Identification of Unstable Ellagitannin Metabolites in the Leaves of Quercus dentata by Chemical Derivatization

The identification of unstable metabolites of ellagitannins having ortho-quinone structures or reactive carbonyl groups is important to clarify the biosynthesis and degradation of ellagitannins. Our previous studies on the degradation of vescalagin, a major ellagitannin of oak young leaves, suggested that the initial step of the degradation is regioselective oxidation to generate a putative quinone intermediate. However, this intermediate has not been identified yet. In this study, young leaves of Quercus dentata were extracted with 80% acetonitrile containing 1,2-phenylenediamine to trap unstable ortho-quinone metabolites, and subsequent chromatographic separation afforded a phenazine derivative of the elusive quinone intermediate of vescalagin. In addition, phenylenediamine adducts of liquidambin and dehydroascorbic acid were obtained, which is significant because liquidambin is a possible biogenetic precursor of C-glycosidic ellagitannins and ascorbic acid participates in the production of another C-glycosidic ellagitannin in matured oak leaves.

Automated summary

The identification of unstable metabolites of ellagitannins with ortho-quinone structures or reactive carbonyl groups is crucial for understanding their biosynthesis and degradation. The degradation of vescalagin, a major ellagitannin in oak young leaves, involves regioselective oxidation to produce a putative quinone intermediate, which has not been identified yet. In this study, phenazine derivatives of the elusive quinone intermediate were obtained from the extraction of young leaves of Quercus dentata, using 80% acetonitrile containing 1,2-phenylenediamine to trap the unstable ortho-quinone metabolites. Additionally, phenylenediamine adducts of liquidambin and dehydroascorbic acid were also obtained, providing insights into the biosynthesis of C-glycosidic ellagitannins in oak leaves.