General procedure for the synthesis of (S)-2-((S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)propionylamino)propanoic acid from N-fluorenylmethoxycarbonyl-L-alanine (Fmoc-L-Ala-OH): Fmoc-L-Ala-OH (0.93 g, 3.0 mmol) was dissolved in dichloromethane (25 mL), added 2 -chlorotrityl chloride resin (6 g, loading 0.5 mmol/g) to N,N-diisopropylethylamine (DIPEA, 0.52 mL, 3.0 mmol). The resin mixture was stirred for 14 min, then DIPEA (0.78 mL, 4.5 mmol) was added and the reaction continued to be shaken for 2.5 h. The reaction was carried out for 2.5 min. After completion of the reaction, the reaction solution was drained and the resin was washed with N,N-dimethylformamide (DMF, 20 mL). Next, a mixture of dichloromethane/methanol/DIPEA (40 mL, 80:15:5, v/v) was added and shaken for 30 min, the solution was drained and this washing process was repeated. After that, the resin was washed with DMF (20 mL). A DMF solution of piperidine (15.0 mL, piperidine:DMF=1:4) was added to the resin mixture, shaken for 10 min, drained the liquid and repeated the piperidine wash for 40 min. Subsequently, the resin loaded with amino acids was thoroughly washed with DMF (35 mL), isopropanol (35 mL), and hexane (35 mL) in that order. The resin was dried under vacuum for 30 min and then placed in a desiccator overnight. Dichloromethane (50 mL) was added to the dried resin and left to stand for 1 hour before draining the solution. Next, HBTU (4.27 g, 11.25 mmol), HOBt (1.72 g, 11.25 mmol), Fmoc-L-Ala-OH (2.34 g, 7.5 mmol), and DIPEA (2.61 mL, 15.0 mmol) were dissolved in DMF (11.3 mL), and added to the resin mixture and stirred to react for 4 hours After the reaction was completed, the reaction solution was discharged. After the reaction was completed, the reaction solution was drained and the resin was washed with DMF (30 mL), isopropanol (30 mL) and hexane (30 mL) sequentially. Finally, a dichloromethane solution of 2,2,2-trifluoroethanol (15 mL, 1:4, v/v) was added to the resin, and the solution was discharged after stirring for 2 h. The organic solvent was removed under vacuum to give the target product (0.90 g, 78% yield) as a yellow foam. The melting point of the product was 195-196 °C; the Rf value was 0.53 (unfolding agent was dichloromethane/methanol, 9:1, v/v); the specific optical rotation [α]D20 was -23.9 (c 0.14, methanol); the infrared spectroscopy (ATR) νmax showed that the absorption peaks were located at 3297, 2918, 1692, 1650, 1533, 1450, 1318, 1229 cm-1; NMR hydrogen (CDCl3, 500 MHz) and carbon (CDCl3, 125 MHz) spectral data were consistent with the structure of the target compounds; high-resolution mass spectrometry (HRESIMS) showed the [M + Na]+ peak to be 405.1424 (calculated value C21H22N2NaO5, 405.1421).
