Palladium-Catalyzed Annulation of Allenes with Indole-2-carboxylic Acid Derivatives: Synthesis of Indolo[2,3‑<i>c</i>]pyrane-1-ones via Ar–I Reactivity or C–H Functionalization

Two methodologies, one involving Ar–I reactivity and the other through C–H functionalization, for the formation of indolo­[2,3-c]­pyrane-1-ones via the corresponding allenes, are presented. A highly efficient approach to indolo­[2,3-c]­pyrane-1-one derivatives through the Pd-catalyzed regioselective annulation of allenes with 3-iodo-1-alkylindole-2-carboxylic acids is described. This method is fairly general for a wide range of allenes affording the respective indolo­[2,3-c]­pyrane-1-ones in good to excellent yields. In addition, a Pd­(II)-catalyzed oxidative coupling of indole-2-caboxylic acid derivatives with allenes via direct C–H functionalization to afford the corresponding indolo­[2,3-c]­pyrane-1-ones in moderate to good yields has been developed.