Triethylamine (9.71 g) and magnesium chloride (7.61 g) were successively added to a suspension of potassium ethyl malonate (10.8 g) in tetrahydrofuran with stirring at room temperature. After stirring at 60-70° C. for 1.5 hr, a solution of 5-(3-chloro-2-fluorobenzyl)-2-fluoro-4-methoxybenzoyl chloride prepared in Step V in toluene was added dropwise, and the mixture was further stirred for 30 min. After cooling, toluene (50 ml) and 2N hydrochloric acid (60 ml) were added to the reaction mixture, and the mixture was stirred for 1 hr. The organic layer was washed successively with water (50 ml), 5% sodium hydrogencarbonate (50 ml, twice), and water (50 ml) again, and the organic layer was concentrated under reduced pressure. Toluene (50 ml) was added to the residue, and the mixture was concentrated under reduced pressure. The residue was dissolved in toluene (50 ml) to give a solution of 3-(5-(3-chloro-2-fluorobenzyl)-2-fluoro-4-methoxyphenyl)-3-oxopropionic acid ethyl ester in toluene. This was identified as the title compound by HPLC.
