LIGNIN-HYDROXYCINNAMYL MODEL COMPOUNDS RELATED TO FORAGE CELL-WALL STRUCTURE .1. ETHER-LINKED STRUCTURES

A series of lignin-hydroxycinnamyl methyl ester model compounds have been synthesized and characterized by NMR spectroscopy. The materials represent several of the possible ether linkages analogous to the predominant beta-O-4 structure of native lignins. The alpha-etherified model trimers were prepared by nucleophilic attack of the phenolate anion of methyl p-coumarate and methyl ferulate on the quinone methide of guaiacylglycerol-beta-guaiacyl ether. Combined threo/erythro yields ranged from 40 to 67%, with methyl p-coumarate addition affording a significantly higher yield. The beta-O-4-cinnamyl ether dimers were prepared according to established strategies for the beta-O-4 lignin model dimers, with a modified hydroxymethylation step providing an alternative to the classical method. Zinc borohydride reduction of the alpha-keto-beta-cinnamyl ethers proved to be quite useful for the enrichment of the erythro-beta-ethers. Complete spectroscopic characterization of the models as well as their peracetates by one- and two-dimensional NMR spectroscopy provided important chemical shift data for the evolving lignin NMR database as well as for comparison with native tissue isolates and synthetic DHP lignins.