[Cl:1][C:2]1[CH:7]=[CH:6][C:5]([C:8]2[C:12]([C:13]([O:15]CC)=[O:14])=[C:11]([C:18]([O:20]CC)=[O:19])[S:10][N:9]=2)=[CH:4][CH:3]=1.[OH-].[Na+].Cl>C(O)C.O>[Cl:1][C:2]1[CH:3]=[CH:4][C:5]([C:8]2[C:12]([C:13]([OH:15])=[O:14])=[C:11]([C:18]([OH:20])=[O:19])[S:10][N:9]=2)=[CH:6][CH:7]=1Into a 100-mL round-bottom flask, was placed a solution of 4,5-diethyl 3-(4-chlorophenyl)-1,2-thiazole-4,5-dicarboxylate (3.28 g, 9.84 mmol, 1.00 equiv) in ethanol (20 mL) and a solution of sodium hydroxide (2.36 g, 59.00 mmol, 6.00 equiv) in water (20 mL). The resulting solution was stirred for 2.5 h at 90° C. in an oil bath. The pH value of the solution was adjusted to 3 with hydrogen chloride (1 mol/L). The resulting solution was extracted with 3×150 mL of dichloromethane. The organic layers were combined, washed with 1×10 mL of brine, dried over anhydrous sodium sulfate and concentrated under vacuum. This resulted in 1.86 g (69%) of 3-(4-chlororphenyl)-1,2-thiazole-4,5-dicarboxylic acid as a yellow solid.
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