A solution of benzyl 3-[(tert-butoxycarbonyl)amino]azetidine-1-carboxylate obtained in Example (227a) (500 mg, 1.63 mmol) in THF (6 mL) was added to a suspension of sodium hydride (55%) (142 mg, 3.26 mmol) in THF (10 mL) at 0° C. Subsequently, butyl iodide (0.93 mL, 8.15 mmol) was added at 0° C., and the mixture was stirred at room temperature for 19 hours. DMF (2 mL) was added, followed by stirring for four hours. Then, sodium hydride (55%) (36 mg, 0.82 mmol) and butyl iodide (0.37 mL, 3.26 mmol) were added, followed by stirring for 2.5 hours. Water was added to the reaction solution, and the mixture was extracted with ethyl acetate. Thereafter, the combined organic layers were washed with brine and dried over anhydrous magnesium sulfate. Following filtration, the filtrate was concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (elution solvent: hexane/ethyl acetate=2/1) to obtain 288 mg of the title compound as a colorless oily substance (49%).
