Sulfuric Acid → 3,7-Diaminodibenzothiophene-5,5-dioxide

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In accordance with Scheme IX, benzidine sulfate 37 as described in U.S. Pat. Nos. 2,590,632 and 2,620,343 was added in portions at 10° C. to fuming sulfuric acid and the reaction maintained at 70° C. for 18 hours, cooled and poured into ice. The resulting solid was collected and washed with water and saturated sodium bicarbonate and dried to give 3,7-diamino dibenzothiophene-5,5-dioxide 38, which was further treated with 20% oleum at 25-200° C. for 0.5-3 hours, followed by cooling to room temperature. The solution was made basic to pH from 8 to 12 with excess base which include but are not limited to triethylamine, N,N-diethylmethylamine, N,N-diethylaniline, N,N-diethylethylenediamine, N,N-diisopropylethylamine or pyridine and the like; and the resulting solution was evaporated to a thick syrup. The syrup was treated with absolute ethyl alcohol and the formed precipitate was collected to give 3,7-diamino-5,5-dioxo-5λ6 -dibenzothiophene-2,8-disulfonic acid di-triethylamine salt 39.

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