[Cl:1][C:2]1[CH:10]=[C:9]([Cl:11])[CH:8]=[C:7]2[C:3]=1[C:4](=[O:13])[C:5](=[O:12])[NH:6]2.C(O)(=[O:16])C>C(OC(=O)C)(=O)C.O.[O-2].[O-2].[O-2].[Cr+6]>[Cl:1][C:2]1[C:3]2[C:4](=[O:13])[O:12][C:5](=[O:16])[NH:6][C:7]=2[CH:8]=[C:9]([Cl:11])[CH:10]=1To a stirred solution of 4,6-dichloro-1H-indole-2,3-dione (5.00 g, 23.2 mM) in acetic acid (21 mL) and acetic anhydride (21 mL) at 80° C. was added in small portions chromium trioxide (4.12 g, 41.3 mM). The temperature of the reaction mixture was maintained between 80°-90° C. during the addition of the chromium trioxide. After the addition was complete, the reaction mixture was diluted with water (100 mL) and then filtered to separate the precipitated solids. The solids were washed thoroughly with water and then dried to obtain the title compound as a yellow solid (4.13 g, 72%); MS(CI): 232 (M+H).
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