Isopropylidene acetals of 5-Thiopentopyranoses

Both 5-thio-D-ribose and 5-thio-D-xylose react with acetone and 2,2-dimethoxypropane, respectively, in the presence of acids to give 1,2:3,4-di-O-isopropylidene-5-thio-α-D-ribo- and -xylo-pyranoses (9 and 8); no furanoid products were detected. Partial hydrolysis of the xylo-diacetal 8 gave 1,2-O-isopropylidene-5-thio-α-D-xylopyranose, but a monoacetal could not be obtained from the ribo-diacetal 9. The methyl 5-thio-D-ribopyranosides (12) also react with acetone, giving only the 3,4-acetal from the α anomer 12a, and a separable mixture of 2,3- and 3,4-acetals from the β anomer 12b.