Tailoring the Oxidative Chemistry of Hydroxycinnamyl Alcohols: New and Green Access Routes to Lignans and Lignin Functional Derivatives

Abstract

The oxidative reactivity of coniferyl alcohol (CA), sinapyl alcohol (SA), and p-coumaryl alcohol (PCA), the main lignin precursors, was systematically investigated under enzymatic and non-enzymatic conditions in both solution and solid-state. For CA, aqueous oxidation promoted the formation of β-O-4 and β-5 dimers, whereas organic solvents favored selective β-5 dimer formation. Remarkably, solvent-free mechanochemical oxidation with silver oxide provided access to the trimeric guaiacylglycerol-α,β-bisconiferyl derivative. Comparative studies showed that SA readily formed β-β dimer and β-O-4 dimers, whereas PCA remained largely unreactive. Co-oxidation of CA and SA favored a cross-coupling reaction yielding a β-5 heterodimer. Finally, oxidation of CA or SA in the presence of nucleophiles (e.g., lysine, glutathione) afforded new conjugation products, which could be exploited as precursors of water-soluble lignin polymers. Overall, these results provide insights into the oxidative chemistry of lignin monomers and establish a versatile platform for the design of lignin-inspired building blocks and functional materials.