General procedure for the synthesis of 5-chlorothiophene-3-carbaldehyde from 3-thiophenecarboxaldehyde: The reaction was carried out to a solution of thiophene-3-carbaldehyde (20.0 g, 178.3 mmol) and N-chlorosuccinimide (23.8 g, 178.3 mmol) in acetic acid (180 mL) with stirring for 4 hours at 110 °C. After completion of the reaction, the reaction solution was cooled to room temperature, then diluted with ethyl acetate (120 mL), washed sequentially with water (100 mL x 3), saturated sodium bicarbonate solution (50 mL x 2) and brine, and the organic phase was dried with anhydrous sodium sulfate. After concentration under reduced pressure, 5-chlorothiophene-3-carbaldehyde (8.0 g, 54.6 mmol, 31% yield) was obtained as a yellow solid, which could be used for subsequent reactions without further purification.
