L-tryptophan (10.24 g, 50 mmol) was dissolved in 1 M aqueous sodium hydroxide solution (50 mL) and stirred at 0 °C. Subsequently, benzyl chloroformate (7.15 mL, 50.1 mmol) was added slowly and dropwise into the reaction system simultaneously with 1 M aqueous sodium hydroxide solution (50 mL). After the dropwise addition, the reaction mixture was continued to be stirred at room temperature for 1 hour. After completion of the reaction, the reaction solution was acidified to pH 1 with 6 M hydrochloric acid, and then the target product was extracted with ethyl acetate (3 x 200 mL). The organic phases were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure to give a yellow solid product (16.72 g, 99% yield), which could be used in the next reaction without further purification. The analytical data of the product were as follows: thin layer chromatography (TLC) Rf value of 0.43 (unfolding agent ratio of 95:4:1 dichloromethane/methanol/acetic acid); melting point (Mp): 132-134 °C; 1H NMR (DMSO-d6, 400 MHz) δ: 7.57-7.53 (m, 2H), 7.35-7.22 (m, 6H), 7.16- 7.15 (m, 1H), 7.09-7.04 (m, 1H), 7.00-6.95 (m, 1H), 5.02-4.92 (m, 2H), 4.28-4.21 (m, 1H), 3.22-3.16 (m, 1H), 3.04-2.96 (m, 1H); 13C NMR (DMSO-d6, 100 MHz) δ : 174.2, 156.5, 137.4, 136.6, 128.8, 128.2, 128.0, 127.6, 124.2, 121.4, 118.8, 118.6, 111.9, 110.5, 65.8, 55.5, 27.4; low resolution mass spectrometry (LRMS, ESI+): C19H19N2O4 [M+ H]+ calculated value 339.13, measured value 339.35.
