Formal [4+2] cycloaddition reactions of N-sulfonyl-2,2′-biindoles: synthesis of indolo[2,3-a]carbazoles and indigo azines

The formal [4+2] cycloaddition reactions of readily available N-sulfonyl-2,2′-biindoles with various dienophiles is reported. The reaction with carbon-centered dienophiles leading to indolo[2,3-a]carbazoles is facilitated by the sulfonyl group, which is lost as part of the aromatization process. Reaction with aza-dienophiles gives access to indigo azines.