[摘要]:A phytochem. investigation of Ainsliaea macrocephala led to the isolation of a new dimeric sesquiterpene lactone, ainsliadimer A (I). The structure of I was elucidated by spectroscopic anal., and confirmed by single crystal X-ray diffraction. I represents an unusual carbon skeleton with a cyclopentane system connecting the two monomeric sesquiterpene lactone units. This unique mol. exerted potent inhibitory activity against the prodn. of nitric oxide in RAW264.7 stimulated by LPS.