Biomimetic Oxidation of Monolignol Acetate and p-Coumarate by Silver Oxide in 1,4-Dioxane

Abstract

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.

Existence of Syringyl α-Carbonyl-Type Tetrahydrofuran β–β Structure in Hardwood Lignins

“…The in vitro dehydrogenative polymerization of monolignols using various laccases and peroxidases, such as horseradish peroxidase (HRP), − or one-electron oxidizing metal reagents, such as Ag 2 O and FeCl 3 , − has been used for elucidating the chemical structure and polymerization mechanism of lignin. The observation of the formation of ligninlike polymers in the oxidation of CA using a mushroom laccase was the first demonstration of the nature of lignin as a dehydrogenation polymer derived from monolignols .…”

Initial Stage of Syringyl Lignin Formation from Sinapyl Alcohol

“…In our previous investigation, the β−β/CαO dimer (3) and its hemiacetal (4) were identified from the dehydrogenation products of SA using laccases, and the HSQC NMR evidence for these α-oxidized β−β structures in hardwood lignins has been demonstrated. 20 The coupling of the SA radical with β−β/CαO dimer (3) selectively at a non-α-oxidized moiety (A ring) in the β−β structure can form the (β-O-4)(β−β/CαO) trimer (9). The αoxidation of the β−β end unit (C ring) of the (β-O-4)(β−β) trimer ( 5) can also yield the (β-O-4)(β−β/CαO) trimer (9).…”

Effect of pH on the Dehydrogenative Polymerization of Monolignols by Laccases fromTrametes versicolorandRhus vernicifera

“…Monolignol acylates, including coniferyl acetate, sinapyl acetate, and sinapyl p -coumarate were used, and the effects of acyl groups on the lignin substructures were studied. 6 , 7 …”

“…Monolignol acylates, including coniferyl acetate, sinapyl acetate, and sinapyl p-coumarate were used, and the effects of acyl groups on the lignin substructures were studied. 6,7 Recent investigations on lignin biosynthesis suggest that peroxidase/H 2 O 2 and laccase/O 2 play an essential role in lignin polymerization in many plant species. 8,9 Especially, laccase draws increasing attention, and the vital role of laccases in the lignification was shown through the knockout mutants in Arabidopsis.…”

“…Silver­(I) oxide was also used to investigate the radical coupling of monolignols. Monolignol acylates, including coniferyl acetate, sinapyl acetate, and sinapyl p -coumarate were used, and the effects of acyl groups on the lignin substructures were studied. , …”