Towards a synthetic strategy for the ten canonical carrageenan oligosaccharides – synthesis of a protected γ-carrageenan tetrasaccharide

We herein report a synthetic strategy aiming at synthesizing the ten canonical carrageenan oligosaccharides from one single precursor. The key β-(1→4)-linked disaccharide was synthesized from commercially available galactose pentaacetate. The notoriously difficult formation of β-(1→4)-D-galactan linkages was successfully optimized on the differentially substituted monosaccharides to afford the desired disaccharide in 55% yield. Following a convergent strategy, two disaccharides were then glycosylated to form the fully protected α-(1→4)-linked tetrasaccharide backbone of the carrageenans. The careful selection of protecting groups provides the opportunity to access all ten carrageenan substructures identified in polysaccharides isolated from red algae. Here, we demonstrate how one such target oligosaccharide can be obtained in a protected form.