benzyl 3,5-di-O-benzyl-2-deoxy-1,4-dithio-d-erythro-pentofuranoside → 3',5'-di-O-benzyl-4'-thio-thymidine

A suspension of benzyl 3,5-di-O-benzyl-2-deoxy-1,4-dithio-D-erythro-pentofuranoside (22.5 g, 51.6 mmol), bis TMS-thymine (46 g, 170 mmol), mercuric bromide (20.5 g, 56.7 mmol), cadmium carbonate (29.3 g, 170 mmol) and dry toluene (1 L) was boiled under reflux, with stiring, for 24 hours. The hot mixture was then filtered and the solids were washed with toluene. The filtrate was successively washed with potassium iodide solution (30%) and water and then evaporated. The residue was taken up in 4:1 methanol-water, stirred for 30 minutes, the suspension filtered and the filtrate evaporated. The residue was applied to a silica gel column (hexane-ethyl acetate (1:1)) and combination of the appropriate fractions gave the title product as a clear colourless syrup. 1H NMR indicated the ratio of α- to β- anomers to be 2.8:1. Further column chromatography gave more of the separated anomers. The first compound to be eluted from the column was 3',5'-di-O-benzyl-4'-thio-thymidine as a colourless syrup. This could be crystallised from methanol to give colourless crystals m.p. 140°-142° C.