Ring-closing methods, electrocyclic

The cyclobutene-butadiene interconversion involves four v electrons and is designated a process. Note that by the principle of microscopic reversibility, the number of tt electrons involved in the transformation is the same for ring opening as for ring closing. Once we know the number of tt electrons involved in an electrocyclic reaction and the method of activation, the stereochemistry of the process is fixed according to the rules outlined in Table 6.1. 

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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