| The main challenge in synthetic carbohydrate chemistry is the development of procedures for the regioand stereoselective introduction of glycosidic bonds between donor and acceptor saccharides to give (oligo)saccharides. Among the types of glycosidic bonds that can be formed in a glycosylation event, namely 1,2-cis bonds and 1,2-trans bonds, the introduction of the first one is the most problematic. Anchimeric assistance of ester protecting groups at the C-2 hydroxyl of glycosyl donors usually steers condensations with acceptors to the sole formation of 1,2-trans bonds. The introduction of 1,2-cis bonds requires the presence of a non-participating group at the C-2 hydroxyl of a glycosyl donor and mostly leads to the formation of both cis and trans products. Different mechanisms based on either an SN1 or an SN2-like pathway are thought to play part in directing the stereochemical outcome in a glycosylation event, and in which the magnitude of the anomeric effect is often considered to be the deciding factor. Until recently the most difficult glycosidic linkage to install by organic synthesis was the β-mannosidic linkage, as both the anomeric effect and the stereoelectronic effect of the axially oriented C-2 function work against the formation of this 1,2-cis linkage. We recently discovered that glycosylations of mannuronic ester donors (D-mannopyranosides of which the C6-alcohol function is oxidized to a carboxylic acid) as well as L-gulose donors (the C5 epimers of D-mannopyranosides) with various acceptors predominantly led to 1,2-cis products. Intrigued by the unexpected cis selectivity of the Dmannuronic ester and L-gulose donors and unsatisfied with the explanation of a SN2 like mechanism we investigated the influence of the C-5 carboxyl ester on the stereochemical outcome of glycosylations by the condensations of a ?stripped? donor having an isolated C-5 ester function. On the basis of these results we postulated a SN1 type mechanism, in which attack of the acceptor occurs on the most stable oxacarbenium ion conformer of the donor, thereby explaining the found 1,2-cis products. The present proposal aims to deepen our insight in the mechanism of the glycosylation reactions by assessing the role that oxacarbenium ion conformers play in the stereochemical outcome. Thorough low temperature NMR experiments of various donors will be executed to identify the nature of the reactive intermediates in these glycoslations. Kinetic isotope effects of selected 1,2-cis glycosylations will be determined to assess the amount of oxacarbenium ion character. The influence of the nature, position and configuration of isolated substituents (in particular on the C-2 and C-5) in the pyran core on the stability of the oxacarbenium ion conformers and thereby on stereochemical outcome will be evaluated. The mechanistic studies will be applied and put to the test in the synthesis of fragments of the cell-wall polysaccharides of Neisseria meningitidis K and I, having repeating units with cis-linked N-acetyl mannosaminuronic acid and cis-linked N-acetyl gulosaminuronic acid. The prepared oligosaccharides will be evaluated as potential immunomodulating agents. |
