LAUSR

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. Introduction In order to increase understanding of plant cell wall biology and physiology, well-defined plant cell wall oligosaccharides have been used as model compounds to represent domains from larger, more complex polysaccharides. Therefore, synthetic chemists have devoted attention to developing efficient strategies for the chemical synthesis of such oligosaccharide fragments.[1]–[8] However, as outlined by a recent review summarizing all plant and algal cell wall oligosaccharides synthesized to date,[9] there are relatively few examples of synthetic fragments representative of marine polysaccharides. Therefore, we are interested in the synthesis of carrageenans, which are sulfated polysaccharides found in red algae seaweed.[10]–[13] Not only are these targets potentially very valuable for understanding the algal cell wall, but they would also help advance the development of new methodologies in carbohydrate chemistry and more specifically in relation to sulfated oligosaccharides.[14]–[17] Carrageenans are a family of highly sulfated galactans found in the cell walls of red seaweeds of the Rhodophyceae class. These polysaccharides serve as gelling, stabilizing and viscosity-enhancing agents in many sectors including the pharmaceutical and food industry and represent one of the major texturizing ingredients in processed foods.[13]-[18] Their use in pharmaceutical products continues to be investigated, and recent applications in drug delivery systems are summarized in a review by S. Liang Li et al.[19] Carrageenan are divided into ten different idealized repeating disaccharides, which differ mainly by their degree of sulfation and the presence or absence of 3,6-anhydrogalactose residues (Figure 1). Naturally occurring carrageenan is a mixture of non-homologous polysaccharides, which makes it challenging to obtain samples of homogenous oligosaccharides by either chemical or enzymatic degradation of the algae cell wall components. We present herein a synthetic strategy aiming at producing the ten canonical carrageenan oligosaccharides from one single protected oligosaccharide precursor.