A solution of L-PHENYLALANINE BENZYL ESTER HYDROCHLORIDE (2 g, 6.85 mmol), Z-Phe-OH (2.257 g, 7.54 mmol), DIPEA (2.99 mL, 17.14 mmol) and DCM (100 mL) was treated with 1-HYDROXY-7-AZABENZOTRIAZOLE (1.120 g, 8.23 mmol) and EDC (1.577 g, 8.23 mmol) and stirred at rt for 15 h. The mixture was washed with saturated NaHCO3 (2×100 mL), 1 M HCl (2×50 mL) and brine (50 mL), dried over sodium sulfate, and concentrated under reduced pressure to give (S)-benzyl 2-((S)-2-(((benzyloxy)carbonyl)amino)-3-phenylpropanamido)-3-phenylpropanoate (3.2 g, 5.96 mmol, 87% yield) as a white solid. 1H NMR (400 MHz, DMSO-d6) δ 7.42-7.11 (m, 21H), 4.96 (d, J=2.0 Hz, 4H), 4.63-4.45 (m, 1H), 4.06-3.91 (m, 2H), 3.34-2.96 (m, 3H), 2.85 (br. s., 4H)
