Methyl(2S,4S)-1-BOC-4-[(4,4-dimethylcyclohexyl)amino]pyrrolidine-2-carboxylate (1.01 g, 2.84 mmol) obtained in Step A, was dissolved in pyridine (5 mL), and Ac2O (1.34 mL, 14.20 mmol) was added thereto at room temperature. The reaction solution was heated to 90° C. and was stirred for 2 hours. After the reaction was completed, CuSO4.5H2O aqueous solution was added and the solution was extracted with EtOAc. The extracted organic solution was dried over MgSO4 and concentrated in vacuo. The obtained residue was purified by column chromatography (eluent: EtOAc/Hex=1/1) to give the title compound (0.98 g, 87%).
