Facile Synthesis of Trisaccharide Moiety Corresponding to Antitumor Activity in Triterpenoid Saponins Isolated from Pullsatilla Roots

Abstract

A trisaccharide found in triterpenoid saponins isolated from Pullsatilla roots appears as an important promoiety for the enhancement of anticancer activity of their aglycones. Thus a facile synthetic method for a trisaccharide moiety, allyl-2,3,4-tri-O-benzoyl-α-L-rhamnopyranosyl-(1→2)-[2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl-(1→4)]-3-O-benzoyl-β-L-arabinopyranoside (3), has been firstly developed through the regio- and stereoselective glycosylations from arabinose in total 16% yield via route 2 (eight steps). In this synthetic procedure, the protection of anomeric –OH of L-arabinose with equatorially oriented allyl group unlike with the axial 4-methoxybenzyl protecting group well promoted glycosyl bond formation between α-L-rhamnopyranosyl trichloroacetimidate and 2-OH of arabinose. As expected, the synthesized trisaccharide moiety 3 has no cytotoxicity (ED₅₀: >100 μM) against three human cancer cell lines (A-549, SK-OV-3, and SK-MEL-2), respectively.