Reaction of p-hydroxycinnamyl alcohols with transition metal salts. IV. Tailored syntheses of beta-O-4 trimers
2001
Landucci, L.L. | Ralph, S.A.
Reaction of coniferyl alcohol or sinapyl alcohol with (beta-O-4 dimeric model compounds in the presence of manganese(III), copper(II), or vanadium(V) gave trimeric compounds that served as superior models for 13C chemical shift assignments in natural and synthetic lignins. By appropriate choice of dimer and monomer, seven of the eight possible sequences of guaiacyl (G) and syringyl (S) units were prepared: GGG, GGS, GSG, GSS, SSS, SSG, and SGG. Preparation of the missing sequence (SGS) by this method was not successful, so it was obtained by conventional synthetic techniques. Stereochemistry in the trimers was also controlled to some extent by utilizing dimers that were predominantly erythro (e) or threo (t), and by performing the oxidative coupling under conditions of high stereo-selectivity. The maximum number of geometric isomers in (beta-O-4 trimers is eight (one pair each of ee, et, te, tt). In this study the GGG and GGS trimers contained all eight isomers, the SGG trimer contained four isomers, and GSG, GSS, SSS, SSG, and SGS (synthetic) each contained only two isomers. The chemical shifts of the sidechain carbons in the trimers were compared with corresponding chemical shifts in natural lignins isolated from spruce (Picea mariana), birch (Betula papyrifera), and hickory (Carya ovata). The comparison indicated that GG and SG entities in lignin had e/t ratios ranging from 2/1 to 1/1, but GS and SS entities were predominantly e. This observation was consistent with the isomer composition of linkages formed by oxidative coupling of coniferyl alcohol or sinapyl alcohol with dimers.
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