Intramolecular hetero Diels-Alder (Povarov) approach to the synthesis of the alkaloids luotonin A and camptothecin

[reaction: see text] Pyrrolo[3,4-b]quinolines can be formed through the coupling of anilines with N-propargylic substituted heterocyclic aldehydes in the presence of mild Lewis acid catalysts (Ln(OTf)3). The coupling proceeds through sequential imine formation and a formal intramolecular aza-Diels-Alder (Povarov) reaction. This approach was applied in a total synthesis of luotonin A and a formal synthesis of camptothecin.