Anti-Inflammatory Diterpenoids from the Aerial Parts of Isodon suzhouensis

A phytochemical investigation of the aerial parts of Isodon suzhouensis resulted in the isolation of nine ent-kauranoid diterpenes, six of which were previously undescribed. The structures of these compounds were elucidated comprehensively using a combination of 1D/2D NMR spectroscopy and high-resolution electrospray ionization mass spectrometry (HRESIMS). Among the isolated compounds, compounds 2 and 3 exhibited intramolecular SN ₂ tautomerism, a phenomenon further explained through the Fukui function and free energy calculations of the transition states of the reactions via the DFT calculation. Compound 7 is particularly noteworthy as it represents a novel case of C-3/C-4 carbon bond breakage in an ent-kauranoid diterpene, highlighting a unique structural modification in this class of natural products. All of the isolates were evaluated for their inhibitory effects on LPS-induced RAW 264.7 macrophages. Compound 8 emerged as a promising candidate, demonstrating significant inhibitory effects. Further studies confirmed that compound 8 suppresses the gene expression of the pro-inflammatory cytokines. The underlying anti-inflammatory mechanism of compound 8 was therefore investigated and shown to be related to the inhibition of the NF-κB signaling pathway activation.