Cyclohepta[<i>b</i>]indole Core Construction <i>via</i> the Michael Addition/Friedel–Crafts Condensation Sequence

An approach to cyclohepta[<i>b</i>]indoles <i>via</i> the formal (4 + 3) annulation of readily synthesized 2-indolyl-derived 1,3-dicarbonyl compounds to acrolein and enones was developed. The process is initiated by catalytic Michael addition, wherein the adduct was generated <i>in situ</i>. The subsequent addition of a Bro̷nsted acid catalyst triggers a cascade process that includes intramolecular Friedel-Crafts hydroxylalkylation followed by dehydration. The cascade reaction is relatively undemanding since even degrading chloroform is able to initiate it. The obtained tetrahydrocyclohepta[<i>b</i>]indoles were prone toward easy dimerization, underscoring the high reactivity of the double bond in the seven-membered ring.