Diphenolate radical coupling

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

These findings show that by dimerization to diphenol, structures are being formed which are less stable towards oxidation (radical one-electron transfer reactions), explaining dimerization by radical coupling during oxidative degradation from the energetic point of view. 

At about the same time, Nicolaou and Chen et al. independently reported the synthesis of haplophytine [85]. Retrosynthetically, haplophytine was envisioned by a sequence of Suzuki-Miyaura Coupling, Vilsmeier-Haack reaction, and radical cyclization from indole 205 and vinyl iodide 206. The left-hand domain 205 could arise through the oxidative skeletal rearrangement of enamine 207, which could be obtained from the oxidative coupling of tetrahydro-jS-carboline 208 and diphenol 209 (Scheme 37). 

Extract of rosemary is currently approved in the EU as a natural antioxidant (E392). Camosic acid has a typical o-diphenol structure and is easily oxidised. The antioxidation mechanism is based on a coupling reaction with the peroxyl radical at the 12- or 14-position of camosic acid and subsequent transformation reactions of intermediates to an o-quinone and a hydroxy p-quinone (Figure 10.13). 

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