Auto-oxidation of phenolic compounds

Auto-oxidation refers to the formation of cross-linked structures as a result of exposure to light and oxygen. Under the influence of light, oxygen can abstract a proton, thereby generating a radical. This is particularly likely to occur if the proton is adjacent to a double bond, because the radical electron can be delocalized, thus lowering the energy. 

Given their aromatic nature, phenolic compounds are easily auto-oxidized. The radical that is generated can subsequently react with other radicals to form a dimer. Since the radical electron is delocalized, several structures can be formed depending on the precise location of the radical electrons at the time of the reaction. 

A variety of complex compounds can arise through these mechanisms, including biflavonyls and bianthraquinones. An example of the latter is the compound iridoskyrim (2.39) formed by the fungus Penicillium islandicum. 

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