4-Hydrazinobenzenesulfonic acid → 2,3-Dimethyl-3-(4-sulfobutyl)-3H-indole-5-sulfonic acid

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4-Hydrazinobenzenesulphonic acid (7.5 g), 5-methyl-6-oxoheptane-1-sulphonic acid (11.0 g) and acetic acid (50 ml) were heated under reflux under nitrogen for 6 hrs, during which time all of the suspended solid dissolved. The solvent was then evaporated under vacuum and the residue triturated with 2-propanol at 80° C. to give a light brown solid in suspension. The mixture was allowed to cool to ambient temperature, the solid collected by filtration, washed with 2-propanol and diethyl ether and dried under vacuum. The product was purified by HPLC, collecting the major peak detected at 270 nm. (Phenomenex Jupiter 15 μC18 300 A, 250×50 mm. 100 ml/min. 0.5 g per run. Eluant isocratic water+0.1% TFA). Product fractions were pooled and evaporated to give 11.1 g.

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