We have performed an investigation on the cyclotrimerisation of molecules having exo- and endo-benzotricyclo[188.8.131.52(2,5)]nonene skeletons (3 and 4) with the aim of producing their respective cyclotrimers 2 that feature unusual geometries and electronic properties. Activation towards the cyclotrimerisation reaction was performed using the vic-bromostannyl vinyl derivatives and was accomplished under copper-mediated or palladium-catalysed reaction conditions. While the exo isomer 3 proved to be quite reactive and afforded variable amounts of the syn and anti cyclotrimers, the endo isomer 4 turned out to be quite resistant to cyclotrimerisation because of steric hindrance. Only dimers and acyclic trimers were obtained from reactions using this substrate. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).