Electrochemical degradation of quercetin: Isolation and structural elucidation of the degradation products

This paper reports the first complete isolation, characterization and structural elucidation of 18 intermediates following the electrochemical oxidation of quercetin and provides a mechanism of reaction. Bulk electrolysis was employed to induce the oxidation of quercetin in ethanol–PBS buffered neutral solution (1:1, v/v, pH 7.40). Simultaneous UV–Vis, cyclic and differential pulse voltammetric studies indicated the formation of new products due to the induced electrochemical oxidation of quercetin. Preparative separation and the verification of the oxidized products were achieved using a combination of column chromatography, GC–MS and LC–MS–MS analyses. A possible mechanism for the degradation pathways of quercetin was proposed including an electrochemical step leading to the formation of semi-quinone, ortho-quinone and quinone methide, as well as a chemical step corresponding to the formation of complex degradation products of taxifolin, depside, small phenolic acids, quercetin–ethanol adduct, benzofuronone and dimer.