Diels-Alder reactivity of pyrano[3,4-b]indol-3-ones. Part 3. Intramolecular Diels-Alder reactions

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Abstract
Heating the 1-alkynylpyrano[3,4-b]indol-3-ones (5), prepared from the alkynoic acids (3), results in intramolecular Diels–Alder reaction involving the indole-2,3-quinodimethane diene with subsequent extrusion of carbon dioxide to give the cycloalka[a]carbazoles (6). The pyranoindolones need not be isolated, since the 2-acylindol-3-ylacetic acids (8) undergo cyclodehydration and intramolecular Diels–Alder reaction when heated in an acid anhydride as solvent. In contrast to the intramolecular Diels–Alder reaction involving the acetylenic compound (5a), the olefin (9) was a much poorer substrate.

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