Thio-sugars. Part 8. Methyl 2,3-anhydro-5-thio-α-D-ribopyranoside and methyl 3,4-anhydro-5-thio-α-D-ribopyranoside

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Abstract
Selective reaction of the primary sulphonate group in 1,2-O-isopropylidene-3,5-di-O-tosyl-α-D-xylofuranose with potassium thiobenzoate gives 5-S-benzoyl-1,2-O-isopropylidene-3-O-tosyl-5-thio-α-D-xylofuranose, which is converted by hydrogen chloride in methanol into methyl 3-O-tosyl-5-thio-α-D-xylopyranoside, from which the title epoxides are obtained by treatment with base. The constitutions of these epoxides are deduced from the 1H n.m.r. spectra of their acetates. Reaction of the 4-O-methyl derivative of the 2,3-epoxide with sodium O-methyl dithiocarbonate gives methyl 2,3-dideoxy-4-O-methyl-2,3-thiocarbonyldithio-5-thio-α-D-arabinopyranoside, and similar treatment of the 2-O-methyl ether of the 3,4-epoxide gives methyl 3,4-dideoxy-2-O-methyl-3,4-thiocarbonyldithio-5-thio-α-D-xylopyranoside; the configurations of these cyclic trithiocarbonates are established from their 1H n.m.r. spectra. Methyl 5-S-benzoyl-3-O-tosyl-5-thio-α-D-xylofuranoside is converted by base into methyl 3,5-dideoxy-3,5-epithio-α-D-xylofuranoside; the β-anomer behaves similarly and also gives 5,5′-S-methylenebis(methyl 2,3-anhydro-5-thio-β-D-ribofuranoside) by competitive interaction of the intermediate 5-thiol with the dichloromethane solvent.

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