To 3 ml of tetrahydrofuran solution containing 221 mg (0.450 mmol) of 2-(3-cyclopropoxy-4-difluoromethoxyphenyl)-3-formyl-1-(2-trimethylsilylethoxymethyl)-1,5-dihydropyrrolo[2,3-d]pyridazin-4-one obtained in Example 10-(a) was added 58 mg (1.53 mmol) of sodium borohydride under ice-cooling, and the reaction mixture was raised to room temperature. After completion of the reaction, water was added to the reaction mixture, and the mixture was extracted with ethyl acetate. The organic layer after separation was washed successively with water and then with a saturated aqueous solution of sodium chloride, dried over anhydrous magnesium sulfate, and then concentrated under reduced pressure. The obtained residue was applied to silica gel column chromatography (Eluent; hexane:ethyl acetate=1:1 (V/V)), and the fractions containing the desired compound were concentrated under reduced pressure to obtain 155 mg of the title compound as a pale yellowish foam. (Yield: 70%)
