Abstract
The first chemical synthesis of the natural triterpenoid saponin Patrinia-glycoside B-II, namely oleanolic acid 3-O-α-l-rhamnopyranosyl-(1→2)-[β-d-gluco-pyranosyl-(1→3)]-α-l-arabinopyranoside, has been accomplished in a linear 11-step sequence 11 with 9.4% overall yield. The abnormal 1C4 conformation of the arabinose residue was found to occur via conformational fluctuation during preparation of the intermediates. Molecular mechanism and quantum chemistry calculations showed that Patrinia-glycoside B-II and its conformer 1 cannot interconvert under normal conditions. Preliminary structure-activity relationships studies indicated that the 4C1 chair conformation of the arabinose residue in the unique α-l-rhamnopyranosyl-(1→2)-α-l-arabinopyranosyl disaccharide moiety is one of the chief positive factors responsible for its cytotoxic activity against tumors.
Highlights
Triterpenoid saponins from terrestrial plants possess enormous structural diversity and varied biological activities [1,2,3]
To investigate the structure-activity relationships (SARs) of this type of saponin, we have developed a convenient method for the chemical preparation of Patrinia-glycoside B-II
The C1-C2 chemical bond needs to be broken and created, as shown by the transition state (Figure 3). Both molecular mechanism and quantum chemistry calculations showed that compound 1 and Patrinia-glycoside B-II cannot interconvert under normal conditions
Summary
Triterpenoid saponins from terrestrial plants possess enormous structural diversity and varied biological activities [1,2,3]. Some oleanane-type triterpenoid saponins bearing the Molecules 2013, 18 α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl disaccharide moiety at the C3-OH of the aglycone often present significant antitumor activity [4,5,6,7,8]. Previous research has identified this unique disaccharide moiety as a characteristic sugar sequence that can strongly induce the antitumor activity in some oleanane-type glycosides such as β-hederin [9,10]. Patrinia-glycoside B-II (PB-II, Figure 1), a typical triterpenoid saponin isolated from the seeds of Patrinia scabiosaefolia Fischer [11], contains the unique oligosaccharide substructure and displays prominent inhibitory activity against many tumor cell lines [7]. To investigate the structure-activity relationships (SARs) of this type of saponin, we have developed a convenient method for the chemical preparation of Patrinia-glycoside B-II
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