Construction of the Pyrrolo[2,3-d]carbazole Core of Spiroindoline Alkaloids by Gold-Catalyzed Cascade Cyclization of Ynamide.

We achieved direct construction of the common pyrrolo[2,3-d]carbazole core of aspidosperma and malagasy alkaloids by a gold-catalyzed cascade cyclization of ynamide. This reaction involves intramolecular cyclization from indole to ynamide followed by trapping of the resulting iminium intermediate. Through the use of chiral gold complexes, an enantiomerically enriched pyrrolo[2,3-d]carbazole was obtained in up to 74% ee. This methodology was successfully applied to the asymmetric formal synthesis of vindorosine.