Synthesis and biological activity of new homolupanes and homolupane saponins

A concise synthesis of 28a-homolupane triterpenes and the corresponding saponins containing d -mannose, d -idose, d -arabinose, and l -rhamnose moieties was elaborated. The overall synthesis of the new triterpenes involved three linear steps starting from readily available 3-O-acetyl-betulinal: elongation of the carbon chain by Wittig reaction followed by enol ether hydrolysis and reduction (or oxidation) of the elongated aldehyde. Saponins were obtained by glycosylation of triterpenes with classical Schmidt donors. Cytotoxic activities of new lupane and homolupane compounds were evaluated in vitro. Several triterpenes and the corresponding saponins exhibited an interesting cytotoxic activity profile against human cancer cell lines. Influence of the side-chain structure and substituents on the cytotoxicity of betulin and homobetulin derivatives was investigated. These results open the way to the synthesis of various lupane-type triterpene and saponin derivatives as potential anticancer compounds.