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Modulation of Stereoselectivity of Glycosylation: A Supramer Approach (pp. 143-148) $0.00
Authors:  (Leonid O. Kononov, N. K. Kochetkov Laboratory of Carbohydrate Chemistry, N. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences, Leninsky prosp., Moscow, Russian Federation)
Glycoconjugate carbohydrate chains terminated by sialic acids are involved in a wide
range of biological phenomena ranging from cell–cell adhesion and mobility to
oncogenesis and recognition by viruses and bacteria. Therefore, the synthesis and the
biomedical investigation of sialic acid-containing glycoconjugates, oligosaccharides and
their analogs is a very important area of research aiming at understanding their biological
roles and determining their therapeutic relevance.
Sialic acid residues are introduced into an oligosaccharide by a glycosylation
reaction called sialylation. Although substantial progress has been achieved in this area,
poor predictability and reproducibility of results are still typical of the sialylation
reaction. Especially notorious in this respect is the stereoselectivity of sialylation which
can sometimes vary considerably. There has been a long-standing need for an approach to
rationalize the outcome of a particular glycosylation experiment, which can be confusing.
In this Chapter, we describe a novel approach for modulation of yield and
stereoselectivity of glycosylation, and sialylation in particular, which is based on the
recently introduced supramer concept, which emphasizes the importance of
supramolecular aggregation in the reaction mixture leading to formation of supramers
(supramolecular isomers), which are differently arranged supramolecular assemblies of
the same molecular entities. Depending on the molecular structure and reaction
conditions, molecules of reagents can form fundamentally different supramers, which can
be distinguished by physical methods and differ in their chemical properties: the
accessibility of the reaction center in the supramers present would determine the apparent
(macroscopic) reactivity and the outcome of a reaction [product yield and reaction
(stereo/regio)-selectivity]. Using the supramer approach, new variables, which can
modulate stereochemical outcome of glycosylation, have been discovered. The
previously unrecognized factors include (1) concentrations of reagents, (2) presence of
other compounds in the reaction mixture (including "non-reacting" additives or
impurities), and (3) reaction time. All these parameters may influence the structure of
supramers formed by the molecules of reagents in the reaction solution as was
demonstrated by IR-spectroscopy, polarimetry and laser light scattering on a series of
representative examples. The changes in the structure of supramers, and hence their
reactivity, were shown to correlate with outcome of glycosylation influencing both the
yield of glycosylation product and its anomeric ratio. Better understanding of the real
reasons that determine stereoselectivity of glycosylation can be obtained by consideration
of the nature and supramolecular structure of all components of the reaction mixture
rather than by analysis of molecular structure of glycosyl donor only. The supramer
approach was shown to be useful for explanation, prediction and discovery of a series of
unexpected phenomena and allowed the development of highly efficient and
stereoselective glycosylation reactions that lead to formation of Neu-(α-2-3)-Gal and
Neu-(α-2-6)-Gal linkages found in many natural sialo-oligosaccharides of biological and
medical significance. 

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Modulation of Stereoselectivity of Glycosylation: A Supramer Approach (pp. 143-148)