Diverse Sesquiterpenoids From <i>Michelia alba</i> and Their Cytotoxic and Nitric Oxide Inhibitory Activities

Two previously undescribed sesquiterpenoids, including one germacrane sesquiterpenoid (1) and one guaiane sesquiterpenoid (2), were isolated and characterized from the branches and leaves of Michelia alba DC., along with 11 known sesquiterpenoids (3-13). The structures of the new compounds were elucidated using comprehensive high-resolution electrospray ionization mass spectroscopy, infrared, ultraviolet, and nuclear magnetic resonance analyses, with the absolute configurations being determined through single-crystal X-ray diffraction analysis and electronic circular dichroism calculations. Furthermore, this study also presented the proton and carbon-13 nuclear magnetic resonance data for 3 for the first time. All isolated compounds were evaluated for their cytotoxic effects against HL-60, A549, HepG2, MDA-MB-231, and SW480 cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-5(3-carboxymethoxyphenyl)-2-(4-sulfopheny)-2H-tetrazolium bromide assay, as well as their ability to inhibit lipopolysaccharide-induced nitric oxide (NO) production in RAW264.7 macrophages. Notably, compound 7 showed remarkable cytotoxic activity against HL-60, HepG2, MDA-MB-231, and SW480 cancer cell lines, with half-maximal inhibitory concentration (IC₅₀) values of 3.44 ± 0.19, 8.13 ± 0.43, 3.69 ± 0.14, and 3.50 ± 0.21 µM, respectively. Additionally, compounds 4, 7, and 13 demonstrated notable NO inhibitory activities, with IC₅₀ values of 3.26 ± 0.04, 1.48 ± 0.07, and 8.54 ± 0.37 µM, respectively.