Isoflavonoids from Erythrina poeppigiana: Evaluation of Their Binding Affinity for the Estrogen Receptor

Five new isoflavones, named 5,4'-dihydroxy-7-metboxy-3'-(3-methylbuten-2-yl)isoflavone (1), 5,2',4'-trihydroxy-7-methoxy-5'-(3-methylbuten-2-yl)isoflavone (2), 5,4'-dihydroxy-7-methoxy-3'-(3-methyl-2-hydroxybuten-3-yl)isoflavone (3), 3'-formyl-5,4'-dihydroxy-7-methoxyisoflavone (4), and 5-hydroxy-3 ''-hydroxy-2 '',2 ''-dimethyldihydropyrano[5 '',6 '': 3',4']isoflavone (5), as well as six known compounds, wighteone (6), 3'-isoprenylgenistein (7), isolupabigenin (8), alpinumisoflavone (9), erypoegin D (10), and crystacarpin (11), were isolated from Erythrina poeppigiana. The structures of the isolated compounds were elucidated on the basis of chemical and spectroscopic analysis. The affinity of these compounds for the estrogen receptors ER alpha and ER beta was evaluated using a receptor binding assay. While isoprenyl and dimethylpyrano substituents in ring A reduced the affinity of binding to ER beta ca. 100-fold compared to genistein, the isoprenyl substituent in ring B was better accommodated, allowing 7 to bind with ca. 10-fold lower affinity than genistein.