To a solution of the obtained 2-methyl-2-propanyl (3R)-3-[({4-[2-(dibenzylamino)ethyl]-2-(hydroxymethyl)-2,7-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-6-yl}carbonyl)(2-propanyl)amino]-1-piperidine carboxylate (22.5 g) in N,N-dimethylformamide (80 ml) were added sodium hydride (1.6 g) and methyl iodide (5.5 g) under ice-cooling. The mixture was stirred for 30 minutes, and then, cooled to room temperature, and further stirred for 2 hours. To the reaction solution was added a saturated aqueous ammonium chloride solution, and the mixture was ethyl acetate. The organic layer was washed with a saturated aqueous sodium chloride solution, and dried over sodium sulfate. The sodium sulfate was removed by filtration, and the filtrate was concentrated under reduced pressure, followed by purification with silica gel chromatography to give the title compound (19.2 g).
