Lignin-Hydroxycinnamyl Model Compounds Related to Forage Cell Wall Structure. 1. Ether-Linked Structures

Abstract

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 β-O-4 structure of native lignins. The α-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-β-guaiacyl ether. Combined threo/erythro yields ranged from 40 to 67%, with methyl p-coumarate addition affording a significantly higher yield. The β-O-4-cinnamyl ether dimers were prepared according to established strategies for the β-O-4 lignin model dimers, with a modified hydroxymethylation step providing an alternative to the classical method. Zinc borohydride reduction of the α-keto-β-cinnamyl ethers proved to be quite useful for the enrichment of the erythro- β-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. © 1992, American Chemical Society. All rights reserved.