The diversity of cycloaromatization reactions

Certainly, the role of photochemical excitation may be displayed in different ways19 and even in those cases when no aromatic system is formed, topologically 

4 The relative role of a- versus 71-effects at the early reaction stage 

Although data presented in the previous section illustrate the diversity of cycloaromatization reactions, most of the following discussion will concentrate on the Bergman cyclization - a reaction that has been studied intensively in recent decades due to its role in the mechanism of biological activity of natural anticancer antibiotics.8,9 We will take advantage of the large body of data produced by this 

The above analysis illustrates why it is helpful to consider the enediyne moiety as two independent re-systems. As discussed above, the conjugated out-of-plane 7r-system of the reactant is smoothly transformed into the conjugate 7r-system of the product (e.g., the aromatic system of benzene) without an overall change in the number of bonds. We will refer to this group of electrons as out-of-plane MOs or simply as the re-system . In contrast, the two in-plane re-bonds are transformed in a more drastic way to the new cr-bond and a pair of radical centers. We will refer to this system of orbitals as in-plane re-bonds or as the a system . 

Due to the different role of a- and re-effects, we will separate the general description of electronic effects into a- and re-effects in this study. c-Effects include 

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