Diels-Alder reactions of fused 5-, 6- and 7-membered ring 2-vinylindoles: Synthesis of annulated tetrahydrocarbazoles

Scaffolds of tetrahydrocarbazoles are an important class of biologically active compounds found abundant in nature. In the present investigation, functionalized polycyclic tetrahydrocarbazoles were synthesized using Diels-Alder cycloaddition between fused 5-, 6- and 7-membered ring 2-vinylindoles and various dienophiles. The dienophiles used were N-phenylmaleimides, 1,4-naphthoquinone, and dimethyl acetylenedicarboxylate (DMAD). When 1,4-naphthoquinone was used as a dienophile, the 5-membered ring Diels-Alder adduct was obtained in the oxidized form, but this form was not observed in the case of 6- and 7-membered ring Diels-Alder adducts. The desired product yields ranged from 60–70% and the structure of final compounds were determined using 1D and 2D NMR analysis. ¹H NMR studies confirmed excellent diastereoselectivity (>99% de).