Oxidative Coupling of Phenols and the Biosynthesis of Lignin

Our understanding of the structure of lignin is based on our knowledge of its biosynthesis. Lignification in the plant cell wall is initiated by the enzymatic formation of phenoxy radicals from cinnamyl alcohol precursors. The radicals are thought to react without the intervention of enzymes. An essential element in this process is the cross coupling of radicals formed from the monomeric precursors with radicals formed on the polymer chain (end-wise polymerization). The rates of formation of C C and C-O bonds are governed by the effective concentrations of the different radicals and by the regioselectivity of the coupling reaction. Finally, the stabilization reactions of intermediate quinone methides will, in many cases, determine the outcome of the coupling reaction.