Syntheses Of New Lignin-related Model Compounds And Their Uses For Structural Characterization Of Lignin

Lignin, the phenylpropanoid polymers produced primarily from oxidative polymerizationof three4-hydroxycinnamyl alcohols differing in their degrees of methoxylation, is the secondabundant and important natural plant biopolymers accounting for approximately30%of theorganic carbon in the biosphere. As a highly abundant natural polymer, the biosynthesis oflignins and its chemical/mechanical properties, have attracted significant attention for decades.However, because of its complexity and heterogeneity, it has been a most challengingproblem for scientists to understand the composition and structure of lignin, to some extent,which also hinders the research of high-value utilization of lignin. Lignin model compounds,which could be used as reference standard compounds, plays essential roles in the ligninstructural analysis. In this study, we have synthesized a series of lignin model compound andused for NMR characterization, chemical degradation analysis and GC/GC-MSdetermination.Analytical thioacidolysis releases diagnostic monomers from lignins by selectivelycleaving β-aryl ether units. Described in this work (chapter2) are high yielding syntheses ofthe three lignin thioacidolysis monomers and their application (as standard compounds) forGC-MS and GC-FID quantification of released monomers from lignocellulosics (softwoods,hardwoods and grasses). First, sinapyl (S), guaiacyl (G), and p-hydroxyphenyl (H) glycerolswere synthesized from the corresponding ethyl cinnamates and used as substrates to preparelignin-derived thioacidolysis monomers by the standard thioacidolysis method in high yields.Then the synthesized and purified thioacidolysis monomers were used as standard compoundsto measure their GC-MS and GC-FID response factors against two internal standards,4,4′-ethylidenebisphenol (EBP) and tetracosane (C₂₄). For quantification of lignin-derivedthioacidolysis monomers, it was shown that EBP is a better internal standard than C₂₄whenGC-MS is used, whereas C₂₄remains the better choice for GC-FID. Finally, when theobtained response factors were applied, the yields of thioacidolysis monomers from whitespruce, loblolly pine, poplar, bamboo, and sugarcane bagasse were determined by GC-MS.The obtained results were found to be consistent with those reported in the literature.Thioacidolysis also releases dimeric fragments and other degradation products in addition to diagnostic monomers from lignins by selectively cleaving β-aryl ether units.Identification of these dimeric fragments recovered from thioacidolysis followed by RaneyNickel desulfurization by GC or GC-MS can provide interunit linkage information betweenmonolignols. We have got10dimeric lignin model compounds (G) via different synthetic andpurification methods according to the possible dimer structures proposed by the thioacidolysisfollowed by Raney Nickel desulfurization analysis, including five β-5, two β-1, one5-5, one4-O-5and one β-β linked dimeric model compounds. The structures of abovementioned10lignin dimeric model compounds were verified by applying the corresponding10dimermodel compounds as standards combined with the GC-MS determination of dimers recoveredfrom thioacidolysis followed by Raney Nickel desulfurization. In addition,5new possibledimeric structures were put forward based upon the literature report and GC-MS data analysis.Meanwhile,the NMR data for these compounds are also valuable for the structural analysis ofnatural or synthetic lignins.Pinoresinol, being a dimer of coniferyl alcohol, is one of the structurally simplest lignanand plays an important role in pinoresinol-related structures in lignin analysis. In this study,5-bromoconiferyl alcohol was used as starting material instead of coniferyl alcohol that usedin traditional synthetic methods to protect the5-position during the coupling reaction. Thecrystal yield of5-bromopinoresinol achieved24.46%by using this new, facile and efficientsynthetic method, which was twice more than the yield from traditional synthetic method.With the relative higher