5-Thiopyranoses. Part 11. Isopropylidene acetals of 5-thio-D-glucose, 5-thio-D-allose, and 5-thio-D-altrose and some of their methyl glycosides
5-Thio-D-glucose (9) reacts with acetone in the presence of an acid catalyst to give 1,2-O-5,6-S,O-di-isopropylidene-5-thio-α-D-glucofuranose (10). 5-Thio-D-altrose (37) similarly gives a 1,2 : 5,6-diacetal (39) but 5-thio-D-allose (27) affords 2,3-O-5,6-S,O-di-isopropylidene-5-thio-β-D-allofuranose (28). Under kinetic conditions, using 2,2-dimethoxypropane, the major products from 5-thio-D-glucose (9) and 5-thio-D-allose (27) are 2,3 : 4,6-di-O-isopropylidene-5-thin-D-gluco- and allopyranoses (11a) and (32), even though the former possesses a trans-fused dioxolane ring. 1,2 : 4,6-Di-O-isopropylidene-5-thio-α-D-glucopyranose (14), which is also produced in the former reaction, has been synthesised by an alternative route. 2,2-Dimethoxypropane and acetone react with methyl 5-thio-α- or β-D-glucopyranosides (3) or the related methyl 5-thio-α- or β-D-allopyranosides (25), to give the 2,3 : 4,6-diacetals (5) and (26) respectively. With acetone alone, the glucosides (3) give mainly the 4,6-monoacetals (4) together with smaller amounts of the 2,3 : 4,6- diacetals (5); the allosides (25) give complex mixtures and methyl 5-thio-α-D-altropyranoside (40) gives first the 4,6-acetal (41) which then isomerises into the 3,4-acetal (42). Methylation of either of the pyranose 2,3 : 4,6-diacetals (11a) or (32) leads to the corresponding 2,3 : 4,6-di-O-isopropylidene-5-S-methyl-5-thio-aldehydo compounds (44) and (45), respectively.