2.41 g (5.94 mmol) of Compound 51 obtained in Example 51 was dissolved in 60 mL of dimethylformamide under argon atmosphere, 237 mg (5.94 mmol) of sodium hydride (60% oil dispersion) and 0.47 mL (6.24mmol) of chloromethyl methyl ether were added under ice-cooling and the mixture was stirred at room temperature for 40 minutes. The reaction solution was cooled on ice, water was added and the mixture was extracted with ethyl acetate. The organic layer was washed once with water and once with an aqueous saturated solution of sodium chloride and dried over anhydrous sodium sulfate. The solvent was distilled off under reduced pressure and the residue was purified by silica gel column chromatography (chloroform:acetonitrile=30:l) to give 2.47 g of 5-amino-2-[4-[N-(tert-butoxycarbonyl)-N-methoxymethylamino]-3-fluorophenyl)-6,8-difluoro-4H-1-benzopyran-4-one (yield: 92%).
