Rearrangement of Bridged Hexadienes

In semibullvalene, Ci and Ce of hexadiene are bonded to one carbon atom, C3 and C4 are bonded to another, and the two additional C-atoms (C7 and Cg respectively) are linked to one another. Barbaralane differs from it only in that C7 and Cs are linked by a third aliphatic carbon atom, C9. Both molecules have C5 symmetry, with the sole mirror plane passing through C7 and Cg, whereas a perpendicular mirror plane and a two-fold rotational axis appear at the TS, that necessarily has the boat (C y) geometry. 

The a frame and the CH-bonding orbitals retain their identity across the diagram in Fig. 8.4, so only three occupied orbitals on each side of the diagram need be considered explicitly. On the left they are the three uppermost MOs the acc orbital of the bond broken (a in and the symmetric (a ) 

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