12 QUINONE METHIDES IN LIGNIFICATION JOHN RALPH 1,2,3 ,P AUL F. SCHATZ 3 ,F ACHUANG LU 1 ,HOON KIM 1 , T AKUYA AKIYAMA 3,4 , AND STEPHEN F. NELSEN 5 1 Department of Biochemistry and Great Lakes Bioenergy Research Center, University of Wisconsin, Madison, WI, USA 2 Department of Biological Systems Engineering, University of Wisconsin, Madison, WI, USA 3 Dairy Forage Research Center, USDA-Agricultural Research Service, Madison, WI, USA 4 RIKEN Plant Science Center, Suehiro, Tsurumi, Yokohama, Kanagawa 230-0045, Japan 5 Department of Chemistry, University of Wisconsin, Madison, WI, USA 12.1 INTRODUCTION Reactive quinone methide (QM) intermediates are implicated in lignin and lignan biosynthesis, as well as in degradation reactions of lignins such as those that occur during alkaline pulping of wood. 1 Dehydrodimeric lignans are plant- extractive products derived from radical coupling reactions of monlignols, 2,3 primarily the two hydroxycinnamyl alcohols coniferyl MG (Fig. 12.1) and sinapyl alcohol MS—here we shall largely ignore the reactions of p-coumaryl alcohol, typically a minor monolignol. Lignins are the polymers produced by combinatorial radical coupling reactions of primarily the monolignols M with the growing polymer P (Fig. 12.2); 4–8 dehydrodimerization reactions are in- volved in chain initiation. Although lignin dimers may have the same structures as their lignan analogues and are produced from the same monolignols, it is generally considered that lignan formation and lignin biosynthesis are well separated in time and space. Importantly, lignans are invariably chiral 2 whereas Quinone Methides, Edited by Steven E. Rokita Copyright Ó 2009 John Wiley & Sons, Inc., Publication. 385