To a solution of (3-(1-cyclopentyl-4-oxo-4,5-dihydro-1H-pyrazolo[4,3-c]pyridin-3-yl)phenyl)acetonitrile (58.3 mg) obtained in Example 128 in DMSO (5 mL) were added potassium carbonate (30.4 mg) and 30% aqueous hydrogen peroxide (0.056 mL), and the mixture was stirred for 3 days. To the reaction mixture was added water, and the mixture was extracted with ethyl acetate. The organic layer was washed successively with water and saturated brine, dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (methanol/ethyl acetate), and the obtained solid was washed with ethyl acetate/diisopropyl ether to give the title compound (43.2 mg).
