3-[2-(Trifluoromethanesulfonylamino)benzyl]-[1,3]-oxazinan-2-one (0.13 g, 0.38 mmol) and sodium hydrogen carbonate (0.06 g, 0.71 mmol) was suspended in acetonitrile (15 ml), added with propyl chloroformate (0.09 g, 0.77 mmol) and heated to reflux for six hours. After cooling to room temperature, the reaction mixture was added with water and extracted with ethyl acetate. The organic layer was washed with a saturated brine and then dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (hexane:ethyl acetate=1:4) to obtain 3-{2-[N-(propoxycarbonyl)-N-(trifluoromethanesulfonyl)amino]benzyl)-[1,3]-oxazinan-2-one (0.11 g).
