Structure and bioactivity of protolimonoids from the root bark of Dictamnus dasycarpus Turcz

Eight previously undescribed protolimonoids, including seven apotirucallane-type (1, 3, 6, and 14-17) and one tirucallane-type (11) triterpenoids, along with ten known analogues, were isolated from the root bark of Dictamnus dasycarpus Turcz. Their structures were unambiguously determined by comprehensive spectroscopic analysis and single-crystal X-ray diffraction. The absolute configurations of the flexible tetrahydrofuran side chains in 1 and 10 were definitively established, resolving a longstanding stereochemical challenge in this class of triterpenoids. All isolated compounds were evaluated for hepatoprotective, antifibrotic, and cardioprotective activities. Compounds 2, 3, 7, 8, 12-14, and 16-18 exhibited striking hepatoprotective activity against acetaminophen-induced toxicity in HepG2 cells. Compounds 4, 5, 11-13, and 15-17 showed significant inhibitory activity against transforming growth factor-β-induced fibrosis in HSC-T6 cells. Additionally, compounds 1-3, 7, 8, 10, and 11 demonstrated protective effects against hypoxia/reoxygenation-induced injury in H9c2 cells. These findings highlight the therapeutic potential of protolimonoids from D. dasycarpus and provide valuable structure-activity relationship insights for future drug development.