1-Ethylpropylamine (124 μL, 1.06 mmol) was added to a solution of tert-butyl 4-{[(4-chloro-5-ethyl-1H-imidazol-2-yl)carbonyl]amino}-3-oxopiperidine-1-carboxylate obtained in Example (201c) (197 mg, 0.531 mmol) in tetrahydrofuran (4 mL), and the mixture was heated under reflux at 50° C. for four hours. Methanol (3 mL) was added to the reaction solution. Sodium cyanoborohydride (100 mg, 1.59 mmol) and acetic acid (304 μL, 5.31 mmol) were added, and the mixture was stirred at room temperature for 15 hours. The reaction solution was concentrated under reduced pressure. Saturated aqueous sodium bicarbonate solution was added to the residue, followed by extraction with ethyl acetate. The resulting organic layer was washed with brine, and dried over anhydrous sodium sulfate, and then the insoluble matter was separated by filtration. The filtrate was concentrated under reduced pressure, and the resulting residue was purified by silica gel column chromatography and preparative TLC (eluent: chloroform:methanol=10:1 v/v) to obtain 49 mg of the cis isomer of the title compound (21%) as a pale yellow oily substance and 16 mg of the trans isomer of the title compound (7%) as a pale yellow oily substance.
