Chalcone-flavanone equilibrium in the isomeric mixture: 2',3,4,4',6'-Pentahydroxychalcone and eriodictyol from Stevia lucida

From the leaves and stems of Stevia lucida Lagasca (Asteraceae), an equilibrium chalcone-flavanone isomeric mixture, composed of 2',3,4,4',6'-pentahydroxychalcone (8) (eriodictyol-chalcone) and eriodictyol (9) [ratio 8/9 5:3], was isolated as an isomorphic crystal. The mixture was structurally characterized by spectroscopic methods, including 1D- and 2D-NMR experiments. The presence of chalcones in the genus Stevia is reported here for the second time.