To a solution of (S)-2-[[(2S,3S,4R,5R)-5-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-2,4-difluoro-3-hydroxy-4-methyl-tetrahydro-furan-2-ylmethoxy]-(naphthalen-1-yloxy)-phosphorylamino]-propionic acid isopropyl ester prepared in Example 2 (86 mg, 144 μmol) in THF (10 mL) were added propionyl chloride (66.6 mg, 720 μmol) and DMAP (87.9 mg, 720 μmol). The reaction mixture was stirred at room temperature for 5 h. The mixture was diluted with ethyl acetate, washed with water, brine. The organic layer was separated, dried over MgSO4, and concentrated. The residue was purified by flash chromatography (silica gel, 40 g, 0-15% MeOH in DCM) to give the title compound as white solid (45 mg, 48%).
