Chlorosulfonic acid → 2-oxo-2,3-dihydro-1H-indole-5-sulfonyl chloride

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Reaction Steps

To a solution of chlorosulfonic acid (25 mL, 37.5 mmol) at 0° C. was slowly added oxindole over 40 min. The resulting solution was stirred at RT for 1.5 h and then heated to 50° C. for 10 min until gas evolution ceased. After cooling to RT, the reaction mixture was carefully poured onto ice. A white precipitate was formed which was filtered, washed with ice-water and dried under HV. Yield: 14.35 g (82%). LC/MS (10-99%): M/Z: M+(obs)=232; tR=2.24 min.

Provenance & Evidence