An alternative β-ether cleavage pathway for a non-phenolic β-O-4 lignin model dimer catalyzed by a laccase-mediator system

A non-phenolic β-O-4 lignin substructure model dimer, 1-(4-ethoxy-3-methoxyphenyl)-2-(2,6-dimethoxyphenoxy)ethanol (I) was oxidized by Trametes versicolor laccase in the presence of 1-hydroxybenzotriazole (1-HBT) to give many degradation products (II–VII). When the degradation was performed under 18O2, one oxygen atom (18O) was incorporated into the C2(β)-OH of 1-(4-ethoxy-3-methoxyphenyl)-2-hydroxyethanone (III) formed via a β-ether cleavage reaction. When the C2(β)-2H2 labeled substrate I-2H was synthesized and oxidized by Laccase/1-HBT couple, deuterium atoms were retained in the product ethanone III. It is clearly indicated that ethanone III might be formed from substrate I without the abstraction of C2(β)-protons during the oxidation. We conclude that ethanone III was formed via a peroxy radical intermediate produced from the reaction between the benzylic radical of substrate I catalyzed by the laccase/1-HBT couple and molecular oxygen.