Abstract
Saccharides are polyhydroxy compounds, and their synthesis requires complex protecting group manipulations. Protecting groups are usually used to temporarily mask a functional group which may interfere with a certain reaction, but protecting groups in carbohydrate chemistry do more than protecting groups usually do. Particularly, protecting groups can participate in reactions directly or indirectly, thus affecting the stereochemical outcomes, which is important for synthesis of oligosaccharides. Herein we present an overview of recent advances in protecting groups influencing stereoselectivity in glycosylation reactions, including participating protecting groups, and conformation-constraining protecting groups in general.
Highlights
It has been widely recognized that oligosaccharides and glycoconjugates play important roles in diverse biological processes, including viral and bacterial infections, cell growth and proliferation, cell–cell communication, as well as immune response [1,2,3,4,5]
To extend the scope of armed–disarmed strategy and allowed stereoselective introduction of a 1,2-trans linkage prior to another 1,2-trans or 1,2-cis linkage (Scheme 5a) [31]. This protecting group is capable of efficient participation through a six-membered intermediate 36 (Scheme 5b) as acyl participating group to show complete 1,2-trans selectivity, but it belongs to the ether protecting groups that are capable of activation to retain the glycosyl donor in the armed state as opposed to conventional acyl participating moieties
In 2005, Boons and coworkers reported a novel strategy for stereoselective glycosylations in which a chiral auxiliary at the C2 position of a glycosyl donor was used (Scheme 6) [32]
Summary
It has been widely recognized that oligosaccharides and glycoconjugates play important roles in diverse biological processes, including viral and bacterial infections, cell growth and proliferation, cell–cell communication, as well as immune response [1,2,3,4,5]. Protecting groups in carbohydrates differentiate the same sort of functional groups to expose the one needed to be reacted, and confer other effects to the molecules in glycosylation reactions. They can increase or decrease the reactivity and they can participate in the reactions, affecting the stereochemical outcomes of glycosylations. As there have been excellent reviews involving oligosaccharide synthesis [13,14], especially about mannosides [15], 2-amino-2-deoxysugars [16] and sialic acid-containing sugars [17,18,19,20], this review will mainly summarize the latest advances in stereoselective glycosylations from the protecting group point of view
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