[CH2:1]([O:3][C:4]([C:6]1[N:7]=[C:8]([C:11]2[CH:16]=[CH:15][CH:14]=[CH:13][CH:12]=2)[O:9][CH:10]=1)=[O:5])[CH3:2].[Cl:17]N1C(=O)CCC1=O>S(=O)(=O)(O)O.C(Cl)(Cl)Cl>[CH2:1]([O:3][C:4]([C:6]1[N:7]=[C:8]([C:11]2[CH:16]=[CH:15][CH:14]=[CH:13][CH:12]=2)[O:9][C:10]=1[Cl:17])=[O:5])[CH3:2]Mixture of 2-phenyl-oxazole-4-carboxylic acid ethyl ester (217 mg, 1 mmol), 1-chloro-pyrrolidine-2,5-dione (400 mg, 3 mmol) and two drops of sulfuric acid in chloroform (10 mL) was heated at 90 degrees for overnight. After the reaction was complete, solvent was evaporated. To the residue, water was added, and then the mixture was extracted with ethyl acetate twice. The organic layers were collected, combined, washed with saturated sodium bicarbonate, dried over sodium sulfate, filtered, and concentrated in vacuo. Flash chromatography (Merck silica gel 60, 230-400 mesh, 5-40% ethyl acetate in hexane for 30 min) gave 5-chloro-2-phenyl-oxazole-4-carboxylic acid ethyl ester (95 mg, 37%) as a white solid. LCMS calcd for C12H10ClNO3 (m/e) 251, obsd 252 (M+H).
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