Diels-Alder reactivity of pyrano[3,4-b]indol-3-ones, stable analogues of indole-2,3-quinodimethanes

The pyrano[3,4-b] indol-3-ones (13a,b), stable synthetic analogues of indole-2,3-quinodimethanes, undergo Diels–Alder reactions with acetylenes to give, with concomitant loss of carbon dioxide, carbazoles. Thus, reaction with dimethyl acetylenedicarboxylate and dibenzoylacetylene gives the carbazole diesters (15a,c) and dibenzoylcarbazoles (15b,d) respectively. The dibenzoylcarbazoles (15b,d) react with hydrazine to give pyridazo[4,5-b]carbazoles (16). The pyranoindolones (13) also react with benzyne to give the benzo[b]carbazoles (18).