Structurally Diverse Sesquiterpenoids from the Endangered Ornamental Plant Michelia shiluensis

A preliminary phytochemical investigation on the MeOH extract of the leaves and twigs of the endangered ornamental plant Michelia shiluensis led to the isolation of 16 sesquiterpenoids. The isolated compounds comprised germacrane- (1–4, 13, 14), guaiane- (5–9, 15), amorphane- (10), and eudesmane-type (11, 12, 16) sesquiterpenoids. The new structures (1–12) were elucidated by spectroscopic and computational methods, and their absolute configurations (except for 9) were assigned by single-crystal X-ray diffraction crystallographic data and/or electronic circular dichroism spectra. Shiluolides (A–D, 1–4) are unprecedented C16 or C17 homogermacranolides, and their putative biosynthetic pathways are briefly discussed. Shiluone D (8) is a rare 1,10-seco-guaiane sesquiterpenoid featuring a new ether-containing spirocyclic ring, whereas shiluone E (9) represents the first example of a 1,5–4,5-di-seco-guaiane with a rare 5,11-lactone moiety. Shiluone F (10) is the first amorphane-type sesquiterpenoid possessing an oxetane ring bridging C-1 and C-7. Bioassay evaluations indicated that lipiferolide (13) showed noteworthy cytotoxicities toward human cancer cell lines MCF-7 and A-549, with IC50 values of 1.5 and 7.3 μM, respectively. Shiluone D (8) exerted inhibition against protein tyrosine phosphatase 1B (IC50: 46.3 μM).