Resolution and identification of scalemic caged xanthones from the leaf extract of Garcinia propinqua having potent cytotoxicities against colon cancer cells

A new scalemic 8,8a-dihydro caged xanthone (1) was isolated from the leaf extract of Garcinia propinqua. Five other known natural products, the three caged xanthones (2, 5 and 6) and the two neocaged xanthones, (3 and 4) were also isolated as scalemic mixtures. Their structures were characterized by spectroscopic methods. The enantiomeric ratios (er) of compounds 1-6 ranged from 1:0.7 to 1:0.9. These compounds were also resolved by semipreparative chiral HPLC. The absolute configurations of (+)-2 and (+)-3 were determined by single-crystal X-ray diffraction analysis using Cu Kα radiation while the absolute configurations of the other compounds were determined by comparisons of their ECD spectra. Compounds (-)-4, (+)-4, (-)-5, (+)-5, and (-)-6 showed potent cytotoxicities against a colon cancer cell line HCT116 with IC50 values of 2.60, 7.02, 1.47, 3.37, and 4.14μM, respectively, which were better than the standard control doxorubicin (IC50 9.74μM).