Bupleuri Radix, which is one of the most important component in Chinese traditional medicine, has been used as an antiinflammatory, antipyretic, and antihepatotoxic agent. The botanical origin of this natural medicine was prescribed as the roots of Bupleurum falcatum ("Mishimasaiko" in Japanese) in Japanese Pharmacopea XIII. In recent years, due to the poor supply of Japanese Bupleuri Radix, various Chinese Bupleuri Radix have been imported and commonly used in the traditional preparations. In the course of our studies in search of bioactive principles from Chinese natural medicine, we have found that the glycosidic fraction from a Chinese Bupleuri Radix, the roots of B. scorzonerfolium, showed potent protective effect on liver injury induced by D-galactosamine and lipopolysaccharide. From the active glycosidic fraction, sixteen new triterpene saponins called bupleurosides I (1)-XVI (16) were isolated together with sixteen known saponins. The structures of bupleurosides were elucidated on the basis of physicochemical evidence using 2D-NMR technique (H-H, C-H COSY, NOESY, HMBC) and chemical findings, which included the synthesis of the adonitol-glucoside part of bupleuroside XVI (16). The hepatoprotective effect of bupleurosides and known related saponins was examined by monitoring the inhibition of cytotoxicity induced by D-galactosamine in primary cultured rat hepatocytes. Among them, bupleurosides III (3), VI (6), IX (9), and XIII (13) and saikosaponin b_3 (28) were found to exhibit the inhibitory activity. In addition, by examination of the structure requirement in saponin for the hepatocytoprotective effect, the 11-oxygenated functional group, the 16β- and 28-hydroxyl groups, and the 3-O-diglycoside structure were found to be essential to exerting the activity. By utilizing anodic oxidation as the key reaction, bupleuroside-type saponins were readily synthesized from naturally abundant olean-12-ene oligoglycosides without previous protection of the hydroxyl groups in starting oligoglycosides.
