5'-O-(t-Butyldimethylsilyl)-2',3'-dideoxy-3'-(N,N-dimethylamino-methylene)-2'-oxo-uridine (1.78 g, 4.5 mmol), from Step 6, was added to the trifluoromethanesulfonyl azide solution and the reaction mixture was heated at 60° C. for 2.5 h under a nitrogen atmosphere. The solvents were evaporated in vacuo to a syrup (2.8 g) which was dissolved in 10 mL of methylene chloride and purified by flash chromatography (5 psi) on a silica gel column (2×35 cm) eluted with 200 mL of methylene chloride, 200 mL of 1% methanol in methylene chloride, 200 mL of 2% methanol in methylene chloride and 3% methanol in methylene chloride to give 1.27 g (77% yield) of the title compound. MS DCI: 367 (M+H)+.
