Chlorosulfonic acid → Methyl N-[6-(chlorosulfonyl)-1H-benzimidazol-2-yl]carbamate

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A solution of 2-carbomethoxyamino-benzimidazole (5.0 g, 0.026 mole) in chlorosulfonic acid (35.00 mL) was stirred at 0° C. for 30 minutes and at room temperature for 3 hours. The resulting dark colored reaction mixture was poured into an ice-water mixture (200 mL), and stirred at room temperature for 30 minutes. The resulting precipitate was filtered and washed with cold water (500 mL). The solid was dried overnight under high vacuum in a desiccator over NaOH pellets to give 5-chlorosulfonyl-2-carbomethoxyamino-benzimidazole (5.9 g, 78%) as a gray powder. 1H NMR (DMSO-d6) d: 3.89 (s, 3H), 7.55 (d, J=8.4 Hz, 1H), 7.65 (d, J=8.4 Hz, 1H), 7.88 (s, 1H). (German Patent DE 3826036)

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