Synthesis of the pentasaccharide moiety of starfish asterosaponin luidiaquinoside and its conformational analysis

Abstract

The synthesis of the pentasaccharide moiety of starfish asterosaponin luidiaquinoside as their p-methoxyphenyl (PMP) glycosides was achieved by following a sequential glycosylation strategy using suitably functionalized thioglycoside donors. Sulfuric acid immobilized on silica (H₂SO₄–silica) was used successfully as a Brönsted acid catalyst to work as a promoter for all glycosylation reactions. A Qui III donor (5) equipped with a neighbouring participating chloroacetyl (CA) group at the 2-position was first coupled with a trisaccharide acceptor (10) to construct the β-(1-4) glycosidic bond. Then the chloroacetyl (CA) group was selectively removed and subsequent glycosylation with a peracetylated D-fucopyranosyl thioglycoside donor gave the desired pentasaccharide. To gain an understanding about the structural properties of these pentasacharides in a biologically relevant environment, NMR studies were carried out in aqueous solution and the restrained molecular dynamics (MD) were carried using the experimental restraints derived from the NMR studies.