Kinetic benzylidenation. Part II. Rearrangement of the kinetic products from benzylidenation of aldose diethyl dithioacetals

Abstract

Terminal five-membered O-benzylidene derivatives of aldose diethyl dithioacetals can be rearranged at room temperature in N,N-dimethylformamide, often in high yields. Derivatives with the arabino configuration for their three terminal secondary hydroxyl groups, i.e. D-glucose, D-mannose, and D-arabinose derivatives, rearranged to structures containing terminal six-membered O-benzylidene rings. 4,5-O-Benzylidene-D-ribose diethyl dithioacetal rearranged chiefly to the 2,4 isomer, which was obtained by crystallization. Chromatography yielded some of the 3,5 isomer. 5,6-O-Benzylidene-D-galactose diethyl dithioacetal rearranged to a mixture of the two 4,5-O-benzylidene diastereomers, contrary to predictions based on the Hann–Hudson rules. A revised set of rules for acetal stability in N,N-dimethylformamide has been formulated. D-arabinose and D-ribose diethyl dithioacetal were shown to react with α, α-dimethoxytoluene under rearrangement conditions to give the products noted above in good yields.