General procedure for the synthesis of ethyl 5-methoxybenzo[b]thiophene-2-carboxylate from 2-fluoro-5-methoxybenzaldehyde and ethyl thioglycolate: To a solution of N,N-dimethylformamide (DMF, 30 mL) of 2-fluoro-5-methoxybenzaldehyde (4.35 g, 28.2 mmol) was added sequentially ethyl thioglycolate (3.71 mL, 33.9 mmol) and potassium carbonate (K2CO3, 7.86 g, 57.0 mmol). The reaction mixture was stirred at 80 °C for 60 min, and after completion of the reaction, the reaction was quenched with water (300 mL). The reaction mixture was extracted with ethyl acetate (EtOAc, 2 x 200 mL), the organic layers were combined and dried over anhydrous sodium sulfate (Na2SO4). The organic layer was concentrated under reduced pressure and the resulting crude product was purified by silica gel column chromatography (eluent: 10% ethyl acetate/hexane) to afford ethyl 5-methoxybenzo[b]thiophene-2-carboxylate as an oil (4.40 g, 66% yield). Nuclear magnetic resonance hydrogen spectrum (1H-NMR, CDCl3, δ ppm): 7.97 (1H, s), 7.70 (1H, d, J = 8.8 Hz), 7.27 (1H, d, J = 2.6 Hz), 7.09 (1H, dd, J = 2.6, 8.8 Hz), 4.41 (2H, q, J = 7.0 Hz), 3.88 (3H, s), and 1.42 (3H, t, J = 7.0 Hz).
![Ethyl 5-methoxybenzo[b]thiophene-2-carboxylate](/CAS/20200331/GIF/40862-88-2.gif)