Isolation, Structure Elucidation, and Absolute Configuration of Germacrane Isomers from Carpesium divaricatum

Five sets of germacrane isomers (1/8/17, 2/7/10/11/13/16/18, 3/4/5/14/20, 6/12/15, and 9/19) with different skeletal types, including seven new ones (1–3, 8–9, and 15–16) were isolated from the whole plant of Carpesium divaricatum. Among them, there are six pairs of stereoisomers (1/8, 2/13, 4/14, 6/12, 7/11 and 10/11). The planar structures and relative configurations of the new compounds were elucidated by detailed spectroscopic analysis. The absolute configurations of 4, 10, 11, and 17 were established by circular dichroism (CD) spectra and X-ray crystallographic analyses, and the stereochemistry of the new compounds 1–3, 8–9, and 15–16 were determined by similar CD spectra with 4, 10, 11, and 17, respectively. The confusion in the literature about subtypes I and II of germacranolides was clarified in this paper. The NMR data of 10–11, and the absolute configurations of the known compounds 4–6, 13–14, and 17–20 were reported for the first time. Compounds 13, 17, and 18 showed cytotoxicity against human cervical (HeLa), colon (LoVo) and stomach cancer (BGC-823) cell lines with IC50 values in the range 4.72–13.68 μM compared with the control cis-platin (7.90–15.34 μM).