[CH2:1]([O:3][C:4]([C:6]1[C:10]([CH3:11])=[C:9]([CH3:12])[S:8][C:7]=1[NH2:13])=[O:5])[CH3:2].[C:14]1(=[O:20])[O:19][C:17](=[O:18])[CH:16]=[CH:15]1>CCOCC>[CH2:1]([O:3][C:4]([C:6]1[C:10]([CH3:11])=[C:9]([CH3:12])[S:8][C:7]=1[NH:13][C:14](=[O:20])[CH:15]=[CH:16][C:17]([OH:19])=[O:18])=[O:5])[CH3:2]A solution of 2-Amino-4,5-dimethyl-thiophene-3-carboxylic acid ethyl ester (5.53 g, 27.8 mmol) in 25 mL of anhydrous ether was added dropwise to a solution of maleic anhydride (2.72 g, 27.8 mmol) in 25 mL of anhydrous ether. The reaction mixture was stirred at room temperature for 10 h. A yellow precipitate was formed. 2-(3-Carboxy-acryloylamino)-4,5-dimethyl-thiophene-3-carboxylic acid ethyl ester (5.96 g, 72% yield) was obtained as a yellow solid by filtration.
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