Intramolecular N-acyliminium ion versus Friedel-Crafts cyclization onto 3-indoles: synthesis of the novel rings pyrrolizino[2,1-b]indole and homologues

Acid treatment of indole-2-carboxylic acid β- and γ-oxoamides causes Friedel-Crafts intramolecular cyclization to β-carbolinones and dihydro-2H-azepino3,4-b]indol-1-ones, in contrast to secondary δ-,?-, and ζ-oxoamides, which cyclize to the novel heterocyclic rings pyrrolizino2,1-b]indole, indolizino2,1-b]indole, and 9a,11-diaza-indeno1,2-a]azulene, via an intermediate N-acyliminium ion. Tertiary amides lead only the Friedel-Crafts ring closure, thus allowing the synthesis of larger fused rings.