Synthesis of anhydro psicofuranosyl nucleosides

Methyl 1-O-mesyl-5-O-toluoyl-β-d-psicofuranoside (5) was synthesised from the known 1,3;4,5-di-O-isopropylidene-β-d-psicofuranose (1) as a key carbohydrate precursor for the preparation of anhydro psicofuranosyl nucleosides. Transformation of 5 into acetate 7 or bromide 10 followed by (i) coupling with persilylated N6-benzoyladenine in the presence of SnCl4, and (ii) treatment with MeONa/MeOH gave 1′,3′-anhydro nucleoside 9. Employment of silylated thymine in a similar sequence of reactions afforded the 1′,4′-anhydro nucleoside 12. Reaction of blocked 11 with NH3/MeOH gave O2,1′-anhydro nucleoside 13, which was rearranged into 12 upon MeONa/MeOH treatment. Condensation of the 1′,3′-anhydro sugar 15 with silylated thymine gave, after deprotection, the 1′,3′-anhydro nucleoside 16.