Step 1: Synthesis of 7-chloro-1-benzothiophene-2-carboxylic acid
In a reaction flask containing potassium hydroxide (1.3 g, 22 mmol), 2-mercaptoacetic acid (0.70 mL, 10 mmol) and 10 mL of water, 2,3-dichlorobenzaldehyde (1.8 g, 10 mmol) was added. The reaction mixture was stirred at 120 °C for 2 hours. Upon completion of the reaction, the reaction solution was cooled to room temperature and distilled water was added until the precipitated solid was completely dissolved. Subsequently, ether was added to the reaction solution and the mixture was separated into an organic layer and an aqueous layer. To the aqueous layer, 1.0 N hydrochloric acid was added dropwise to adjust the pH to 5, and then the aqueous layer was extracted with ethyl acetate. The organic layers were combined, washed with saturated brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give 7-chloro-1-benzothiophene-2-carboxylic acid (1.7 g, 82% yield) as a colorless liquid. The product was confirmed by 1H-NMR (400 MHz, DMSO-d6) and mass spectrometry (ESI): 1H-NMR δ 7.48-7.54 (1H, m), 7.62-7.70 (1H, m), 7.98-8.04 (1H, m), 8.16-8.22 (1H, m); MS (ESI) [M+H]+ 213.
