Total synthesis and biological activities of (+)- and (-)-boscialin and their 1'-epimers

Natural (-)-boscialin [(-)-1] has recently been described as one of the constituents of various medicinal plants. To obtain more material for investigations of its biological activities, we carried out the synthesis of (-)-1 and its isomers. Starting from the chiral building block 2, the key steps of the synthesis involved a regioselective reduction and a nucleophilic addition. The enantiomer of the natural product, (+)-boscialin [(+)-1], could be obtained via acid-catalyzed epimerization of hydroxyketone 4 to (+)-3. Starting the synthesis with (-)-3 led to (-)-boscialin [(-)-1] with the natural absolute configuration. In addition to (+)- and (-)-boscialin, the corresponding 1'-epimers (+)- and (-)-epiboscialin were also obtained. In vitro assays with (-)-boscialin [(-)-1] and its three stereoisomers were carried out to test for activity against microbes, parasites, and human fibroblasts. The investigations revealed activity against various microbes and against Trypanosoma brucei rhodesiense and also revealed cytotoxicity against human cancer cells.