Under nitrogen protection, (S)-N-benzyloxycarbonyl-L-tryptophan (2.45 g, 7.2 mmol) was suspended in 10 mL of anhydrous dichloromethane and 4-dimethylaminopyridine (DMAP, 0.1 g, 0.82 mmol) and methanol (0.28 g, 11.8 mmol) were added. The reaction mixture was cooled to 0 °C and a solution of N,N'-dicyclohexylcarbodiimide (DCC) dissolved in dichloromethane was slowly added dropwise. After the dropwise addition, the reaction system was gradually warmed up to room temperature and stirred continuously for 12 hours. After completion of the reaction, the insoluble material was removed by filtration and the filtrate was concentrated under reduced pressure. The crude product was purified by fast column chromatography (eluent: n-hexane/ethyl acetate, 4:1 to 2:1 gradient elution) to afford the target compound (S)-methyl (S)-2-(((benzyloxy)carbonyl)amino)-3-(1H-indol-3-yl)propanoate (2.5 g, 98%) as a colorless oil. Thin layer chromatography (TLC) Rf = 0.19 (Expanding agent: ethyl acetate/hexane = 1:2). Nuclear magnetic resonance hydrogen spectrum (1H NMR, 300 MHz, CDCl3): δ 8.94 (broad peak, 1H), 7.49 (d, J = 7.7 Hz, 1H), 7.25-6.98 (multiple peaks, 9H), 6.81 (single peak, 1H), 5.84 (d, J = 8.7 Hz, 1H), 4.98 (d, J = 3.2 Hz, 2H), 4.68 (quadruple peak, 2H). 4.68 (quadruple peak, J = 7.3 Hz, 1H), 3.25 (single peak, 3H), 3.21 (multiple peaks, 2H). Nuclear magnetic resonance carbon spectrum (13C NMR, 75 MHz, CDCl3): δ 172.2, 155.9, 141.8, 136.8, 134.9, 128.6, 127.8, 126.9, 123.4, 122.3, 120.0, 118.4, 112.1, 109.4, 67.0, 65.1, 55.1. Fourier Transform Infrared Spectroscopy (FTIR, KBr, pure sample): ν 1707, 1508, 1456, 1436, 1215, 742 cm-1. High Resolution Mass Spectrometry (HRMS, EI): Calculated value C20H20O4N2 [M]+ 352.1418, measured value 352.1425.
