Alcoholyses of 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide; kinetics and products at 25–40°

Abstract

The alcoholysis of 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide with four primary and two secondary alcohols was investigated at 25–40°. Rate measurements, including salt effects and the effect of bromide salts, were made. The anomeric composition of the glucopyranosidic products was analysed at intervals. In all cases the acetylated alkyl β-D-glucopyranoside was the main product but in the majority of the alcoholyses the α-glucopyranoside was also formed. It is concluded that the primary alcoholyses proceed by an S_N1 mechanism. An increase in the time-independent fraction of α-glucopyranoside as an apparent function of alcohol size for the primary alcoholyses is attributed to a steric factor. An S_N2 mechanism is suggested for the secondary alcoholyses. The time-dependent increase in the fraction of α-glucopyranoside formed in the secondary alcoholyses is attributed to reaction of the α-glucosyl bromide with liberated bromide ions to form the β-glucosyl bromide which subsequently forms the α-glucopyranoside. Calculations for this theoretical model agree with experimental results.