Sulfuric acid, fuming ethyl chlorosulfonate

Ethyl-7-hydroxyindoxazenium tetrafluoroborate in boiling sulfuryl chloride yields a monochloro derivative (either the 4- or 6-) in 77% yield.45 Similarly, sulfonation of the tetrafluoroborate in fuming 30% sulfuric acid affords the zwitterionic 4- or 6-sulfonic acid (29) in 90% yield. In contrast, chlorosulfonation of indoxazene at 100°C yields indoxazene-5-sulfonyl chloride (67%).46... 

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