5-Bromo-1-tert-butoxycarbonyl dihydroindole (2.0 g, 6.7 mmol) and N,N-dimethylformamide (0.53 mL, 6.7 mmol) were used as raw materials and dissolved in tetrahydrofuran (30 mL). The resulting solution was cooled to -78 °C under stirring and a pentane solution of tert-butyllithium (1.7 M, 10.3 mL, 17.5 mmol) was added slowly and dropwise over 5 min under nitrogen protection. After the reaction mixture was stirred at -78 °C for 2 h, acetic acid (3 mL) was added. Subsequently, the cloudy mixture was slowly warmed to room temperature over 30 min and diluted with ethyl acetate (100 mL). The organic phase was washed sequentially with saturated sodium carbonate solution (2 × 10 mL) and brine (10 mL). The organic layer was dried over anhydrous sodium sulfate, filtered and concentrated in vacuum to give the crude product as an oil. Purification by column chromatography (eluent: ethyl acetate/hexane, 1:9 to 1:1 gradient elution) and vacuum removal of the solvent resulted in the isolation of two products, both white solids: (1) 1-tert-butoxycarbonyldihydroindole (270 mg, 18% yield): 1H NMR (CDCl3, 300 MHz): δ 7.18-7.10 (m, 2H), 6.92 (t 1H, J = 7 Hz), 3.97 (t, 2H, J = 7 Hz), 3.08 (t, 2H, J = 7 Hz), 1.57 (s, 9H); MS (APCI+): m/z 220 ([C13H18NO2 + H]+); (2) 1-Boc-5-formylindoline (650 mg, 39% yield): 1H NMR (CDCl3 , 300 MHz): δ 9.86 (s, 1H), 7.9 (br s, 1H), 7.70-7.65 (m, 2H), 4.06 (t, 2H, J = 7 Hz), 3.15 (t, 2H, J = 7 Hz), 1.58 (s, 9H); MS (ES+): m/z 248 ([C14H18NO3 + H]+).
