An efficient and highly regioselective synthesis of 4-deoxy- and 2-acetamido-2,4-dideoxy-alfa-D-threo-hex-3-enopyranosides

The preparation of the previously undescribed class of 4-deoxy- and 2,4-dideoxy-2-acetamido-beta-D-threo-hex-3-enopyranosides was accomplished with a very high yield and a complete regioselectivity by means of a simultaneous activation–elimination process of the OH-4 group of -D-talopyranosides (5a,b) and 2-acetamido-2-deoxy-beta-D-talopyranosides (5c,d) with NaH/N,N'-sulfuryldiimidazole. The same reaction of analogous beta-D-galactopyranosides (5e,f) is not regioselective, leading to mixtures of 3- and 4-hexeno derivatives. This difference is evidently determined by the orientation of the C-2 substituent, which, in the talo series, is anti diaxially disposed to the H-3 eliminating group.