Generation of α-D-glucopyranosylacetonitrilium ions. Concerning the reverse anomeric effect

Abstract

Reaction of the α- and β-anomers of the pent-4-enyl D-glucopyranoside (10) with N-bromosuccinimide in dry acetonitrile generated stereospecifically the α-D-glucopyranosylacetonitrilium ion (5α), which reacts in situ with 2-chlorobenzoic acid to afford the α-imide (8α). The result is in contrast to that predicted by the reverse anomeric effect and previous work on trapping carbohydrate acetonitrilium ions with 2-chlorobenzoic acid. The unusually large J₁₂ 7.3 Hz for 1-H of (8α) is rationalised by a substantial flattening of the pyranose ring at C-1 and C-2. Molecular dynamic studies on the model α-imide (12) support a flattened ⁴C₁, conformation. Treatment of imide (8α) with sodium methoxide leads to the α-2-chlorobenzamide (9α), which was substantiated by independent synthesis of the β-2-chlorobenzamide (9β).