β anomer.