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

The pyrano[4,3-b]indol-3-ones (7) are stable indole-2,3-quinodimethane type dienes, which undergo Diels–Alder reaction with alkynes to give, after loss of carbon dioxide, carbazoles. The reactivity of the diene is increased by the presence of the electron-withdrawing t-butoxycarbonyl group on the indole nitrogen. The pyranoindolones (7) are less reactive, and exhibit the opposite regiochemistry in their Diels–Alder reactions than the isomeric pyrano[3,4-b]indol-3-ones (1). Factors which affect the regiochemistry of the Diels–Alder reaction are discussed.