Methyl 2,2,2-trichloroacetimidate → 2-(2-Trichloromethyl-1H-benzoimidazole-5-sulfonyl)-ethanol

500 mg (2.31 mmol) 3,4-diamino-benzene-sulfonylethan-2-ol were dissolved in 10 mL concentrated acetic acid. 0.4 mL (1.4 equiv.) methyl-2,2,2-trichloroacetimidate were added slowly and the resulting mixture was stirred at room temperature for 4 h. The mixture was diluted with 100 mL toluene and the solvent was removed under reduced pressure. The residue was rinsed with toluene, filtered and dried under vacuo to give a brown crystalline solid pure enough for all further reactions. Yield: 680 mg MS (ES+): m/e=345, chloro pattern.