Hexatriene, electrocyclic ring closing

Deprotonation of Cl gives an enolate ion, which in this compound is actually a 1,3,5-hexatriene. As such it can undergo an electrocyclic ring closing. Protonation gives the product. 

The electrocyclic ring closure of hexatriene (Eq. 5.13) can also be considered in terms of its transition state, shown in Figure 5.13a, where a complete loop of overlap is followed with a dashed line from atom to atom through all six. This as drawn with a minimum of nodal zones resembles the lowest molecular orbital of aromatic benzene since there are six electrons and no nodes. This transition state is favored by aromatic stabilization and is reached only by disrotation. Conrotation would have given a transition state with one nodal zone (Fig. 5.13b) that would be part of a Mobius orbital set, but six electrons do not give a closed shell in this set, and the transition state does not have aromatic stabilization and is not allowed. 

The reaction of hex-3-ene-1,5-diyne derivatives (enediynes) to p-benzyne, the Bergman cyclization, is related to the electrocyclic ring closure of hexatriene. Following the discovery of a new class of potent antitumor antibiotics that possess an enediyne unit, this reaction has received considerable attention in recent years. Although the reaction has been investigated using semiempirical methods, the use of more advanced methods for the study of a closed-shell species reacting to a diradical is clearly desirable. 

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