190 mg (0.412 mmol) of the above 5-(4-aminobutylamino)-6,8-difluoro-2-(3-fluoro-4-pivaloylaminophenyl)-4H-1-benzopyran-4-one was dissolved in 10 mL of methanol, 0.23 mL (8.2 mmol) of a 37% aqueous solution of formaldehyde, 259 mg (4.12 mmol) of sodium cyanoborohydride and a solution of 0.24 mL (4.1 mmol) of acetic acid in 0.48 mL of methanol were added and the mixture was stirred at room temperature for 24 hours. Ethyl acetate was added to the reaction solution, and the organic layer was separated, washed twice with a 1N aqueous solution of sodium hydroxide and twice 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 high performance liquid chromatography (column: YMC SH-365-15 S-15, acetonitrile: a 0.1M aqueous solution of ammonium acetate=7:3, 40 mL/min.) to give 116 mg of 5-(4-dimethylaminobutylamino)-6,8-difluoro-2-(3-fluoro-4-pivaloylaminophenyl)-4H-1-benzopyran-4-one (yield: 57%).
