Reductive smiles rearrangement of 1-[(5-chloro-2-nitrophenyl)sulfonyl]-1H-pyrrole-2-carbohydrazide to 1-amino-6-chloro-2-(1H-pyrrol-2-yl)benzimidazole

Treatment of 1-[(5-chloro-2-nitrophenyl)sulfonyl]-1H-pyrrole-2-carbohydrazide with powdered iron in glacial acetic acid afforded 1-amino-6-chloro-2-(1H-pyrrol-2-yl)benzimidazole as the sole product. This compound was also obtained by iron-acetic acid reduction of 1-(5-chloro-2-nitrophenyl)-1H-pyrrole-2-carbohydrazide. Splitting of the sulfone group occurred only in the presence of the carbohydrazide group. In fact, iron-acetic acid reduction of 5-chloro-2-nitrophenyl 2-ethoxycarbonyl-1H-pyrrol-1-yl sulfone gave the expected 2-amino-5-chlorophenyl 2-ethoxycarbonyl-1H-pyrrol-1-yl sulfone. Treatment of this compound with hydrazine yielded the corresponding carbohydrazide, which failed cyclization when reacted with phosphorus pentoxide. Formation of 7-chloropyrrolo[1,2-b] [1,2,5]benzothiadiazepin-11(10H)-one 5,5-dioxide with loss of hydrazine was observed when the above amino hydrazide was reacted with 2-hydroxypyridine or with glacial acetic acid.