A mixture of 0.20 g of 1-[2-(4-chloro-2-phenyl-1H-imidazo[4,5-c]quinolin-1-yl)ethyl]-4-piperidinone, 0.04 g of hydroxylamine hydrochloride, 0.09 g of sodium acetate and 4 ml of methanol was stirred at room temperature for 1 hour. The reaction solution was concentrated, and the residue was added with aqueous sodium hydrogencarbonate solution, and extracted with ethyl acetate. The extract was washed with saturated aqueous sodium hydrogencarbonate solution, and dried, and the solvent was evaporated to give 0.25 g of a colorless solid. Recrystallization from ethyl acetate gave colorless crystals having the melting point of from 201 to 207° C. (decomposition).
