Oxalyl chloride → 4,5,6,7-Tetrahydro-benzo[b]thiophene-2-carbonyl chloride

4,5,6,7-Tetrahydro-benzo[b]thiophene-2-carboxylic acid (1.0 g, 5.50 mmol) is dissolved in dichloromethane [DCM] (25 mL) that contains 5 drops of N,N-dimethylformamide [DMF] under nitrogen and cooled to 0° C. Oxalyl chloride (13.7 mL of a 2.0M solution in DCM) is added via syringe and allowed to warm to RT over 1 hour. All solvent is then removed under reduced pressure, and the resultant oil is reduced from toluene (3×20 mL) to remove residual oxalyl chloride. The residue is then dissolved in ethyl acetate and washed with saturated sodium bicarbonate (1×100 mL), then washed with saturated sodium chloride (1×100 mL) and dried over sodium sulfate. The solution is then filtered and concentrated under reduced pressure to give 4,5,6,7-tetrahydro-benzo[b]thiophene-2-carbonyl chloride (1) as an off-white solid (1.03 g).