In a similar manner to Step 2 of Example 6, 4-hydroxy-5-methoxy-7-[1-(tert-butoxycarbonyl)-5-formylindol-2-yl]isoindolinone (0.494 g, 1.17 mmol) was dissolved in acetonitrile (16.0 mL), and the solution was treated with methylamine hydrochloride (1.60 mL, 23.4 mmol), triethylamine (3.30 mL, 23.4 mmol), acetic acid (1.30 mL, 23.4 mmol) and sodium triacetoxyborohydride (0.744 g, 3.51 mmol), followed by purification by flash column chromatography (chloroform/methanol=85/15) to obtain 4-hydroxy-5-methoxy-7-[1-(tert-butoxycarbonyl)-5-(methylaminomethyl)indol-2-yl]isoindolinone (0.159 g, 31%).
