Cyclic acetals of ketoses : Part III. Re-investigation of the synthesis of the isomeric DI-O-isopropylidene-β-d-fructopyranoses

Abstract

Condensation of d-fructose with acetone in the presence of sulfuric acid gave first 1,2:4,5-di-O-isopropylidene-β-d-fructopyranose. This compound isomerizes, at a rate dependent on the concentration of the acid, to 2,3:4,5-di-O-isopropylidene-β-d-fructopyranose. Careful selection of reaction conditions permits the synthesis of either isomer in satisfactory yield. In acetone containing 5% of sulfuric acid, either compound is converted into an equilibrium mixture (3:47) of 1,2:4,5- and 2,3:4,5-diacetal within 5 min at room temperature, but, when anhydrous zinc chloride is used as the catalyst, the 1,2:4,5-diacetal does not rearrange to the 2,3:4,5-diacetal.