To 133 mg (0.270 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) and 112 mg of Molecular sieve (3A) was added 5 ml of tetrahydrofuran solution containing 2M dimethylamine at room temperature, and the mixture was stirred for 10 minutes. Then, 156 mg (0.740 mmol) of sodium triacetoxyborohydride was added to the mixture, and the mixture was stirred for further 17 hours. 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; ethyl acetate→chloroform:methanol=4:1 (V/V)), and the fractions containing the desired compound were concentrated under reduced pressure to obtain 74 mg of the title compound as a pale yellowish solid. (Yield: 53%)
