Diels‐alder reactions of methyl‐ and π‐acceptor‐substituted 2‐vinylindoles with dimethyl acetylenedicarboxylate and tetracyanoethylene: Novel functionalized carbazoles

The Diels‐Alder reactions of the 2‐vinylindoles 1a‐1d, which are now readily accessible, with dimethyl acetylenedicarboxylate and tetracyanoethylene give rise to the novel 1,2‐dihydro‐ and 1,2,3,4‐tetrahydrocarbazoles 2, 4, and 5 as well as the fully aromatized carbazoles 3. With regard to the product spectrum, the mechanistic rationale comprises a Diels‐Alder step, formal 1,3‐hydrogen shift, ene reaction, and dehydrogenation. Conformational aspects of the 1,2‐dihydrocarbazoles 2b and 2c are also discussed.