N-carbobenzoxy-L-phenylalanine (Cbz-Phe: 7.16 g, 25.4 mmol), N-t-butoxycarbonyl-ethylenediamine hydrochloride (5.00 g, 25.4 mmol), 1-hydroxybenzotriazole hydrate (HOBT: 4.28 g, 28.0 mmol), and N-methylmorpholine (NMM: 3.07 mL, 28.0 mmol) were dissolved in 100 mL of dimethylformamide (DMF), to which 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC: 5.36 g, 28.0 mmol) was then added under stirring and cooling with ice, followed by stirring at room temperature for one day. A 10% citric acid aqueous solution was added to the reaction solution, and the precipitated solid was dissolved in chloroform and a small amount of methanol, followed by washing the solution with a saturated sodium bicarbonate solution and a saturated saline solution before drying with sodium sulfate. It was concentrated under reduced pressure, followed by purifying the resultant residue using silica gel column chromatography (elution solvent: chloroform:methanol=95:5) to provide 9.69 g of the title compound as a white solid.
