Heterocyclic Systems Having Eight π‐Electrons. Synthesis, Properties, and Significance

The terms “aromatic” and “antiaromatic” have attracted widespread interest in recent years. After all the discussion about the meaning of words, one fact remains: the energy content of planar systems displaying cyclic electron delocalization depends upon the number of, and the orbital interactions between, the participating electrons. The concept underlying this progress report is that the electronic destabilization of systems having eight π‐electrons should be especially pronounced in the case of six‐membered rings for steric and electronic reasons. Such systems are found primarily in anionic, but also in neutral, heterocycles containing hetero ring members which possess electron pairs capable of delocalizing into the π‐system. The production and reactivity of such systems are investigated and their biological significance discussed.