Synthesis of lupane-type saponins bearing mannosyl and 3,6-branched trimannosyl residues and their evaluation as anticancer agents

The saponins modified with mono- or trimannosyl residues can provide a convenient means of delivering drugs to certain human cells via interactions with mannose receptors. In the study reported therein, we developed a convenient approach for the synthesis of 3-O-mannoside and branched trimannoside derivatives of the saponin lupeol and of C-28 acyl esters of 3-O-acetyl-betulinic acid bearing the same mannosyl entities. Lupeol and 3-O-acetyl-betulinic acid were mannosylated with tetra-O-benzoyl- or tetra-O-acetyl-α-d-mannopyranosyl trichloroacetimidates. De-esterification followed by regioselective dimannosylation of the unprotected monosaccharide derivatives with 2 equiv of tetra-O-benzoyl-α-d-mannopyranosyl trichloroacetimidate selectively yielded O-3,O-6-linked trimannosides. The cytotoxic activity of selected lupane-type saponins (derivatives of lupeol, betulinic acid, and betulin) toward normal human fibroblasts and various cancer cell lines was also compared.

Graphical abstractMannosylation of lupeol and 3-O-acetyl betulinic acid with tetra-O-benzoyl- or tetra-O-acetyl-α-d-mannopyranosyl trichloroacetimidate followed by de-esterification and regioselective dimannosylation selectively yields O-3,O-6-linked trimannosides.Figure optionsDownload full-size imageDownload as PowerPoint slide