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-azepino[3,4-b]indol-1-ones, in contrast to secondary δ-,ɛ-, and ζ-oxoamides, which cyclize to the novel heterocyclic rings pyrrolizino[2,1-b]indole, indolizino[2,1-b]indole, and 9a,11-diaza-indeno[1,2-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.