An unexpected bispericyclic transition structure leading to 4+2 and 2+4 cycloadducts in the endo dimerization of cyclopentadiene

The stereospecific endo dimerization of cyclopentadiene takes place through an asynchronous and symmetrical bispericyclic transition structure, which shows a merging of the 4+2 and 2+4 cycloaddition paths. The shape of the transition structure testifies to the presence of attractive Salem/Houk secondary orbital interactions assisting the endo approach.