Electrocyclic ring-closure anionic cyclization

Dienyl cations and dienyl anions both undergo electrocyclic ring closure—a nice example occurs when this cyclooctadiene is deprotonated with butyllithium. There are still five p orbitals involved in the cyclization, but now there are six n electrons, so the reaction is disrotatory. 

In this case, it is the conrotatory photochemical cyclization that is prevented by strain (it was tried—cyclooctadienyl anion is stable for at least a week at -78 °C in broad daylight) as the product would be a 5,5 traws-fused system. The same strain prevents thermal electrocyclic ring closure of cyclooctadienyl cations. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

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