Reaction of dicyanomethylides with 3-(3′,3′-dimethyltriazene-1-yl)- pyridine-4-carboxylic acid. Unexpected preferential formation of pyrido[4,3-a]indolizines

Contrary to theoretical predictions on the basis of a discrete hetaryne intermediate (frontier orbitals, total atomic charges on C-3 and C-4, and activation energies for the concerted process) from the MO calculations, reactions of dicyanomethylides with 3-(3′,3′-dimethyltriazene-1-yl)pyridine-4-carboxylic acid, a formal precursor of 3,4-didehydropyridine, afforded exclusively the pyrido[4,3-a]indolizines. The results are better in accord with a mechanism involving a zwitterionic hetaryne precursor.