Steric and electrostatic effects on the elimination of 2- and 3-sulphonyloxy-groups from methyl 4,6-O–benzylidenehexopyranosides

Abstract

The elimination of 2- and 3-sulphonyloxy-groups from methyl 4,6-O–benzylidenehexopyranosides by the Tipson–Cohen reaction (sodium iodide–zinc in NN-dimethylformamide) has bsen studied. The β-glucoside 2,3-disulphonate (3) or (4) is rapidly changed into the 2,3-unsaturated sugar (8) in high yield, whereas the yield of 2-enoside (7) from the α-anomer (1) is only 55%. The α-mannoside sugar disulphonate (6) was converted into the unsaturated sugar (8) in 66% yield, whereas the β-isomer gave the enoside (7) in low yield. The differences in reactivity are discussed in the light of steric and electrostatic effects.