A concise synthesis of 3-fluoro-5-thio-xylo- and glucopyranoses, useful precursors towards their corresponding pyranonucleoside derivatives

The chemical synthesis of 1,2,4-tri-O-acetyl-3-deoxy-3-fluoro-5-thio-d-xylopyranose, 1,2,4,6-tetra-O-acetyl-3-deoxy-3-fluoro-5-thio-α-d-glucopyranose and their corresponding nucleosides of thymine is described. Treatment of 3-fluoro-5-S-acetyl-5-thio-d-xylofuranose, obtained by hydrolysis of the isopropylidene group of 3-fluoro-1,2-O-isopropylidene-5-S-acetyl-5-thio-d-xylofuranose, with methanolic ammonia and direct acetylation, led to triacetylated 3-deoxy-3-fluoro-5-thio-d-xylopyranose. Condensation of acetylated 3-fluoro-5-thio-d-xylopyranose with silylated thymine afforded the corresponding nucleoside. Selective benzoylation and direct methanesulfonylation of 3-fluoro-1,2-O-isopropylidene-α-d-glucofuranose gave the 6-O-benzoyl-5-O-methylsulfonyl derivative, which on treatment with sodium methoxide afforded the 5,6-anhydro derivative. Treatment of the latter with thiourea, followed by acetolysis, gave the 3-fluoro-5-S-acetyl-6-O-acetyl-1,2-O-isopropylidene-5-thio-α-d-glucofuranose. 3-Fluoro-5-S-acetyl-6-O-acetyl-5-thio-d-glucofuranose, obtained after hydrolysis of 5-thiofuranose isopropylidene, was treated with ammonia in methanol and directly acetylated, giving tetraacetylated 3-deoxy-3-fluoro-5-thio-α-d-glucopyranose. Condensation of the latter with silylated thymine afforded the desired 3-deoxy-3-fluoro-5-thio-β-d-glucopyranonucleoside analogue.