In a similar manner to Step 2 of Example 6, 4-chloro-7-[1-(tert-butoxycarbonyl)-5-formylindol-2-yl]isoindolinone (40.0 mg, 0.0974 mmol) was dissolved in acetonitrile (3 mL), and the solution was treated with isonipecotamide (154 mg, 1.20 mmol), acetic acid (0.896 mL, 15.7 mmol) and sodium triacetoxyborohydride (188 mg, 0.887 mmol). The reaction mixture was added with water and sodium carbonate to adjust the pH to 9. The mixture was extracted with ethyl acetate. The organic layer was washed with saturated brine and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure to obtain 4-chloro-7-[1-(tert-butoxycarbonyl)-5-(4-methoxycarbonylpiperidinomethyl)indol-2-yl]isoindolinone (35.5 mg, yield 70%).
