Route to a kinetically stabilized protonated spirocyclopentane with a pentacoordinated carbon atom: the missing link between bicyclo(3.2.0)hept-3-yl and 7-norbornyl cation

An ethano-bridged derivative, the tricyclo(4.1.0.0)heptyl cation, of the center-protonated spirocyclopentyl cation is much more stabilized than its parent compound. The pyramidal geometry and the state of coordination of carbon atom C3 are apparently not affected by the ethano-bridge. The ethano-bridge, however, increases the activation energy of a rearrangement to cyclopentyl derivatives.