11.6 g (0.090 mole) of 5-fluorocytosine, 19 ml of hexamethyldisilazane and 24 ml of acetonitrile were mixed, and 0.2 g of ammonium sulfate was added to the mixture, which was refluxed for 1 hr. After cooling the reaction mixture to room temperature, 72 ml of acetonitrile was added thereto, followed by subjecting the resulting mixture to distillation to remove about 60 ml of the solvent. The resulting solution was cooled to room temperature, mixed with 27.7 g (0.090 mole) of the compound obtained in Example 1 and 72 ml of acetonitrile, and the resulting mixture was cooled to 20° C. After adding 16.3 ml (0.090 mole) of trimethylsilyl trifluoromethanesulfonate dropwise thereto at 25° C., the reaction mixture was stirred at room temperature overnight, cooled to 10° C., mixed with 45.4 g of sodium hydrogen carbonate, and stirred for 30 min. 9.8 g of water and 72 ml of dichloromethane were added dropwise thereto, the resulting solution was stirred for 2 hrs, filtered, and the isolated solid was washed with 72 ml of dichloromethane. The filtrate was washed with 120 ml of 4% sodium bicarbonate, dried over anhydrous sodium sulfate, filtered, and concentrated under a reduced pressure, to obtain 35.8 g of the title compound.
