Synthetic sulfated saccharides in cell signalling

Abstract

Access to heparan sulfate-like oligosaccharides, displaying a specific sulfation pattern, is chemically challenging but highly desirable for further insight into the relationship between structure (including sulfation pattern) and function of heparan sulfates. One of the difficulties encountered in heparan sulfate oligosaccharide synthesis is the incorporation of uronic acids, due to reduced reactivity of these saccharides. Another difficulty is the design of a protecting group strategy to give selective access to sites for sulfation. To investigate the use of uronic acids as donors in oligosaccharide synthesis, a series of comparative glycosylation reactions was carried out using both hexose and uronate, donor and acceptor disaccharides. An orthogonally protected disaccharide was synthesised to access these donors and acceptors. It was found that acetimidate donor systems performed very well in the hexose case, whereas using a uronate acetimidate donor significantly reduced yields of the reaction. A thioglycoside donor performed similarly in both cases but was overall significantly lower yielding than the hexose acetimidate reaction. Later, two fully protected octasaccharide targets were synthesised. The compounds were designed to investigate the structural requirements for FGF/FGF2 binding. Considering the previous results, the synthesis of these octasaccharides was carried out using hexose donors, necessitating an oxidation step following chain assembly. Firstly, two monosaccharide starting materials, incorporating a novel protecting group allowing orthogonal access to the 3-O-position of glucosamine residues, were synthesised. The monosaccharides were then glycosylated to produce disaccharide building blocks, from which the octasaccharide chains were assembled. Considerable difficulty was encountered in oligosaccharide assembly reactions using tetrasaccharide and especially hexasaccharide acceptors. This difficulty may have arisen from the novel protecting group, which is proximal to the reaction site. Some optimisation was achieved by changing the donor system used, but ultimately a revised glycosylation strategy was used to overcome this difficulty. In future work, the fully protected octasaccharides will be oxidised, deprotected and sulfated in sequence to produce an octasaccharide heparan sulfate mimetic with a specific sulfation pattern.