The unexpected conversion of a thiophene ring into a pyrrole ring via a putative nitrene intermediate

Triethylphosphite induced reductive cyclisation of 1-(2-nitrophenyl)-3,5-di(2-thienyl)pyrazole 6 afforded the expected 1,3-di(2-thienyl)pyrazolo[1,2-a]benzotriazole 7 (14%) together with an unexpected product which was shown to be a 2-(2-thienyl)pyrazolo[1,5-a]pyrrolo[2,1-c]quinoxaline 8 derivative (27%). A mechanism for the extrusion of sulfur during the transformation of heterocycle 6 into product 8 is proposed.