Newest Strategies in the Search for Bioactive Saponins from the Tropical Plant Biodiversity

This review will focus on newest results leading to the discovery of new bioactive saponins by using a combination of successive advanced procedures in extraction, isolation, structure elucidation and bioassays. Microwave- and ultrasonic-assisted extractions, two recent advanced methods have been increasingly used in the last decade. Then, a multistep purification procedure was achieved by flash chromatography, vacuum liquid chromatography, low, medium- and high-pressure liquid chromatography on silica gel and reversed-phase silica gel RP-18 (VLC, LPLC, MPLC, HPLC). These successive chromatographic steps have been implemented in the author's laboratory in order to avoid the time-consuming traditional partitions between butanol and water, dialysis procedures or precipitations in diethyl/ether. The structural elucidation of complex saponins possessing from 5 to 8 sugar units is performed by a combination of extensive spectroscopic techniques including 1D- and 2D-NMR experiments (1H, 13C, DEPT, COSY, NOESY, TOCSY, HSQC, HMBC) and mass spectrometry (FAB-MS HRESIMS). The bioassays have been mainly carried out in the field of cancerology and inflammation, two closely related areas, and also in the field of immunology with recent literature results on Quillaja saponins in order to explore some structure/activity relationships. The more recent results of the author's laboratory will be presented with examples of saponins from the tropical plant biodiversity (Pittosporaceae, Polygalaceae, Mimosaceae, Sapindaceae, Apiaceae, Dioscoreaceae, and Asparagaceae). Furthermore, some new trends reported in the literature will be briefly reviewed concerning dereplication, and metabolomic approachs which are currently of considerable importance in the field of natural product discovery.