General procedure for the synthesis of 1-(5-hydroxy-1-indoline)ethanone from 1-acetyl-5-amino dihydroindole: 1-(5-hydroxy-1-indoline)ethanone (13.5 g, 76.70 mmol) was dissolved in sulfuric acid (100 mL) at 5-10 °C and an aqueous solution of sodium nitrite (6.35 g, 92.02 mmol) was added slowly ( 30mL). After addition, the reaction mixture was stirred at the same temperature for 1 hour. Subsequently, the mixture was poured into a 30% sulfuric acid solution (100 mL) of cuprous oxide (38.39 g, 268.46 mmol), heated to 100 °C and stirred at this temperature for 1 hour. After the reaction was completed, it was cooled to 25 °C and extracted with ethyl acetate (200 mL x 3). The organic layers were combined, washed with brine, filtered, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography to give 5.3 g of brown solid crude product. After grinding with acetone (100 mL), the pure product 1-(5-hydroxy-1-indoline)ethanone (3.00 g, 22% yield) was obtained. NMR hydrogen spectrum (DMSO-d6, 400MHz) δ 9.11 (s, 1H), 7.83 (d, J = 8.4Hz, 1H), 6.63 (s, 1H), 6.50-6.52 (m, 1H), 4.02 (t, J = 8.8Hz, 2H), 3.05 (t, J = 8.8Hz, 2H), 2.09 (s, 3H). LC-MS: 178.0 [M + H]+.
