Dimethylformamide → 5-Chloro-3-methylbenzo[b]thiophene-2-carbaldehyde

To a solution of 5-chloro-3-methyl-benzo[b]thiophene (4.98 g, 27.3 mmol) in THF (50 mL) at −40° C. was added n-butyllithium (18.7 mL of a 1.6 M solution in hexanes, 30.0 mmol) dropwise. The resulting yellow solution was stirred for 1 h and then DMF (6.3 mL, 81.9 mmol) was added in one portion. The solution was warmed slowly to room temperature and stirred overnight. Saturated aqueous NH4Cl was added and the mixture was extracted with ethyl acetate (3×200 mL). The combined organics were washed with water (100 mL) and brine (100 mL), dried over Na2SO4, filtered and concentrated under reduced pressure to give the title compound (6.62 g, 89%) as a yellow solid: 1H NMR (300 MHz, CDCl3) δ 10.32 (s, 1H), 7.85 (s, 1H), 7.79 (d, J=8.7 Hz, 1H), 7.46 (dd, J=8.7, 2.0 Hz, 1H), 2.74 (s, 3H).