Siegesbeckia pubescens Makino (Compositae) is an annual herb widely distributed in Korea, China, and Japan. The aerial parts of this plant have been used as traditional medicine to treat rheumatic arthritis, asthma, hypertension, and malaria.[1] Previous phytochemical investigations of the genus Sigesbeckia have revealed the presence of sesquiterpenoids, ent-kaurane and ent-pimarane type diterpenoids.[2] In the course of ongoing research for the discovery of inhibitors of nitric oxide (NO) production, the methanolic extracts of S. pubescens were partitioned with n-hexane, CH2Cl2, EtoAc, and water, successively. The CH2Cl2 fraction and ethyl acetate fraction were chromatographed on silica gel, Sephadex LH-20, RP-18, and preparative HPLC. As a result, two new germacrane sesquiterpenoids (1-2) and a new ent-pimarane diterpenoid (3) along with eighteen known compounds were isolated. The structure of these compounds was determined by 1D-(1H, 13C), 2D-(HSQC and HMBC) NMR, and HR-ESI-MS spectrum. Highly oxygenated germacrane type sesquiterpenoids (1-2 and 13-14) showed significant inhibitory effects with IC50 values ranging from 3.9 to 16.8 μM.
[1] Li H, Kim JY, Hyeon J, Lee HJ, Ryu JH. Phytotherapy reseach 2011; 25: 1323-1327
[2] Wang JP, Zhou YM, Ye YJ, Shan XM, Cai YL, Xiong CM, Hu HX. Journal of ethnopharmacology 2011; 137: 1089-1094