A solution of 3-tert-butoxycarbonylaminoazetidine (136 mg, 0.79 mmol), ethyl 2-bromo-4-isopropyl-1,3-thiazole-5-carboxylate obtained in Example (23b) (200 mg, 0.72 mmol) and diisopropylethylamine (0.25 mL, 1.44 mmol) in DMF (8 mL) was stirred at 90° C. for 11 hours. The reaction solution was diluted with ethyl acetate, and then the mixture was washed with 10% saline and dried over anhydrous magnesium sulfate. Following filtration, the solvent was evaporated under reduced pressure. The resulting residue was purified by silica gel column chromatography (elution solvent: hexane/ethyl acetate=4/1) to obtain 175 mg of the title compound as a white solid (66%).
