A solution of benzyl 3-[(tert-butoxycarbonyl)amino]azetidine-1-carboxylate obtained in Example (227a) (472 mg, 1.54 mmol) in THF (5 mL) was added to a suspension of sodium hydride (55%) (134 mg, 3.08 mmol) in THF (10 mL) at 0° C. Subsequently, propyl iodide (0.75 mL, 7.70 mmol) was added at 0° C., and the mixture was stirred at room temperature for 70 minutes. THF (4 mL) and DMF (5 mL) were added, followed by further stirring for 18 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 499 mg of the title compound as a colorless oily substance (93%).
