Phenolic Oxidative Coupling

Many natural products are produced by the coupling of two or more phenolic systems, in a process readily rationalized by means of free radical reactions. The reactions can be brought

Baeyer-Villiger oxidations nucleophilic attack of peracid on to carbonyl

ketone sO

peracid

carbonyl reforms: alkyl group migrates from carbon to adjacent oxygen

Baeyer-Villiger oxidation

O2, NADPH

HO t

ester

Phenolic oxidative coupling

resonance-stabilized free radical

OH H® V ^ - e Ay resonance-stabilized free radical

Phenolic Oxidative Coupling

ether linkage ortho-ortho coupling ortho-para coupling para-para coupling

Figure 2.26

ether linkage

ortho-ortho coupling ortho-para coupling para-para coupling

Figure 2.26

about by oxidase enzymes, including peroxidase and laccase systems, known to be radical generators. Other enzymes catalysing phenolic oxidative coupling have been characterized as cytochrome P-450-dependent proteins, requiring NADPH and O2 cofactors, but no oxygen is incorporated into the substrate. A one-electron oxidation of a phenol gives the free radical, and the unpaired electron can then be delocalized via resonance forms in which the free electron is dispersed to positions ortho and para to the original oxygen function (Figure 2.26). Coupling of two of these mesomeric structures gives a range of dimeric systems as exemplified in Figure 2.26. The final products indicated are then derived by enolization, which restores aromaticity to the rings. Thus, carbon-carbon bonds involving positions ortho or para to the original phenols, or ether linkages, may be formed. The reactive dienone systems formed as intermediates may in some cases be attacked by other nucleophilic groupings, extending the range of structures ultimately derived from this basic reaction sequence.

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