To a solution of 2-(5-(3-chloro-2-fluorobenzyl)-2-fluoro-4-methoxybenzoyl)-3-oxobutyric acid ethyl ester prepared in Step VI-1 in ethanol were successively added water (5 ml) and sodium acetate (7.87 g) with stirring at room temperature, and the mixture was stirred for 4 days. Water (20 ml) was added to the reaction suspension at 70° C. and, after confirmation of dissolution, the mixture was cooled to room temperature. A seed crystal (10 mg) of the title compound was added and, after stirring for 1 hr, water (75 ml) was added again, and the mixture was stirred for 4 hr. The precipitated crystals were collected by filtration, washed with 50% ethanol (20 ml) and vacuum dried to give 3-(5-(3-chloro-2-fluorobenzyl)-2-fluoro-4-methoxyphenyl)-3-oxopropionic acid ethyl ester (10.3 g, yield 84.5%). This was identified as the title compound by HPLC.
