Synthesis and Intramolecular Azo Coupling of 4‑Diazopyrrole-2-carboxylates: Selective Approach to Benzo and Hetero [c]‑Fused 6H‑Pyrrolo[3,4‑c]pyridazine-5-carboxylates

A high yield synthesis of fluorescent benzo, thieno, and furo [c]-fused methyl 7-aryl-6H-pyrrolo­[3,4-c]­pyridazine-5-carboxylates, including unprecedented heterocyclic skeletons, was performed by the transformation of methyl 4-aminopyrrole-2-carboxylate into the corresponding diazo compound, followed by intramolecular azo coupling under acid conditions onto a nucleophilic aryl or hetaryl group in the 3-position. Azo coupling is completely regioselective and, according to DFT calculations, a kinetically controlled reaction. N-Methylation of 1,3-disubstituted 2H-pyrrolo­[3,4-c]­cinnolines occurs selectively at N5 under kinetic control, leading exclusively to 5-methyl-5H-pyrrolo­[3,4-c]­cinnoline derivatives.