[Cl:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][C:11]=1[Cl:12])N[C:7](=O)[C:6](=O)[NH:5]2.P(Br)(Br)(Br)(Br)[Br:16].[BrH:21].[NH4+:22].[OH-]>[OH-].[Na+]>[Br:21][C:7]1[C:6]([Br:16])=[N:5][C:4]2[C:9](=[CH:10][C:11]([Cl:12])=[C:2]([Cl:1])[CH:3]=2)[N:22]=1
6,7-Dichloro-1,4-dihydro-quinoxaline-2,3-dione (5.39 g, 23.3 mmol) and phosphorus pentabromide (20.1 g, 46.7 mmol) were charged in a 100 mL round-bottom flask equipped with a condenser with an outlet half immerged in 10% NaOH aqueous solution (to absorb HBr generated during the reaction). The reaction was heated at 155° C. using an oil-bath for 2 hours when the formation of HBr ceased. The reaction content was poured into ice-water (100 mL) and basified with NH4OH. After filtration, the solid was dried and recrystallized using EtOH to give the desired product as a white solid (7.84 g, 94% yield): mp 169-70° C.; Rf=0.50, CH2Cl2; IR (KBr, cm−1) 3088, 1539, 1451, 1240, 1128, 964, 997; 1H NMR (CDCl3) δ 8.13 (s, 2H); 13C NMR (CDCl3) δ 142.4, 139.7, 136.3; MS (ACPI), m/z 338.4 (MH+); Anal. Calcd for C8H2N2Cl2Br2: C, 26.93; H, 0.56; N, 7.85. Found: C, 26.93; H, 0.56; N, 7.86.
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