Carbohydrate-derived spiroketals. stereoselective synthesis of di-D-fructose dianhydrides by boron trifluoride promoted glycosylation-spiroketalization of acetal precursors

Di-D-fructose 1,2':2,1'-dianhydrides, dispiro-tricyclic disaccharides widel y found in food materials, have been stereoselectively prepared in one-pot reaction from O-protected D-fructose 1,2 acetonide precursors by treatment with boron trifluoride diethyl etherate. The dimerization sequence involves (i) cleavage of the anomeric acetal linkage, (ii) autoglycosylation, and ( iii) final spiroketalization, the stereochemical outcome being strongly dep endent on the nature of the hydroxyl protecting groups.