To a solution of tert-butyl (3R,4S)-3-({[4-(4-methoxybutyl)-2,2-dimethyl-3-oxo-7-(trifluoromethyl)-3,4-dihydro-2H-1,4-benzoxazin-6-yl]carbonyl}amino)-4-phenylpiperidine-1-carboxylate (386 mg) in DMF (1 ml) was added sodium hydride (48.7 mg) at 0° C., and the mixture was stirred for 10 minutes. To the mixture was added ethyl iodide (126 μl), and the mixture was stirred at room temperature for 5 hours. To the reaction solution was added a saturated aqueous ammonium chloride solution, and the mixture was extracted with ethyl acetate. The organic layer was washed with water (×2) and a saturated aqueous sodium chloride solution, dried over sodium sulfate, filtered, and concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography (the developing solvent: hexane/ethyl acetate=3/1) to give the title compound (128 g).
