New glucuronic acid donors for the modular synthesis of heparan sulfate oligosaccharides

Although hundreds of heparan sulfate (HS) binding proteins have been implicated in a myriad of physiological and pathological processes, very little information is known about ligand requirements for binding and mediating biological activities by these proteins. We report here a streamlined approach for the preparation of modular disaccharide building blocks that will facilitate the assembly of libraries of HS oligosaccharides for structure-activity relationship studies. In particular, we have found that glucuronic acid donors, which usually perform poorly in glycosylations, can give high yields of coupling products when the C-2 hydroxyl is protected with a permanent 4-acetoxy-2,2-dimethyl butanoyl- (PivOAc) or temporary levulinoyl (Lev) ester and the C-4 hydroxyl modified with a selectively removable 2-methylnaphthyl (Nap) ether. It has been shown that the PivOAc ester can be removed without affecting sulfate esters making it an ideal protecting group for HS oligosaccharide assembly. Iduronic acid donors exhibit more favorable glycosyl donating properties and a compound protected with a Lev ester at C-2 and an Fmoc function at the C-4 hydroxyl gave coupling products in high yield. The new donors avoid post-glycosylation oxidation and therefore allow the facile preparation of modular disaccharide building blocks.