Journal of the Chemical Society. Perkin transactions I p. 2971 - 2980 (1992)
Update date:2022-08-06
Topics:
Ralph, John
Helm, Richard F.
Quideau, Stephane
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-methoxyphenyl)-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 andFA-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 55percent 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.
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