10-bromo-2-hydroxy-5-methyl-4H-pyrido[3,2,1-jk]carbazole-4-one (250 mg) obtained in Example 49 was suspended in dimethyl sulfoxide (10 ml), and potassium carbonate (210 mg) was added to the suspension. The mixture was stirred at room temperature for 30 minutes and t-butyl bromoacetate (0.13 ml) was added to the mixture. The mixture was stirred for 2.5 hours at room temperature, and the reaction mixture was poured into ice water (50 ml) and extracted with ethyl acetate. The ethyl acetate layer was washed with saturated aqueous solution of sodium chloride, and dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (eluent: methylene chloride containing 2% methanol), and the crude purification product was washed with ether and recovered by filtration to obtain the title compound (130 mg, 39%).
