General procedure for the synthesis of 2-oxoindoline-5-sulfonyl chloride from 2-indolone: Chlorosulfonic acid (27 mL, 408 mmol) was slowly added dropwise to indolin-2-one (13.3 g, 100 mmol) under stirring, and the dropwise acceleration was controlled to keep the reaction temperature below 30 °C. After the dropwise addition, the reaction mixture was continued to be stirred at room temperature for 1.5 h, followed by warming up to 68 °C for 1 h. The reaction was then carried out at room temperature for 1.5 h. The reaction temperature was increased to 68 °C. Upon completion of the reaction, the mixture was cooled to room temperature and carefully poured into water. The precipitated solid product was collected by vacuum filtration, washed with water and dried in a vacuum oven to afford 2-oxoindoline-5-sulfonyl chloride (3.02 g, 13.03 mmol, 98% yield), which can be used in subsequent reactions without further purification. The melting point of the product was 180-185 °C. 1H NMR (δ, ppm): 3.46 (s, 2H, CH2), 6.74 (d, 1H, J = 7.7 Hz, Ar), 7.42-7.46 (m, 2H, Ar), 10.48 (br s, 1H). 13C NMR (δ, ppm): 175.89, 143.68, 139.71, 124.55, 121.32, 107.39, 35.16. Elemental analysis (C8H6ClNO3S) Calculated values: C 41.48, H 2.61, N 6.05, S 13.84; measured values: C 41.57, H 2.72, N 6.14, S 13.92. S 13.92.
