Abstract Aldose diethyl dithioacetals react with 1.2 equivalents of 1-ethoxycyclohexene or 2- methoxypropene in N, N-dimethylformamide at 0 with p-toluenesulfonic acid as catalyst to give the five-membered ring acetal attached to the two terminal oxygen atoms as the major product in every case. In most instances, a small proportion of the terminal, six-membered ring acetal was also obtained, and in a few cases, terminal seven-membered ring acetals ...