Absolute configuration of (-)-gambogic acid, an antitumor agent.

(-)-Gambogic acid (1), a biologically active "caged xanthone" from gamboge, the dried resin of Garcinia hanburyi, is of interest as a potential anticancer agent. The planar structure of (-)-gambogic acid has been determined previously by analysis of its detailed NMR data and confirmed by single-crystal X-ray diffraction, with the absolute configuration at C-13 deduced as R through a series of chemical degradations. Using (-)-morellic acid (2), an analogue of (-)-gambogic acid, as a model compound, the 5R, 7S, 10aS, 13R, 27S absolute configuration of (-)-gambogic acid was determined for the first time by comparison of physical and spectroscopic data, especially experimental and calculated electronic circular dichroism.