Sulfuric Acid → 5-Chloro-2-(ethoxycarbonyl)-1-(phenylsulfonyl)-1H-indole-3-sulfonic acid

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Reaction Steps

Concentrated sulfuric acid (2.50 ml, 90 mmol) was added dropwise over 5 min at 0° C. to a stirred solution of 2-carboethoxy-5-chloro-1-phenylsulfonylindole (7.28 g, 20.0 mmol) in acetic anhydride (10 ml) and dry dichloromethane (50 ml). The resulting tan solution was warmed to RT (room temperature) and after 3 hours was poured onto ice and the mixture was extracted with ethyl acetate. The ethyl acetate layer was washed with brine, dried (Na2SO4) and evaporated in vacuo to a syrup. Residual acetic anhydride was removed by azeotroping with toluene (3×50 ml), and the residue was crystallized from dichloromethane. The dichloromethane was evaporated in vacuo to give the title compound as a tan powder:

Provenance & Evidence