Abstract
Mechanistic study of carbohydrate interactions in biological systems calls for the chemical synthesis of these complex structures. Owing to the specific stereo-configuration at each anomeric linkage and diversity in branching, significant breakthroughs in recent years have focused on either stereoselective glycosylation methods or facile assembly of glycan chains. Here, we introduce the unification approach that offers both stereoselective glycosidic bond formation and removal of protection/deprotection steps required for further elongation. Using dialkylboryl triflate as an in situ masking reagent, a wide array of glycosyl donors carrying one to three unprotected hydroxyl groups reacts with various glycosyl acceptors to furnish the desired products with good control over regioselectivity and stereoselectivity. This approach demonstrates the feasibility of straightforward access to important structural scaffolds for complex glycoconjugate synthesis.
Highlights
Mechanistic study of carbohydrate interactions in biological systems calls for the chemical synthesis of these complex structures
The slower pace of breakthroughs in oligosaccharide synthesis is certainly not from limited interest into the field, but rather the inherently complex and diverse structures employed by biological systems[3, 4]
We aim to provide a solution to the aforementioned challenges with a well-designed temporal group that binds to glycosyl donor in situ, prior to activation, to direct the stereooutcome but is removed in the work-up phase (Fig. 1f)
Summary
Mechanistic study of carbohydrate interactions in biological systems calls for the chemical synthesis of these complex structures. Using dialkylboryl triflate as an in situ masking reagent, a wide array of glycosyl donors carrying one to three unprotected hydroxyl groups reacts with various glycosyl acceptors to furnish the desired products with good control over regioselectivity and stereoselectivity. This approach demonstrates the feasibility of straightforward access to important structural scaffolds for complex glycoconjugate synthesis. Nucleophilic substitution at the anomeric center is the crucial bottleneck as clean SN2 transformation is almost always accompanied by competing SN1 due to stabilization of C1 cation by the endocyclic oxygen to generate oxocarbenium ion Controlled outcome of this delicate equilibrium is notoriously difficult since it is influenced by many factors, including reactivity of glycosyl donors, temperature, solvents, other additives, etc. The latter mainly focused on employing diol glycosyl acceptors as binding substrates[25,26,27,28,29,30,31,32]
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