[Cl:1][C:2]1[CH:7]=[C:6]([Cl:8])[C:5]([O:9]C)=[CH:4][C:3]=1[C:11]1[C:12](=[O:22])[N:13]([CH3:21])[C:14]([C:17]([F:20])([F:19])[F:18])=[CH:15][N:16]=1.Br.O>C(O)(=O)C>[Cl:1][C:2]1[CH:7]=[C:6]([Cl:8])[C:5]([OH:9])=[CH:4][C:3]=1[C:11]1[C:12](=[O:22])[N:13]([CH3:21])[C:14]([C:17]([F:18])([F:20])[F:19])=[CH:15][N:16]=1A mixture of 1.00 g of 3-(2,4-dichloro-5-methoxyphenyl)-1-methyl-6-trifluoromethyl-2-oxo-1,2-dihydropyrazine (present compound 1-358) and 16 ml of 48% hydrobromic acid was dissolved in 5.7 ml of acetic acid, and the solution was stirred at 140° C. for 1 hour. After completion of the reaction, the reaction mixture was poured into water, followed by extraction with ethyl acetate. The organic layer was washed with saturated sodium hydrogencarbonate, water, and then saturated sodium chloride solution, dried with anhydrous magnesium sulfate, and concentrated. The residue was subjected to silica gel column chromatography, which afforded 0.74 g (yield, 77%) of 3-(2,4-dichloro-5-hydroxyphenyl)-1-methyl-6-trifluoromethyl-2-oxo-1,2-dihydropyrazine (present compound 1-357; m.p., 173.5° C.).
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