In 26 ml of 1,2-dimethoxyethane, 1.2 g (0.0026 mole) of 1-[N-benzyl-o-(2,6-dichloroanilino)phenyl]-2-methylsulfinyl-2-methylthioethylene was dissolved and then, 13 ml of conc. HCl was added and the mixture was stirred at 80° C. for 4 hours. The reaction mixture was cooled and 80 ml of methylene chloride and 10 ml of water were added to extract the reaction product and the separated water phase was further mixed with 70 ml of methylene chloride to extract the reaction product. Both of methylene chloride phases were combined, dried over anhydrous sodium sulfate, and then concentrated under reduced pressure. The residue was separated by a column chromatography (silica gel: Wako gel Q-23 manufactured by Wako Junyaku and developing solvent:n-hexane:acetone=2:1) to obtain 0.14 g (yield: 14%) of N-benzyl-o-(2,6-dichloroanilino)phenyl acetic acid and 0.88 g (yield: 81%) of methyl N-benzyl-o-(2,6-dichloroanilino)phenylthioacetate.
