Formaldehyde → N-(2-bromo-4-(heptafluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluoro-3-(methylamino)benzamide

0.930 g (1.71 mmol) of 3-amino-N-(2-bromo-4-(heptafluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluorobenzamide was added to 5 ml of concentrated sulfuric acid, and 10 ml of a 37% aqueous formaldehyde solution was charged dropwise thereto at 40° C. The reaction mixture was poured into ice-water, adjusted to pH 10 using an aqueous sodium hydroxide solution, and extracted with the addition of ethyl acetate. The organic layer was washed with a 20% aqueous sodium hydroxide solution and saturated brine, and then dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure and the obtained residue was purified by silica gel column chromatography (eluent solvent; hexane:ethyl acetate=8:1) to prepare 0.690 g (yield 72%) of a target compound.