To 1-(4-methoxybenzyl)piperidin-2-one (50.0 g) was added aqueous 4N sodium hydroxide solution (228 ml) and then the mixture was refluxed for a day. The mixture was cooled to 0° C. and concentrated hydrochloric acid (76 ml) was added thereto. The mixture was neutralized. Then, after adding sodium carbonate (53.8 g) and dimethyl sulfoxide (600 ml), 5-bromo-2-fluorobenzaldehyde (38.6 g) was added dropwise at 135° C. to the mixture. After refluxing for 5 hours, the mixture was neutralized with 3N hydrochloric acid at 0° C., and extracted with ethyl acetate. The organic layer was washed with water and saturated brine, after which it was dried with magnesium sulfate. The solvent was distilled off under reduced pressure to give 5-((4-bromo-2-formylphenyl)(4-methoxybenzyl)amino)pentanoic acid (77.2 g) as a brown oily material.
