The rearrangement of mono- O-isopropylidene derivatives of aldose diethyl dithioacetals

Abstract

Acid-catalyzed rearrangements of terminal mono- O-isopropylidene derivatives of some aldose diethyl dithioacetals in N,N-dimethylformamide have been studied. The 4,5- O-isopropylidene derivatives of d-arabinose and d-xylose diethyl dithioacetal respectively rearrange to the 2,3- and a mixture of the 2,3- and 3,4- O-isopropylidene derivatives. The 5,6- O-isopropylidene derivatives of d-galactose, d-mannose, and d-glucose rearrange to the 4,5-, 3,4-, and a mixture of the 2,3- and 3,4- O-isopropylidene derivatives, respectively. These rearrangements were shown to be intramolecular by means of a cross-over experiment, and by consideration of the structures of the products obtained and of the rates of rearrangement. 3,4- O-Isopropylidene- d-mannose diethyl dithioacetal was also prepared directly from d-mannose diethyl dithioacetal. Consistent ratios of product to starting material indicated that the 4,5- trans-disubstituted 2,2-dimethyl-1,3-dioxolane products are ∼1.2 kcal.mol −1 more stable than the 4-substituted 2,2-dimethyl-1,3-dioxolane starting-materials. Reaction of d-glucose diethyl dithioacetal with an excess of 2-methoxypropene in the presence both of high and low concentrations of acid gave mainly the 2,3:5,6-di- O-isopropylidene derivatives.