Thio-sugars. Part III. The thioseptanose ring

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Abstract
Some derivatives of 6-deoxy-6-mercapto-D-galactopyranose are described; no evidence was found for the existence of the thio-sugar in a septanose form. 6-Deoxy-1,2-O-isopropylidene-6-mercapto-3,5-di-O-methyl-α-D-glucofuranose has been synthesised by two routes; removal of the isopropylidene group by mild acid hydrolysis gives the normal product which retains the free thiol group and the furanose ring, but under more vigorous conditions the 1,6-thioanhydro-compound is formed, containing both a thioseptanose and a furanose ring. A minor product from the latter reaction is a dimer, in which two furanose rings are mutually joined by (probably) 1,6′:1′,6-thioanhydro links. By a sequence of reactions involving acyclic intermediates, D-galactose dibenzyl dithioacetal has been converted into 6-deoxy-6-mercapto-2,3,4,5-tetra-O-methyl-D-galactose ethylene acetal, which on hydrolysis forms the thioseptanose ring to give 6-deoxy-6-mercapto-2,3,4,5-tetra-O-methyl-D-galactoseptanose; by ethanolysis of the acetal the corresponding ethyl αβ-septanoside is obtained. Acid hydrolysis of methyl 4,6-dideoxy-4,6-dimercapto-2,3-di-O-methyl-β-D-glucoside provides 1,6-anhydro-4,6-dideoxy-4,6-dimercapto-2,3-di-O-methyl-β-D-glucofuranose, the first example of the coexistence of the thiofuranose and the thioseptanose ring.

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