Determination of the Absolute Configuration of a Flavanone Isolated from a Madagascar Plant

As part of an International Cooperative Biodiversity Group (ICBG) program, we are focused on discovering biologically active natural products from Madagascar plants. Four antiproliferative flavanones (1-4) were isolated from an ethanol extract of the leaves of a Mundulea sp. (Fabaceae). The structures of 6,8-diprenylaromadendrin (1), lonchocarpol A (2) and mundulea-flavanone B (3) were elucidated as known compounds. Chlorospermin (4) was identified as having been previously isolated from a Glycosmis sp. (Rutaceae), but the optical rotations of the flavanone from the Glycosmis sp and from the Mundulea sp. were of opposite sign. The absolute configuration of 4 was confirmed by analysis of its Electronic Circular Dichroism (ECD) spectroscopic data. The NMR spectra and optical rotation and ECD spectroscopic data of 4 are discussed to confirm the structure and to establish the absolute stereochemistries of the two enantiomeric flavanones.