Sodium hydride (60%, 380 mg) was added to a solution of (4S)-1-(2,4-dimethoxybenzyl)-4-(propan-2-yl)imidazolidin-2-one (2.4 g) in DMF (35 mL), and the mixture was stirred for 20 min. Ethyl bromoacetate (1.14 mL) was added thereto, and the mixture was stirred at 90° C. for 1 hr. Additional ethyl bromoacetate (1.14 g) and sodium hydride (60%, 380 mg) were added thereto, and the resulting mixture was further stirred at 90° C. for 1 hr. After the solvent was distilled off under reduced pressure, the residue was purified by column chromatography (silica gel cartridge, hexane/ethyl acetate) to afford ethyl[(55)-3-(2,4-dimethoxybenzyl)-2-oxo-5-(propan-2-yl)imidazolidin-1-yl]acetate (1.5 g).
