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In a flask equipped with a Dean-Stark trap, a mixture of 2-((3-ethoxy-2-(ethoxycarbonyl)-3-oxoprop-1-enyl)amino)-4-methylthiophene-3-carboxylic acid (Preparation 25, 4.58 g) and phenyl ether (50 mL) is degassed by freeze-pump-thaw method and then heated to reflux. After approximately 30 min, the reaction mixture is allowed to cool to room temperature. The crude mixture is purified by column chromatography (CH2Cl2; CH2Cl2/methanol, 100/1) to afford 1.95 g of the title compound as a yellow solid. Physical characteristics. M.p. 189-192° C.; 1HNMR (400 MHz, CF3CO2D) δ 11.65, 9.10, 7.31, 4.66, 2.76, 1.53; MS (ESI−) m/z 236 (M—H). Anal. Found: C, 55.66; H, 4.67; N, 5.85; S, 13.40.

1. customIn a flask equipped with a Dean-Stark trap
2. customis degassed by freeze-pump-thaw method
3. temperatureheated to reflux
4. customThe crude mixture is purified by column chromatography (CH2Cl2; CH2Cl2/methanol, 100/1)