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To a trifluoroacetic acid solution (47 mL) containing 3-(3-ethoxy-3-oxopropyl)-4-oxo-4,5,6,7-tetrahydro-1H-indole (4, 10.0 g, 42.5 mmol) was added with trimethylorthoformate (13.6 g, 128 mmol) at 0° C. The reaction was stirred at 0° C. for 30 min, warmed up to room temperature and stirred for 2.0 h. The solution was added with ethyl acetate (250 mL), water (200 mL) and NaOH (18 g). The organic layer was collected and washed with saturated aqueous NaHCO3 (200 mL), dried over MgSO4 (s), and concentrated under reduced pressure. The oily residue was purified by gravity column chromatography (50% EtOAc in hexanes) to give 3-(3-ethoxy-3-oxopropyl)-2-formyl-4-oxo-4,5,6,7-tetrahydro-1H-indole (5, 2.1 g, 8.0 mmol) as yellow solids in 19% yield.

1. temperaturewarmed up to room temperature
2. stirringstirred for 2.0 h
3. customThe organic layer was collected
4. washwashed with saturated aqueous NaHCO3 (200 mL)
5. dry with materialdried over MgSO4 (s)
6. concentrationconcentrated under reduced pressure
7. customThe oily residue was purified by gravity column chromatography (50% EtOAc in hexanes)