Polyphosphoric acid (PPA, 0.743 g, 6.83 mmol) was added to a solution of compound 175 (0.656 g, 2.28 mmol) in toluene (4.6 mL) under an argon atmosphere and stirred at room temperature. The reaction mixture was heated to reflux and maintained for 4 hours. Upon completion of the reaction, it was cooled to room temperature and the reaction was quenched with ice water and subsequently extracted with ethyl acetate (EtOAc). The organic layers were combined, washed with saturated brine, dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure. The crude products were purified by silica gel column chromatography (eluent: ethyl acetate/hexane, 2:98) to afford 6-bromobenzofuran (176, main product, 0.206 g, 1.05 mmol, 46% yield) and 4-bromobenzofuran (177, by-product, 0.183 g, 0.935 mmol, 41% yield, colorless oily substance), respectively.
1H-NMR (CDCl3, 400 MHz) data for 6-bromobenzofuran (176): δ 6.75 (d, J = 1.6 Hz, 1H, furan-H), 7.36 (dd, J = 1.2, 8.4 Hz, 1H, Ar-H), 7.46 (d, J = 8.4 Hz, 1H, Ar-H), 7.60 (d, J = 1.6 Hz 1H, furan-H), 7.69 (s, 1H, Ar-H).
1H-NMR (CDCl3, 400 MHz) data for 4-bromobenzofuran (177): δ 6.82 (d, J = 2.4 Hz, 1H, furan-H), 7.17 (dd, J = 7.6, 8.4 Hz, 1H, Ar-H), 7.40 (d, J = 7.6 Hz, 1H, Ar-H), 7.45 (d, J = 8.4 Hz 1H, Ar-H), 7.66 (d, J = 2.4 Hz, 1H, furan-H).
