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

Abstract 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 IC 50 values of 2.60, 7.02, 1.47, 3.37, and 4.14 μM, respectively, which were better than the standard control doxorubicin (IC 50 9.74 μM).