Phosphorus oxychloride (POCl3, 0.31 mL, 3.39 mmol, 1.1 eq.) was slowly added dropwise to dry N,N-dimethylformamide (DMF, 2 mL) under argon protection, and the reaction system was cooled to -20 °C with vigorous stirring.After 30 min, 5-nitroindole (500 mg, 3.08 mmol, 1 eq.) dissolved in 0.5 mL of anhydrous DMF was slowly added to the above solution cooled to -20 °C. The reaction was carried out in the same manner as that for the reaction. mmol, 1 equiv) was slowly added to the above solution cooled to -20 °C. Subsequently, the resulting yellow solution was stirred at room temperature for 1 h. The light yellow compound was observed to precipitate. The reaction mixture was diluted with DMF and an equal volume of ice water was added. Next, the pH was adjusted to 9 by dropwise addition of 50% (v/v) aqueous sodium hydroxide solution, and the resulting red solution was refluxed and subsequently allowed to stand at 4 °C overnight. The precipitated crystals were collected by filtration, washed with cold water and dried. Recrystallization by ethanol-water (EtOH-H2O) system afforded 574 mg of 5-nitroindole-3-carbaldehyde (compound XV) in 98% yield.1H-NMR (400 MHz, DMSO-d6) δ ppm: 7.69 (d, J = 9.0 Hz, 1H), 8.12 (dd, J = 2.3 Hz, 1H), 8.54 (s, 1H ), 8.91 (d, 1H), 9.99 (s, 1H).
