Tricarabrols A-C (1-3), three sesquiterpene lactone trimers, were identified from Carpesium faberi. Tricarabrols A and B are a pair of stereoisomers possessing a novel C44 skeleton featuring a methylene-tethered linkage among the sesquiterpene scaffolds. Besides the unique linkage of the cyclopentane ring, tricarabrol C also manifests a methylene bridge. Their full structures were established on the basis of spectroscopic data and further confirmed by computational methods. The biosynthetic pathways involving the Alder-ene reaction, [3 + 2] cycloaddition, and Michael addition reaction were proposed. Tricarabrols A and B (1-2) significantly inhibited the nitric oxide production on lipopolysaccaride-stimulated RAW264.7 macrophages with IC50 values of 2.90 and 4.52 μM, respectively, compared with the positive control indomethacin. Further studies indicated that the anti-inflammatory effect of tricarabrol A was mediated through inhibiting the phosphorylation and nuclear translocation of nuclear factor-κB.
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The authors are thankful for the financial support from the National Natural Science Foundation of China (21920102003, 81673327, 81872754). Part of the work was supported financially by the China National Key R&D Program (2017YFE0195100) and the Science and Technology Development Fund, Macao S.A.R (FDCT 0031/2019/A1). The authors also acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing computational resources that contributed to the results reported in this paper.