Structural Diversity and Anti-Diabetic Potential of Flavonoids and Phenolic Compounds in Eriobotrya japonica Leaves

<i>Eriobotrya japonica</i> (Thunb.) Lindl., commonly known as loquat, is a plant belonging to the Rosaceae family. While its fruit is widely consumed as food and used in traditional medicine, research on other parts of the plant remains insufficient. Therefore, the chemical constituents and biological activities of its leaves were investigated. Phytochemical analysis of <i>E. japonica</i> leaves identified 30 compounds, including flavonoids, phenolics, and megastigmanes. The flavonoids isolated from the leaves include flavones, flavans, flavolignans, flavonoid glycosides, and coumaroyl flavonoid glycosides. Coumaroyl flavonoid rhamnosides were characteristically present in <i>E. japonica</i> leaves, and the configurations of coumaric acids, as well as the binding position to the rhamnose in each compound, were identified through detailed NMR analysis. Notably, three of them were isolated from this plant for the first time. Phenolic compounds were found to be present as conjugates with organic acids, such as quinic acid, shikimic acid, and glucose. Flavonoid and phenolic compounds demonstrated significant antioxidant and α-glucosidase inhibitory effects, whereas megastigmanes showed little activity. Notably, coumaroyl flavonoid rhamnosides, which consist of flavonoids combined with the phenolic acid, coumaric acid, exhibited excellent anti-diabetic effects. Further molecular docking analysis confirmed that these compounds effectively bind to the α-glucosidase enzyme. In conclusion, the present study identified flavonoid and phenolic components with various structures in <i>E. japonica</i> leaves and clarified their anti-diabetic and antioxidant effects. These findings support the beneficial potential of <i>E. japonica</i> leaves for the treatment and/or prevention of metabolic diseases.