Alashanoids O-S, seco-Humulane and Eremophilane Sesquiterpenoids from Syringa pinnatifolia

Five new sesquiterpenoids, alashanoids O-S (1-5), along with three known analogs (6-8) were isolated from the peeled stems of Syringa pinnatifolia. Their structures were elucidated by analysis of extensive spectroscopic data including ESI-MS, 1D, 2D NMR. The absolute configurations were determined by comparing its experimental and calculated electronic circular dichroism, calculated OR, calculated NMR, and single crystal X-ray diffraction data analysis. Compounds 1 and 2 belong to the seco-humulane type and possess a rare 13-membered oxygen heterocycle framework, and 3-5 belong to eremophilane-type. Compounds 1, 2, and 5 showed inhibitory effects against NO production in LPS-induced RAW264.7 macrophage cells with its IC₅₀ values of 11.86±2.34, 72.08±7.72, and 69.22±15.29 μM, respectively, compared with the positive control indomethacin (IC₅₀ =31.52 μM).