4-Amino-3-fluoro-2-((2-fluorophenyl)amino)-5-hydroxybenzoic acid can be cyclized in the presence of acid (such as p-toluenesulfonic acid, pyridinium toluene-4-sulphonate, formic acid, acetic acid, sulfuric acid) in appropriate solvent (include aliphatic and aromatic hydrocarbon (such as pentane, hexane, heptane, cyclohexane, petroleum ether, petrol, gasoline, benzene, toluene, xylene), aliphatic and aromatic halo-hydrocarbon (such as dichloromethane, 1,2-dichloroethane, chloroform, phenixin, chlorobenzene, o-dichlorobenzene), ether (such as diethyl ether, dibutyl ether, glycol dimethyl ether, 2-methoxyethyl ether, tetrahydrofuran, dioxane), ketone (such as acetone, methyl ethyl ketone, methyl isopropyl ketone, methyl isobutyl ketone), ester (such as ethyl acetate, methyl acetate), nitrile (such as acetonitrile, propiononitrile), amide (such as N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidin-2-one), DMSO, sulfolane, HMPA, DMPU, prefer methyl acetate, ethyl acetate and trimethoxymethane) for some time (0.2-12 h, prefer 0.5-10 h). 4-Fluoro-5-((2-fluorophenyl)amino)benzo[d]oxazole-6-carboxylic acid is obtained after conventional workup.
