[4 + 2] Cycloaddition Reaction of <i>C</i>-Aryl Ketenimines with PTAD as a Synthetic Equivalent of Dinitrogen. Synthesis of Triazolocinnolines and Cinnolines

C,C,N-Triaryl ketenimines and C-alkyl-C,N-diaryl ketenimines react with 2 equiv of PTAD to provide 1,2,4-triazolo[1,2-a]cinnolines with a pendant triazolidindione group by means of a Diels−Alder/ene sequence. The treatment of such adducts with potassium hydroxide affords 3-aminocinnolines.