An antioxidant lignan and other constituents from the root bark of Hibiscus syriacus

A new lignan named as hibiscuside, (+)-pinoresinol 4-O-[β- glucopyranosyl (1→2)-α-rhamnoside] (1), and a known lignan, syringaresinol (2) were isolated from the root bark of Hibiscus syriacus together with two feruloyltyramines (3,4) and three known isoflavonoids (5, 6,7). The structures of these compounds have been established on the basis of their NMR, mass, UV spectra. Among these phenolic compounds, 6_-O-acetyldaidzin (5), 6_-O-acetylgenistin (6), and 3'-hydroxydaidzein (7) with IC₅₀ values of 8.2, 10.6, and 4.1 μM, respectively, significantly inhibited lipid peroxidation in rat liver microsomes. Hibiscuside (1), E- and Z-N-feruloyl tyramines (3,4) exhibited moderate antioxidant activity.