RADICAL-MEDIATED BROMINATION OF PERACETYLATED 5-THIO-D-XYLOPYRANOSYL BROMIDES - AN EASY ACCESS THE CORRESPONDING ANOMERIC ORTHOTHIOLACTONES

On treatment with N-bromosuccinimide in refluxing carbon tetrachloride under irradiation with visible light, both alpha and beta anomers of 2,3,4-tri-O-acetyl-5-thio-D-xylopyranosyl bromide were converted mainl y to 2, 3,4-tri-O-acetyl-5-thio-D-xylopyranosylidene dibromide (5) and to -O-acetyl-5(S)-5-bromo-5-thio-D-xylopyranosylidene dibromide (6). Whereas the more reactive beta anomer could be transformed cleanly int o the dibromide 5 after heating for 2 h, complete conversion of the al pha-bromide required a prolonged treatment(similar to 5 h) leading to a mixture of di-, tri- and tetra-bromides. Similarly, an anomeric mixt ure of 2,3,4,6-tetra-O-acetyl-5-thio-D-glucopyranosyl bromide yielded mainly tribromide 11 after prolonged heating. The reaction rates and t he structure of the products showed again the higher reactivity of axi al C-H bonds at either C-1 or C-5 in pyranosyl rings towards S(H)2 pro cesses, However, activation by the sulfur atom allowed attack of equat orial bonds as well and polybromination at both C-1 and C-5. Treatment of the C-1 dibromide 5 by silver triflate in the presence of either a lcohols or thiols yielded the corresponding 5-thiosugar ortholactones 12-15. Methyl 1-methoxy-5-thio-D-xylopyranoside (18), obtained from 12 on deacetylation, showed no venous antithrombotic activity in rats ac cording to a Wessler test.