Three new C₂₀-norditerpenoids (abietamethinols H–J) and a new sesquiterpenoid (guamethinol A) from Isodon amethystoides and their bioactivity

Three previously undescribed C₂₀-norditerpenoids [abietamethinols H–J (1–3)] and a new guaiane sesquiterpene [guamethinol A (4)] were isolated from Isodon amethystoides (Benth.) H. Hara. Their structures were determined through comprehensive spectroscopic analyses, including single-crystal X-ray diffraction. Compounds 1–4 were evaluated for their cytotoxic, antibacterial, and antiviral activities. Compound 3 showed inhibitory activity against SMMC-7721 cells (IC₅₀ = 36.1±3.4 μM). It also exhibited antibacterial activity against Streptococcus mutans (93.4% inhibition at 174 μM), and antiviral activities against H5N1 avian influenza virus (95.7 %) and vesicular stomatitis virus (76.6 %) at 25 μM. Furthermore, molecular docking studies indicated that compound 3 binds to the active sites of STAT3, glucosyltransferase B (GtfB), neuraminidase (NA), and TLR4 receptor via hydrogen bonding, π–π stacking and hydrophobic interactions.