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 8-0-4 structure of native lignins.The -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-8-guaiacyl ether.Combined threo/erythro yields ranged from 40 to 67%, with methyl p-coumarate addition affording a significantly higher yield.The 8-0-4-cinnamyl ether dimers were prepared according to established strategies for the 8-0-4 lignin model dimers, with a modified hydroxymethylation step providing an alternative to the classical method.Zinc borohydride reduction of the a-keto-8-cinnamyl ethers proved to be quite useful for the enrichment of the erythro- 8-ethers.Complete spectroscopic characterization of the models as well as their peracetates by oneand 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.