(4S)-1-BOC-4-[acetyl(4,4-dimethylcyclohexyl)amino]-L-proline (0.92 g, 2.41 mmol) obtained in Step C of Example 1, was dissolved in DMF (10 mL) and DIPEA (1.05 mL, 6.01 mmol) was added thereto. Piperazine (0.25 g, 2.89 mmol) and HBTU (0.91 g, 2.41 mmol) were also added thereto in order. After the reaction solution was stirred at room temperature for 2 hours, the solution was concentrated in vacuo. The residue was diluted with EtOAc and washed with saturated NaHCO3 aqueous solution. The extracted organic solution was dried over MgSO4, concentrated in vacuo. The residue was purified by column chromatography (eluent: MC/MeOH=10/1) to give the title compound (1.01 g, 93%).
