Lignin acylated with acetate and/or p-coumarate is common in many herbaceous plants. Herein, the biomimetic oxidation of γ-acylated monolignols with Ag2O was studied to understand the effect of γ-acyl groups on monolignol polymerization. The oxidation of sinapyl acetate gave γ-acylated and α-acylated β-O-4 dimers in 71 and 9.5% yields, respectively. The oxidation of sinapyl p-coumarate produced γ-acylated β-O-4 and γ-acylated tetralin β–β dimers in 53 and 16% yields, respectively. Only the sinapyl alcohol moiety in sinapyl p-coumarate reacted, and the p-coumarate moiety remained unchanged, suggesting that p-coumaric acid is not incorporated into the lignin backbone in the acylated lignins. All of the γ-acylated monolignols used in this study produced the γ-acylated β-O-4 dimers, which suggests that the γ-acylated monolignols act as lignin monomers. The relatively high yields of the β-O-4 dimers indicate that Ag2O oxidation of the monolignols can be used as an easy method for synthesizing the β-O-4 dimer model compounds.
Syringyl α-carbonyl-type tetrahydrofuran β–β dimer and its hemiacetal were identified in the dehydrogenation products of sinapyl alcohol (SA), a hardwood lignin monomer, by biomimetic oxidation using laccase from Trametes versicolor (LacT). To investigate the presence of these structures in native lignin, two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D HSQC NMR) analysis was performed on milled wood lignins (MWLs) prepared from eucalyptus and acacia woods and dehydrogenation polymer (S-DHP) prepared by the biomimetic oxidation of SA using horseradish peroxidase (HRP). Diagnostic NMR signals corresponding to the α-carbonyl-type β–β (IIIox1) and its hemiacetal (IIIox2) structures were observed in the HSQC NMR spectra of MWLs and S-DHP, albeit with low signal intensities, demonstrating their existence in hardwood lignins. The formation of IIIox1 and IIIox2 from SA in the biomimetic laccase/O2 and peroxidase/H2O2 systems supported that these α-oxidized β–β structures originated from the lignin polymerization process in plant cell walls.
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