10-bromo-2-hydroxy-4H-pyrido[3,2,1-jk]carbazole-4-one (2.5 g) obtained in Example 59 was suspended in dimethyl sulfoxide (120 ml), and potassium carbonate (2.2 g) was added to the suspension. The mixture was stirred at room temperature for 30 minutes and t-butyl bromoacetate (1.4 ml) was added to the mixture. The mixture was stirred at room temperature for 12 hours, and the reaction mixture was poured into ice water (900 ml), and the crystals precipitated were recovered by filtration. The crude crystals were purified by silica gel flash column chromatography (eluent: methylene chloride containing 2% methanol) to obtain the title compound (1.9 g, 68%).
