In a similar manner to Step 2 of Example 6, 4-methyl-7-[1-(tert-butoxycarbonyl)-5-formylindol-2-yl]isoindolinone (54.0 mg, 0.138 mmol) was dissolved in acetonitrile (4.3 mL), and the solution was treated with ethanolamine (0.034 mL, 0.55 mmol), acetic acid (0.158 mL, 2.76 mmol) and sodium triacetoxyborohydride (58.0 mg, 0.276 mmol). The reaction mixture was added with 1 mol/L hydrochloric acid and extracted with ethyl acetate. The organic layer was washed with sodium carbonate and saturated brine, and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure to obtain 4-methyl-7-{1-(tert-butoxycarbonyl)-5-[(2-hydroxyethylamino)methyl]indol-2-yl}isoindolinone (48.5 mg, yield 81%).
