A novel TFA-mediated cyclo-dimerization of 1-substituted 3-alkenylindole derivatives to cyclopent[b]indoles

Reaction of 1-substituted 3-alkenyl-1H-indoles with an equimolar amount of trifluoroacetic acid in dichloromethane gave cyclic dimers, 1,3-trans-N,N′-disubstituted cyclopent[b]indoles, in 53-84% yields as sole products through an acid-mediated stereoselective cyclo-dimerization process. The structure of the cyclic dimer derived from 3-cyclopentylidenemethyl-1-methyl-1H-indole was elucidated by X-ray crystallographic analysis.