Investigation of the ethanol extract of the whole plant of Ainsliaea macrocephala led to the isolation of five new sesquiterpenoids, namely ainsliadimer C (1), ainsliadimer D (2), ainsliaolide B (3), ainsliatone B (4), and ainsliaolide C (5), together with seventeen known sesquiterpenes and sesquiterpene glycosides (6- 22). Their structures were elucidated by spectroscopic methods. The relative stereochemistry of ainsliadimers C (1) and D (2) were further confirmed by single crystal X-ray diffraction analysis. Total extract of A. macrocephala and compounds 1- 22 were tested for inhibitory activity against the production of nitric oxide in RAW 264.7 cells stimulated by LPS, as well as for cytotoxicity against RAW 264.7 macrophages. Of all samples tested, purified compounds 4, 7, and 12 strongly inhibited the production of nitric oxide with IC50 values of 8.78, 2.50, and 7.11 µM, and simultaneously showed low cytotoxicity against RAW 264.7 macrophages.
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