p-Toluenesulfonic acid → 4-(3-bromophenyl)-2,4-dihydro-3H-1,2,4-triazol-3-one

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A mixture of 3-bromoaniline (1.0 mL, 9.18 mmol), methyl hydrazinocarboxylate (788 mg, 8.75 mmol), triethyl orthoformate (0.96 mL, 8.75 mmol), and TsOH (25 mg) in MeOH (20 mL) was stirred at 65° C. for 3 hr. After cooling to room temperature, NaOMe (1.47 g, 26.2 mmol) was added and the mixture stirred at room temperature for 16 hr. After concentration in vacuo the residue was taken up in EtOAc and H2O then acidified with 1N HCl. The aqueous phase was extracted with EtOAc and the combined organics washed with H2O then extracted twice with 1N NaOH. Combined NaOH extracts were acidified with conc. HCl and aged 5 min. Resulting solids were collected by filtration, washed with H2O and dried to give 4-(3-bromophenyl)-2,4-dihydro-3H-1,2,4-triazol-3-one as a white solid. 1H NMR (400 MHz, DMSO-d6) δ ppm 7.42-7.46 (m, 1H), 7.53 (d, J=7.8 Hz, 1H), 7.72 (d, J=7.8 Hz, 1H), 7.98 (s, 1H), 8.43 (s, 1H), 12.03 (s, 1H).

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