To a solution of diisopropylamine (3.4 mL) in tetrahydrofuran (10 mL) was added dropwise n-butyllithium (1.6 M hexane solution, 15 mL) at −78° C. under nitrogen atmosphere, and the mixture was warmed to 0° C., and stirred for 30 min. The reaction mixture was cooled to −78° C., a solution of 3-ethyl-1-(4-methoxybenzyl)pyrrolidin-2-one (2.8 g) obtained in Step B in tetrahydrofuran (10 mL) was added thereto, and the mixture was stirred at the same temperature for 1 hr. Ethyl bromoacetate (4.0 mL) was added thereto, and the mixture was stirred overnight while it was allowed to be warmed. To the reaction mixture was added saturated aqueous ammonium chloride solution, and the mixture was extracted with ethyl acetate. The obtained organic layer was washed with saturated brine, and dried over anhydrous magnesium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/ethyl acetate) to give the title compound (1.5 g).
