Cross-Coupling of Hydroxycinnamyl Aldehydes into Lignins Hoon Kim, †,‡ John Ralph,* ,†,‡ Nabila Yahiaoui, § Michel Pean, | and Alain-M. Boudet § U.S. Dairy Forage Research Center, USDA-Agricultural Research SerVice, Madison, Wisconsin 53706, Department of Forestry, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706, Po ˆ le de Biotechnologie Ve ´ ge ´ tale, Unite ´ Mixte de Recherche, F31326 Castanet Tolosan, France, and Groupe de Recherches Applique ´ es en Phytotechnologie, Commissariat a ` l’Energie Atomique/Cadarache, DSV-DEVM, Saint-Paul-Lez-Durance Cedex, France jralph@facstaff.wisc.edu Received April 6, 2000 ABSTRACT Pathways for hydroxycinnamyl aldehyde incorporation into lignins are revealed by examining transgenic plants deficient in cinnamyl alcohol dehydrogenase, the enzyme that converts hydroxycinnamyl aldehydes to the hydroxycinnamyl alcohol lignin monomers. In such plants the aldehydes incorporate into lignins via radical coupling reactions. As diagnostically revealed by long-range 13 C- 1 H correlative NMR, sinapyl aldehyde (3,5-dimethoxy-4-hydroxy-cinnamaldehyde) 8-O-4-cross-couples with both guaiacyl (3-methoxy-4-hydroxyphenyl-propanoid) and syringyl (3,5-dimethoxy-4-hydroxyphenyl-propanoid) units, whereas coniferyl aldehyde cross-couples only with syringyl units. Lignins are polymeric natural products present in large quantities in the cell walls of terrestrial plants. They arise principally from one or more of the three monolignols, p-coumaryl (4-hydroxycinnamyl), coniferyl (4-hydroxy-3- methoxy-cinnamyl), and sinapyl (3,5-dimethoxy-4-hy- droxycinnamyl) alcohols, analogues varying in their degrees of methoxylation. 1 However, many other phenolic compo- nents also intimately incorporate into lignins; 2 acylated hydroxycinnamyl alcohols 3 and hydroxycinnamate esters 4 are notable examples. Various monolignol precursors including hydroxycinnamyl aldehydes 5 and 5-hydroxyconiferyl alcohol 6 are incorporated into lignins, particularly when the plant is deficient in certain enzymes on the monolignol biosynthetic ² USDA-Agricultural Research Service. ‡ University of Wisconsin-Madison. § Unite ´ Mixte de Recherche. | Commissariat a ` l’Energie Atomique. (1) (a) Freudenberg, K.; Neish, A. C. Constitution and Biosynthesis of Lignin; Springer-Verlag: Berlin-Heidelberg-New York, 1968. (b) Harkin, J. M. In OxidatiVe Coupling of Phenols; Taylor, W. I., Battersby, A. R., Eds.; Marcel Dekker: New York, 1967; pp 243-321. (c) Li, L.; Popko, J. L.; Umezawa, T.; Chiang, V. L. J. Biol. Chem. 2000, 275, 6537-6545. (2) (a) Boudet, A.-M. Trends Plant Sc. 1998, 3, 67-71. (b) Sederoff, R. R.; MacKay, J. J.; Ralph, J.; Hatfield, R. D. Curr. Opin. Plant Biol. 1999, 2, 145-152. (c) Ralph, J.; Marita, J. M.; Ralph, S. A.; Hatfield, R. D.; Lu, F.; Ede, R. M.; Peng, J.; Quideau, S. p.; Helm, R. F.; Grabber, J. H.; Kim, H.; Jimenez-Monteon, G.; Zhang, Y.; Jung, H.-J. G.; Landucci, L. L.; MacKay, J. J.; Sederoff, R. R.; Chapple, C.; Boudet, A. M. In Progress in Lignocellulosics Characterization; Argyropoulos, D. S., Rials, T., Eds.; TAPPI Press: Atlanta, GA, 1999; pp 55-108. (d) Whetten, R. W.; MacKay, J. J.; Sederoff, R. R. Annu. ReV. Plant Physiol. Plant Mol. Biol. 1998, 49, 585-609. (3) Acetylated monolignols: (a) Ralph, J.; Lu, F. J. Agric. Food Chem. 1998, 46, 4616-4619. (b) Ralph, J. J. Nat. Prod. 1996, 59, 341-342. p-Coumaroylated monolignols: (c) Lu, F.; Ralph, J. J. Agric. Food Chem. 1999, 47, 1988-1992. (d) Ralph, J.; Hatfield, R. D.; Quideau, S.; Helm, R. F.; Grabber, J. H.; Jung, H.-J. G. J. Am. Chem. Soc. 1994, 116, 9448- 9456. (4) (a) Ralph, J.; Hatfield, R. D.; Grabber, J. H.; Jung, H. G.; Quideau, S.; Helm, R. F. In Lignin and Lignan Biosynthesis; Lewis, N. G., Sarkanen, S., Eds.; American Chemical Society: Washington, DC, 1998; ACS Symposium Series no. 697, pp 209-236. (b) Ralph, J.; Grabber, J. H.; Hatfield, R. D. Carbohydr. Res. 1995, 275, 167-178. ORGANIC LETTERS 2000 Vol. 2, No. 15 2197-2200 10.1021/ol005906o CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000