yield, it will make the research of pinoresinol and its relatedstructures much easier to get sufficient pinoresinol-related chemical in quantities for in-depthrelative research.Pinoresinol structures featuring β-β′linkage between lignin units are important parts ofsoftwood lignin. Although being readily detected by NMR, pinoresinol structures escapeddetection from β-ether cleaving degradation analysis presumably due to the5-5′or4-O-5′linkage at5-positions of the pinoresinol moiety. In this study, new lignin model compoundsrelated to such pinoresinol structures were synthesized via a biomimetic peroxidase mediatedoxidative coupling reactions between5-5′or4-O-5′linked coniferyl alcohol and coniferylalcohol aimed to help better understanding5-linked pinoresinol structures by providing therequired data for NMR characterization. It was found that5-5′-or4-O-5′-linked coniferyl alcohol can cross-couple with coniferyl alcohol producing pinoresinol products in addition toother homo-and cross-coupled products. Eight new lignin model compounds were obtainedand characterized by NMR while one tentatively identified cross-coupled β-O-4′product wasformed from4-O-5′-linked coniferyl alcohol. By examining the NMR data of thesecompounds, it was demonstrated that5-5′-and4-O-5′-linked pinoresinol structures could bereadily differentiated via HMBC NMR spectra. With appropriate modification (etherificationor acetylation) to the newly obtained model compounds, it would be possible to identify the5-5′-or4-O-5′-linked pinoresinol structures in softwood lignins by HMBC NMR technique.Identification of the cross-coupled dibenzodioxocin from5-5′-linked coniferyl alcoholsuggested that thioacidolysis or DFRC method could be used to identify or detect the5-5′-linked coniferyl alcohol in lignin.It has been a challenging problem for scientists to distinguish the5-5′and4-O-5′-linkedstructures in the NMR spectra for the analysis of lignin structures. A series of5-linkedtetramer lignin model compounds including pinoresinol-related (β-β), β-5and β-O-4relatedstructures were synthesized via a biomimetic peroxidase mediated oxidative couplingreactions from the dimers of G unit with4-O-5,5-5, β-β, β-5and β-O-4linkage in this study.The linkage types of synthesized tetramers were (β-β)-(5-5)-(β-β),(β-β)-(4-O-5)-(β-β),(β-5)-(5-5)-(β-β),(β-O-4)-(4-O-5)-(β-O-4),(β-O-4)-(5-5)-(β-O-4),(β-5)-(5-5)-(β-5),(β-5)-(4-O-5)-(β-5),(5-5)-(β-β)-(5-5),(4-O-5)-(β-β)-(4-O-5) and (5-5)-(β-O-4)-(5-5), and atrimer model compound with the linkages of (β-O-4)-(β-β) was also obtained at the same time.The NMR data of these synthesized5-5′and4-O-5′-linked model compounds could be usedas reference standard for the analysis of different5-5′and4-O-5′-linked structures in lignin. Itwould be possible to identify and differentiate the5-5′-or4-O-5′-linked structures insoftwood lignins by NMR technique on basis of the NMR data of these model compoundswith appropriate modification (etherification or acetylation) to the newly obtained modelcompounds.Isolation and purification of lignin samples from non-wood materials including ricestraw, bagasse and bamboo via diluted alkali pretreatment were conducted at differenttemperatures, such as room temperature,80℃,120℃,140℃and160℃. The alkali extractswas acidified and precipitated first and then purified by following two steps of 1,4-dioxane-H2O (96/4, v/v) extraction and cyclohexane-EtoAc (5:1, v/v) precipitation toremove the hemicelluloses, p-coumaric acid and other kinds of impurity with low molecularweight produced during the alkali treatment. Totally15pure alkali lignin samples wereobtained after the two-step purification, which would provide the initial substance for thesubsequent lignin structural analysis. It was found that the alkali treatment was much moresuitable for bagasse and rice straw whereas the yield of bamboo alkali lignin was very low.All the lignin samples were used for NMR (HSQC) characterization and the influence onlignin structures caused by temperature difference was also discussed. The results showed thatthe temperature for alkali extraction of three materials should be lower than140℃.