To a solution of the above 1-[4-chloro-2-(trifluoromethyl)phenyl]-2-methyl-N-[4-(methylsulfonyl)phenyl]-5-(2-oxoethyl)-1H-pyrrole-3-carboxamide (0.85 g, 1.7 mmol) in tert-butanol (14 mL)-water (3.4 mL), sodium dihydrogenphosphate (0.40 g, 2.6 mmol), 2-methyl-2-butene (0.77 mL, 6.8 mmol) and sodium chlorite (0.85 g, 5.1 mmol) were added, and the mixture was stirred at room temperature for 1 hour. A 10% aqueous sodium hydrogensulfite solution and 1M hydrochloric acid were added to the mixture to stop the reaction, and the mixture was extracted with dichloromethane. The obtained organic layer was washed with saturated brine, and after the solution was dried with sodium sulfate, it was filtered and the solvent was distilled off under reduced pressure. The residue was purified by silica gel column chromatography to give (1-[4-chloro-2-(trifluoromethyl)phenyl]-5-methyl-4-{[4-(methylsulfonyl)phenyl]carbamoyl}-1H-pyrrol-2-yl)acetic acid (0.50 g, 57%).
