Experimental procedure¹⁾: L-leucine tert-butyl ester hydrochloride (1, 67 mg, 0.3 mmol), N-Fmoc-L-proline (2, 101 mg, 0.3 mmol) and HATU (171 mg, 0.45 mmol) are added to a flask under argon. Anhydrous dichloromethane (1 mL), anhydrous DMF (0.5 mL), and collidine (79 µL, 0.6 mmol) are added and stirred at room temperature for 3 hr. HCl (1 mol/L, 1 mL) is added and the reaction mixture is diluted with dichloromethane (3 mL) and washed with H₂O (3 x 1.5 mL). The crude mixture is concentrated in vacuo and then purified by flash chromatography (hexane:EtOAc = 1:0 ~ 1:1) to yield 3 as a white solid (151 mg, 0.3 mmol) in 99% yield.