The formation of 3,4,6,tri-O-benzyl-2,5-anhydro-L-iditol from 2,3,4,6-tetra-O-benzyl-D-glucopyranose. Intramolecular displacement of a secondary mesyloxy group by a benzyloxy group, in the presence of cesium propionate

Abstract

The reaction between 2,3,4,6-tetra-O-benzyl-1,5-di-O-mesyl-D-glucitol and cesium propionate in dimethylformamide at 100 °C leads to an almost quantitative conversion into 3,4,6-tri-O-benzyl-1-O-propionyl-2,5-anhydro-L-iditol. Closure of the anhydro ring appears to entail anchimeric assistance by the 2-O-benzyl substituent in an SN2 displacement at C-5, leading to an inversion at the latter position. Probable contributors to the marked regio- and stereoselectivity observed for the reaction are a unique property of cesium propionate as compared with other carboxylate salts, large differences in rate between competing reactions, and a geometry highly favorable to an intramolecular displacement process. Conformational properties of 2,5-anhydro-L-iditol, as well as of isomeric 2,5-anhydro-D-mannitol, are described, based on nmr evidence.