Cyclopenta[b]indoles. Part 1. Synthesis of cyclopenta[b]indoles by formal [3 + 2] addition of indolylmethyl cations to alkenes

Treatment of indole-2- or 3-methanols with tin(IV) chloride as Lewis acid in the presence of styrenes or indene results in formal [3 + 2] addition of the indole stabilised cation to the alkene to give cyclopenta[b]indoles with a high degree of stereoselectivity; use of methylcyclohexene as the alkene component gave the cis-fused cyclopenta[b]indole 17, which was independently synthesised as its (–)-enantiomer from the diketone 18.