Sesquiterpene lactones from the aerial parts of Xanthium sibiricum and their cytotoxic effects on human cancer cell lines

► Two new eremophilanolides were isolated from the aerial parts of Xanthium sibiricum. ► The new structures were elucidated by spectroscopic method and the modified Mosher's method. ► Three sesquiterpene lactones exhibited remarkable cytotoxicities against human cancer cells.