5-Thiopyranoses. Part 8. Sulphur participation in displacement reactions of sulphonate esters of 5-thio-D-ribose and 5-thio-D-xylose derivatives

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Abstract
The 4-methanesulphonate group of 1,2-O-isopropylidene-3,4-di-O-methylsulphonyl-5-thio-α-D-xylopyranose (3) is displaced by intramolecular attack by sulphur leading to a cyclic episulphonium ion (22) which then reacts with nucleophiles to give ring contracted 4-thio-β-L-arabinofuranose products (4) and (13)–(17) and, in some cases, products (18) and (19) with retained 5-thio-α-D-xylopyranose stereochemistry. Similar reactions with methyl 2,3(3,4)-O-isopropylidene-4(2)-O-methylsulphonyl-5-thio-D-ribopyranosides (24) and (29) also proceed via episulphonium ions (25) and (30) leading to products of retained stereochemistry such as methyl 4-O-benzoyl-2,3-O-isopropylidene-5-thio-β-D-ribopyranoside (26) or of ring contraction e.g., 2,5-dideoxy-2,5-epithio-3,4-O-isopropylidene-D-arabinose dimethyl acetal (32) respectively. The trans-acetal containing methyl 2,3-O-isopropylidene-4-O-methylsulphonyl-5-thio-α-D-xylopyranoside (36) fails to undergo reaction under the same conditions, presumably because of its inability to form the cyclic sulphonium ion (37).

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