Side reaction pathways in the elimination of vanillin from β-O-4-type vanillin end structure under strong alkaline conditions

The production of vanillin (4-hydroxy-3-methoxybenzaldehyde) via alkaline aerobic oxidation of lignin is a promising strategy for synthesizing low-molecular-weight aromatic compounds from biomass. One of the major pathways for vanillin formation in this process involves the non-oxidative elimination of vanillin from β-O-4-type vanillin end groups formed through oxidative depolymerization of lignin. A previous study revealed that this elimination pathway competes with a side reaction leading to polymerization. To gain further insight into this side reaction, the alkaline degradation behavior of 4-(2-hydroxyethoxy)-3-methoxybenzaldehyde ( HEV ), a simplified model compound of the vanillin end group, was investigated. Under non-oxidative alkaline conditions (4.0 mol/L NaOH aqueous solution, 80 °C, N 2 ), HEV produced vanillin and its disproportionation products in relatively low yields (∼30 mol%), while yielding a group of main products with strong absorption at UV 220 nm . Based on NMR analysis, these products were found to possess α , β -unsaturated aldehyde structures, suggesting degradation of the aromatic ring during the reaction. Upon prolonged reaction, these products gradually converted into high-molecular-weight components. Taken together, the results are consistent with a possible side reaction pathway at the vanillin end group involving nucleophilic addition of OH − to an acetal intermediate, followed by retro-aldol cleavage of the aromatic ring and subsequent polymerization of linear conjugated aldehydes.