5-Thiopyranoses. Part 12. Sulphur participation in displacement reactions of sulphonate esters of 5-thio-D-allose, 5-thio-D-altrose, and 5-thio-D-glucose derivatives

Abstract

Both sulphonate groups in methyl 2,3-anhydro-4,6-di-O-methylsulphonyl-5-thio-α-D-allopyranoside (5) readily undergo displacement reactions. The 4-sulphonate is the more reactive and is displaced with retention of configuration. Participation by the ring sulphur atom and formation of intermediate episulphonium ions is proposed in each displacement. There is evidence to suggest that ring expansion to give methyl 2,3-anhydro-4,5-di-O-methyl-6-thio-β-L-taloseptanoside (19) may occur in the reaction of (5) with methanol. Methyl 4,6-O-isopropylidene-2-O-methylsuphonyl-5-thio-3-O-p-tolylsulphonyl-α-D-glucopyranoside (20) is unreactive until the acetal group is removed, when ring contraction to give 2,5-dideoxy-2,5-epithio-3-O-p-tolylsulphonyl-D-mannose dimethyl acetal (23) occurs. Methyl 3,4-O-isopropylidene-2,6-di-O-methylsulphonyl-5-thio-α-D-altropyranoside (30) was too reactive to be isolated but appeared to react with methanol and chloride ions to give, via ring contraction, the 6-substituted 2,5-dideoxy-2,5-epithio-3,4-O-isopropylidene-D-allose dimethyl acetal derivatives (32) and (36), and, via ring contraction and ring expansion, 2,6-dideoxy-2,6-epithio-3,4-O-isopropylidene-5-O-methyl-L-talose dimethyl acetal (35).