Synthesis of Highly Substituted Indolines and Indoles via Intramolecular [4 + 2] Cycloaddition of Ynamides and Conjugated Enynes

Ynamides react with conjugated enynes in intramolecular [4 + 2] cycloadditions to afford substituted indolines that undergo oxidation with o-chloranil to furnish the corresponding indoles. The cycloaddition substrates are easily assembled from derivatives of 3-butynylamine by Sonogashira coupling with alkenyl halides followed by copper-catalyzed N-alkynylation with acetylenic bromides. Diynamides participate as particularly reactive 2π components in the cycloaddition, providing access to indolines with carbon substituents at the C-7 position. Enynamides serve as 4π components in a complementary version of the cycloaddition strategy which provides access to indoles and indolines substituted with carbon substituents at C-4. These enyne cycloadditions take place upon heating the substrates at 110−210 °C in toluene or 2,2,2-trifluoroethanol and in some cases can be achieved at 0 °C to room temperature in the presence of Lewis acids such as Me₂AlCl.