Compound 1a (9.69 g, 21.9 mmol) was dissolved in 200 mL of methanol, to which 500 mg of 10% palladium carbon was then added, followed by stirring at room temperature for one day under an atmosphere of hydrogen. The catalyst was filtered off from the reaction mixture, followed by concentration under reduced pressure. This residue, Cbz-Phe (6.92 g, 23.1 mmol), HOBT (3.71 g, 24.2 mmol), and NMM (2.66 mL, 24.2 mmol) were dissolved in 50 mL of dimethylformamide (DMF), to which EDC (4.64 g, 24.2 mmol) was then added under stirring and cooling with ice, followed by stirring at room temperature for one day. Water was added to the reaction solution, which was then washed with a 10% citric acid aqueous solution, a saturated sodium bicarbonate solution, and water before drying. The resultant residue was purified using silica gel column chromatography (elution solvent: chloroform:methanol=90:10) to provide 12.8 g of the title compound as a white solid.
