Radical scavenging mechanism of phenol carboxylic acids: reaction of protocatechuic esters

Solvent-dependency of the antiradical reaction of protocatechuic acid (3,4-dihydroxybenzoic acid, PA) and its esters has been investigated. In aprotic solvents, methyl protocatechuate (PAMe) was readily oxidized by two molar equivalents of DPPH radical and converted to protocatechuquinone methyl ester (PQMe). On the other hand, in alcoholic solvents such as methanol, PAMe rapidly consumed five radicals in 30 min and changed to complex oxidized mixtures. A 1H-NMR analysis of the reaction mixture of PAMe and DPPH radical in methanol showed that PAMe was rapidly converted to PQMe and its 3-hemiacetal. In addition, a signal of 2-methoxy-PQMe 3-hemiacetal was also detected in the reaction mixture. The results suggested that PQMe undergoes a nucleophilic attack by the solvent alcohol molecule at the C-2 of the ring in methanol, leading to a regeneration of catechol structure, which accounts well for the higher DPPH radical scavenging activity of PAMe in alcohols than in aprotic solvents.