d-Tagatose derivatives from d-fructose by a facile epimerisation procedure

1,2-O-Isopropylidene-β-d-fructopyranose was directly converted into 5-O-cyclohexylcarbamoyl-1,2-O-isopropylidene-3,4-O-(2,2,2-trichloroethylidene)-β-d-tagatopyranose by treatment with chloral/N,N′-dicyclohexylcarbodiimide. Subsequent acid-catalysed cleavage of the isopropylidene protecting group followed by acetylation afforded, exclusively, 1,2-di-O-acetyl-5-O-cyclohexylcarbamoyl-3,4-O-(2,2,2-trichloroethylidene)-α-d-tagatopyranose. This product was simultaneously dehydrochlorinated and decarbamoylated to 1,2-di-O-acetyl-3,4-O-ethylidene-α-d-tagatopyranose using Bu3SnH/AIBN.