Bioactive scalemic caged xanthones from the leaves of Garcinia bracteata.

Four pairs of previously undescribed caged xanthones (1-4) and twelve known caged xanthones (5-16) were isolated from the leaf extract of Garcinia bracteata. Their structures were unambiguously elucidated on the basis of spectroscopic methods. The planar structure and relative configuration of 1 was confirmed by X-ray crystallographic analysis. The enantiomers of compounds 1, 2, 4 were further resolved by semi-preparative chiral HPLC, and the absolute configurations of enantiomers of compounds 1 and 4 were determined by measurement and calculation of electronic circular dichroism (ECD) spectra and specific rotations. The inhibitory activities of the isolated compounds against human HeLa, A549, PC-3, HT-29, and WPMY-1 cell lines were assayed, and garcibractatin A (4) showed the most potent inhibitory activities in vitro with IC50 values from 1.11 to 2.93 µM. A preliminary structure-activity relationship has been discussed, and some helpful conclusions have been drawn.