Syringenes M - Q, Eremophilane Sesquiterpenoid Dimers from the Peeled Stems of Syringa pinnatifolia.

As a part of systematic studies on Syringa pinnatifolia, a continued phytochemical investigation guided by ¹ H-NMR and LC/MS data on the ethanol extract afforded five new dimeric eremophilane sesquiterpenoids, namely syringenes M-Q (1-5). These structures were elucidated by extensive analysis of spectroscopic data, including infrared (IR), high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), nuclear magnetic resonance (NMR), quantum-mechanics-based computational analysis of NMR chemical shifts, and single-crystal X-ray diffraction. Compounds 4 and 5 showed inhibitory activities against NO production induced by lipopolysaccharide in RAW264.7 macrophage cells, with IC ₅₀ values of 5.1 and 9.3 μM, compared to positive control indomethacin (IC ₅₀ 33.6 μM). These dimeric eremophilane sesquiterpenoids may be potential markers for discriminating this species from the genus Syringa and the Oleaceae family.
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