(2R,3R,4R,5R)-5-(2-Amino-6-azetidin-1-yl-purin-9-yl)-4-fluoro-2-hydroxymethyl-4-methyl-tetrahydro-furan-3-ol (14, 340 mg, 1.0 mmol) was dissolved in anhydrous pyridine (6 ml) at ambient temperature. A solution of 0.45 M 1H-tetrazole in acetonitrile (5.5 mL, 2.5 mmol) was added followed by bis(N,N-diisopropylamino)methylphosphoramidite (317 μL, 1.1 mmol). The mixture was stirred at ambient temperature for 17 h. The solvent was concentrated under reduced pressure and the residue was triturated with ethyl acetate (20 mL). The resulting precipitant of salts was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography using a gradient of ethyl acetate in hexanes (40-80%). The product containing fractions were combined and concentrated to a white solid, 47 mg (12% yield).
