(2S)-3-[(tert-butoxycarbonyl)amino]-2-(5-chloro-2-methoxybenzyl)propionic acid (10g) was dissolved in ethyl acetate (120 ml), then methane sulfonic acid (9.4 ml) was added at room temperature, and the resulting mixture was stirred at that temperature for 2 hours. To the reaction solution was added triethylamine (20.3 ml), and the mixture was stirred for 13 hours. Then 2,4,6-trimethoxybenzaldehyde (5.7 g) was added and the mixture was stirred for 30 minutes. Next, to the reaction solution, sodium triacetoxyborohydride (9.3 g) was added and the mixture was stirred for 2 hours. To the reaction solution, a 1N aqueous sodium hydroxide solution was added, and the layers were separated. The aqueous layer was extracted again with ethyl acetate. The organic layer was washed with a 1N aqueous sodium hydroxide solution, then the appeared precipitated crystals were collected by filtration and dried to obtain the title compound (11.5 g) as a colorless crystal.
