Lignin–feruloyl ester cross-links in grasses. Part 2. Model compound syntheses

Abstract

Five compounds which model the various structures produced when feruloyl esters are copolymerized into lignins have been synthesized. These models represent the lignin–feruloyl–polysaccharide structures which have been theorized to exist in the Graminaceae but have yet to be isolated. Complete spectroscopic characterization provides important chemical-shift information to facilitate the identification of these linkages in native lignins and synthetic DHP polymers. Methyl 5-O-{4-O-[3-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propyl]feruloyl}-α-L-arabinofuranoside, a model for the α-linkage of feruloyl esters to lignin, was prepared as a mixture of threo and erythro isomers by addition of methyl 5-O-(E)-feruloyl-α-L-arabinofuranoside (FA-Ara) to the quinone methide derived from 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (guaiacylglycerol-β-guaiacyl ether). Methyl 5-O-{4-O-[2-hydroxy-2-(4-hydroxy-3-methoxy-phenyl)-1-(hydroxymethyl)ethyl]feruloyl}-α-L-arabinofuranoside, a β-aryl ether model, was prepared by a method analogous to one used for the synthesis of guaiacylglycerol-β-guaiacyl ether; FA-Ara was added to 4-acetoxy-β-bromo-3-methoxyacetophenone, and the product was hydroxymethylated and reduced. The peracetate of methyl 5-O-[3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propenoyl]-α-L-arabinofuranoside, a compound which models the attack of lignin radicals on the β-position of the feruloyl ester, was prepared by elimination of the β-proton from the quinone methide derived from ethyl 3-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-propanoate. As in the preparation of synthetic copolymers between coniferyl alcohol and FA-Ara, only a single geometrical isomer was produced. Synthesis of both isomers of derived compounds and detailed NMR analysis indicated that this was the expected Z-isomer. A model for β-5 coupled products, 3-[3-carboxy-2-(4-hydroxy-3-methoxyphenyl)-7-methoxy-2,3-dihydrobenzofuran-5-yl]acrylic acid bis(methyl 5-deoxy-α-L-arabinofuranosid-5-yl) ester, was obtained as a cis/trans mixture in 55% yield by radical coupling Of FA-Ara using silver(I) oxide. Finally, the crossed β-β compound 4,8-exo-bis(4-hydroxy-3-methoxyphenyl)-3,7-dioxabicyclo[3.3.0]octan-2-one (MEL) was obtained, in admixture with its isomer iso-MEL, pinoresinol, and the dilactone 4,8-exo-bis-(4-hydroxy-3-methoxyphenyl)-3,7-dioxabicyclo[3.3.0]octan-2,6-dione, from mixed radical coupling of coniferyl alcohol and ferulic acid via silver(I) oxide.