Silver-Catalyzed Carbon Dioxide Fixation on Alkynylindoles.

A silver-catalyzed carbon dioxide fixation reaction into 2-alkynylindole derivatives was developed to afford tricyclic indoles. Carbon dioxide was selectively fixed on the N atom of the indole, and only 6-endo-dig cyclization proceeded under mild reaction conditions. Carboxylation on C3 of the indole was not observed. This method was applicable for a variety of 2-alkynylindoles, and the corresponding products were obtained in high yields without the production of side products.