Ambimodal Pericyclic Rearrangements of Dialkenyl-Bullvalenes Give Tetrahydro-1,8-ethenoheptalenes

The
fluxional structure of bullvalene is expanded by the discovery
of a [5,5]-sigmatropic rearrangement of dialkenyl substituted derivatives.
This gives rise to tetrahydro-1,8-ethenoheptalenes (THEH), representing
the first examples of this tricyclic scaffold. Variation of the substitution
pattern alters the product distribution, including one thermodynamically
balanced between THEH and bullvalene isomers. DFT calculations are
used to explore the thermodynamic landscape and reaction mechanism
revealing a pretransition state bifurcation leading to a concerted
ambimodal rearrangement pathway.