Phenoxy radicals coupling

Davin LB, Wang H-B, Crowell AL et al (1997) Stereoselective bimolecular phenoxy radical coupling by an auxiliary (dirigent) protein without an active center. Science 275 362-366... 

Therefore, the radical coupling occurs easily to give a hydroperoxyben-zoquinone intermediate, which is converted to benzoquinone, completing the catalytic cycle. When the phenol is hindered, the inner-sphere coupling between the phenoxide and hydroperoxide does not proceed effectively and homo lysis of the Cu(II)—OPh bond occurs. The resultant phenoxy radicals couple together to give the diphenoquinone. 

The best-known neutral ambident radicals are phenoxy radicals. Many substituted phenoxy radicals couple to give the polymers in wood, and dimerise or couple intramolecularly in the biosynthesis of a number of alkaloids, It is believed that many phenoxy radical couplings are actually the coupling of radical with radical and not of radical with neutral molecule, although the attack of a radical on a phenate ion may occasionally be an important pathway. A radical coupling with a radical is inherently a very fast process, since it is so exothermic, but only... 

Davin, F.B. and Fewis, N.G. (2005a) Dirigent phenoxy radical coupling advances and challenges. Curr. Opin. BiotechnoL, 16, 398-406. 

Most lignans are obtained optically active and, presumably, enantiomerically pure. However, the phenolic coupling processes catalysed by H2C>2-dependent peroxidases, Ch-requiring laccases or phenol oxidases yield racemic products. A protein without an active centre has been isolated that in presence of an oxidase produces stereoselective bimolecular phenoxy radical coupling reactions in in vitro lignan biosynthesis. Its mechanism of action is presumed to involve capture of -coniferyl alcohol-derived free radical intermediates [34],... 

The antitumour properties of fredericamycin A have led to a good deal of interest in the synthesis of spirohydrindanones. One approach to its spirocyclic centre (125) involves a phenoxy radical coupling of the phenol (124) in 67% yield with only 8% of the ortho-product being formed.Another research group has reported two alternative methods. The first involves a Friedel-Crafts... 

Enzyme-Catalyzed Polymerization. Horseradish peroxidase (HRP) has been used for the oxidation of a wide variety of compounds, eg, phenol derivatives in the presence of hydroperoxide (44). HRP is a kind of heme glycoprotein that catalyzes the oxidation of phenol to phenoxy radicals. Subsequently, the resulting phenoxy radicals couple each other to oligo or polyphenol derivatives step by step (see Oxidative Polymerization). 

The random biosynthesis process is imder review. Dirigent proteins, acting as templates, are proposed to assist the orientation of hgnin precursors to afford stereoselective phenoxy radical coupling (2,3). 

Fig. 8 Working hypothesis for the control of the phenoxy radical coupling by the oxidation...

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