Under argon protection, 3.56 g (29.6 mmol) of 2,3-dihydrobenzofuran was added to a 12 mL slurry of 1,2-dichloroethane containing 5.44 g (35.5 mmol) of sulfur trioxide-N,N-dimethylformamide complex. The reaction system was heated to 85 °C, maintained for 1 h and cooled to room temperature. Subsequently, thionyl chloride (2.6 mL, 35.5 mmol, 1.2 eq.) was added slowly and the temperature was gradually increased to 75 °C over 1 hour. After completion of the reaction, the mixture was cooled to room temperature and extracted by adding 100 mL of dichloromethane and 100 mL of water. The organic phase was separated, dried with anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give 6.56 g (100% yield) of 2,3-dihydrobenzofuran-5-sulfonyl chloride as a tan oil (TLC Rf value, chloroform/hexane=1/1).
