The Mechanism of the Anomerization of the Tetra-O-Acetyl- D -Glucopyranosyl Chlorides

The anomerization of the tetra-O-acetyl-D-glucopyranosyl chlorides in acetonitrile is first order in chloride ion. Experiments with 36Cl-labelled chloride ion showed the reaction to involve nucleophilic attack (SN2) by chloride ion at the anomeric center. The reaction is accompanied by a faster dissociation of the β-anomer (I) to the 1,2-acetoxonium ion. Although the rate of anomerization was slightly accelerated by addition of benzene, the ionic reaction was strongly suppressed. Bromide ion was less effective than chloride ion in catalyzing the reaction and iodide ion was least effective. Reaction of tetra-O-acetyl-α-D-glucopyranosyl bromide with tetraethylammonium chloride in acetonitrile rapidly gave I in high yield. The anomeric effect for chlorine appears to be about 2 kcal/mole.