LIGNIN HYDROXYCINNAMYL MODEL COMPOUNDS RELATED TO FORAGE CELL-WALL STRUCTURE .2. ESTER-LINKED STRUCTURES

Model compounds which depict p-coumarate and ferulate attachment at the alpha- and gamma-positions and at the phenolic position of the beta-0-4 interunit linkage of lignin have been synthesized and characterized by NMR spectroscopy. The alpha-ester trimers were prepared by nucleophilic attack of ferulic and p-coumaric acids on the quinone methide generated from guaiacylglycerol beta-guaiacyl ether under slightly basic conditions. The gamma-esters were prepared via 4-acetoxycinnamoylation of 1-(4-acetoxy-3-methoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propanone. Reduction with zinc borohydride was selective for the erythro diacetates, and the threo isomers were obtained by nonselective reduction with sodium cyanoborohydride. Deacetylation with piperidine provided the unprotected gamma-trimers. The phenolate esters were synthesized from the acetates of purified threo- and erythro-1-[4-(benzyloxy)-3-methoxy-phenyl]-1,3-dihydroxy-2-(2-methoxyphenoxy)propane. Debenzylation and 4-acetoxycinnamoylation afforded the peracetylated hydroxycinnamate phenolic esters of guaiacylglycerol beta-guaiacyl ether.