[Synthesis]
The reaction was carried out with 1-(3-thienyl)ethanone oxime (250 mg, 1.77 mmol) in polyphosphoric acid (1.5 mL) at 100 °C with stirring for 30 min. After completion of the reaction, the reaction solution was poured into saturated aqueous sodium carbonate solution and extracted twice with ethyl acetate. The organic phases were combined, washed sequentially with water and saturated saline, and concentrated. The residue was purified by silica gel column chromatography with ethyl acetate:hexane (3:1) as eluent, and the fraction containing the target product was collected and concentrated. The concentrated residue was dissolved in 1,2-dichloroethane (1.5 mL), and a solution of phosphoryl chloride (225 μL, 2.24 mmol) and N,N-dimethylformamide (63 μL, 0.81 mmol) in 1,2-dichloroethane (0.5 mL), which had been pre-cooled to 0 °C, was slowly added. The reaction solution was stirred for 15 min at room temperature and then heated and refluxed for 16 hr. At the end of the reaction, the reaction solution was poured into water (10 mL) containing sodium acetate (550 mg, 4.05 mmol) and the mixture was stirred at 100 °C for 20 min. After cooling, water (25 mL) was added and extracted twice with ethyl acetate. The organic phases were combined, washed sequentially with saturated aqueous sodium bicarbonate and saturated saline, dried over anhydrous sodium sulfate and concentrated to afford 5-chlorothieno[3,2-b]pyridine 115 mg (38% yield) as a light yellow oil, which solidified on standing. The product was characterized by 1H NMR (400 MHz, CDCl3) and ESI-LCMS: 1H NMR δ 8.12 (d, J = 9 Hz, 1H), 7.80 (d, J = 5 Hz, 1H), 7.50 (d, J = 5 Hz, 1H), 7.28 (d, J = 9 Hz, 1H); ESI-LCMS m/z 170 ([M+H]+) . |