The all-syn oligocondensed bicyclo[2.2.2]octenes 3, 5, 20 and 29 with two to five etheno bridges show interaction of their -orbitals by -overlap. This laticyclic conjugation reveals itself in the PE and UV spectra of the series as well as in the site selectivity of the Diels-Alder reaction with inverse electron demand found for the dehydro derivatives 11 and 26. As shown in the crystal structures of 3, 5 and 20, the etheno bridges are positioned face to face at a distance which is smaller than the sum of their van der Waals radii.

Dyotropic Hydrogen Transfer in Dihydrosesqui-and in Dihydro-syn-sesterbicyclo[2.2.2]octenes~*~The intramolecular dyotropic hydrogen transfer is studied in dihydrosesquibicyclo[2.2.2]octenes. In the parent system 4-d2 the process transfers two hydrogen atoms from the ethano to the etheno bridge; it is thermoneutral and reversible at 200°C. In 12a, b a 1,2-dihydrobenzene acts as the hydrogen donor; the reaction is highly exothermic and occurs already at room temperature. In 19-d4 the dyotropic hydrogen exchange from a l,2-dihydrobenzene to benzene is again thermoneutral and occurs at 200°C. The kinetic parameters of these reactions, listed in Table 7, indicate a concerted pericyclic pathway. In the extended dihydro-syn-sesterbicyclo[2.2.2]octenes 6-dz and 15a, b two hydrogen transfers occur in sequence, the central bridge serving as a relais. Here the dyotropic processes establish equilibria between structures with stacked and isolated double bonds, allowing an evaluation of the van der Waals interaction in the former. It is found that this interaction changes with the type of the stakked n-systems from repulsive in 6-d2 to slightly attractive in 16a (see Table 8