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 ...