Two new ent-kaurane glucosides from the fruits of Xanthium strumarium subsp. sibiricum

The chemical investigation of the fruits of Xanthium strumarium (Asteraceae) led to the isolation of two new ent-kauranoid glucosides named 2-O-(6-O-isocaleryl-β-D-glucopyranosyl) atractyligenin (1) and 2-O-(2-O-isovaleryl-β-D-glucopyranosyl) atractyligenin (2), together with one known compound. Their structures were established by comprehensive spectroscopic analysis coupled with single-crystal X-ray diffraction and electronic circular dichroism data. All compounds and their aglycone were evaluated for their anti-proliferative activities in vitro against three human cancer cell lines.