Oleanane-type triterpenoids from Sabia limoniacea and their anti-inflammatory activities

Eighteen new oleanane-type triterpenoids were isolated from the stems of Sabia limoniacea, including sabialimon A (1), a triterpenoid with an unprecedented 6/6/6/7/7 pentacyclic skeleton and seventeen undescribed triterpenoids, sabialimons B-R (2 - 18), along with six previously described analogs (19 - 24). Their structures were fully elucidated via extensive spectroscopic analysis including 1D and 2D NMR, high-resolution electrospray ionization mass spectrometry (HRESIMS), experimental electronic circular dichroism measurements and X-ray crystallographic studies. Compound 1 is the first triterpenoid that possesses a rare ring system (6/6/6/7/7) with an oxygen-bearing bridge between C-17 and C-18 and a hemiketal form at C-17, which is generated a larger ring by the degradation of C-28 and D/E-ring expansion. Biological evaluation revealed that sabialimon I (9), sabialimon K (11), sabialimon P (16) and 11,13(18)-oleanadien-28-hydroxymethyl 3-one (20) exhibited significantly inhibitory activities against nitric oxide (NO) release with IC₅₀ values of 29.65, 23.41, 18.12 and 26.64 μM, respectively, as compared with the positive control (dexamethasone, IC₅₀ value: 40.35 μM). Furthermore, sabialimon P markedly decreased the secretion of TNF-α, iNOS, IL-6 and NF-κB and inhibited the expression of COX-2 and NF-κB/p65 in LPS-induced RAW264.7 cells in a dose-dependent manner.