Oxidation-reduction sequence for the synthesis of peracylated fluorodeoxy pentofuranosides

The oxidation of methyl 5-O-benzyl-3(2)-deoxy-3(2)-fluoro-α-d-pentofuranosides with dimethyl sulfoxide-acetic (trifluoroacetic) anhydride was accompanied by epimerization at the carbon atom bearing a fluoro function, resulting in the formation of the corresponding 2- or 3-keto derivatives as mixtures of two epimers in high combined yield. Reduction of a mixture of the erythro / threo epimeric 2-keto sugars (isolated as stable hydrates) with sodium borohydride in benzene-ethanol proceeded stereoselectively leading to the formation of 3-deoxy-3-fluoro ribo and lyxo-furanosides, respectively. In the case of the ribo and arabino epimers of the 3-keto sugar (isolated as free ketones), reduction stereoselectivity of the former was > 95% for the 2-deoxy-2-fluoro ribo sugar, whereas a ca. 3:1 lyxo / arabino ratio of products was obtained for the latter. Treatment of a mixture of the 2-epimeric 3-keto sugars with triethylamine in carbon tetrachloride at room temperature for 3–5 h afforded the 2-deoxy-2-fluoro ribo ketone (ca. 90%). The synthesis of 1-O-acetyl-2,5-di-O-benzoyl-3-deoxy-3-fluoro-α,β-d-lyxofuranose (8) and 1-O-acetyl-3,5-di-O-benzoyl-2-deoxy-2-fluoro-β-d-ribofuranose (16) and their use as glycosylating agents for bis-trimethylsilylated N6-benzoyladenine is described.