Abstract
Bioconjugations under benign aqueous conditions have the most promise to covalently link carbohydrates onto chosen molecular and macromolecular scaffolds. Chemical methodologies relying on C-C and C-heteroatom bond formations are the methods of choice, coupled with the reaction conditions being under aqueous milieu. A number of methods, including metal-mediated, as well as metal-free azide-alkyne cyclo-addition, photocatalyzed thiol-ene reaction, amidation, reductive amination, disulfide bond formation, conjugate addition, nucleophilic addition to vinyl sulfones and vinyl sulfoxides, native chemical ligation, Staudinger ligation, olefin metathesis, and Suzuki-Miyaura cross coupling reactions have been developed, in efforts to conduct glycoconjugation of chosen molecular and biomolecular structures. Within these, many methods require pre-functionalization of the scaffolds, whereas methods that do not require such pre-functionalization continue to be few and far between. The compilation covers synthetic methodology development for carbohydrate conjugation onto biomolecular and biomacromolecular scaffolds. The importance of such glycoconjugations on the functional properties is also covered.
Highlights
The roles of carbohydrates in a multitude of biological functions are at the forefront of research in glycobiology (Varki et al, 2015)
An early utility of the glycoconjugation pertains to the synthesis of C-glycosyl amino acid through reaction of allyl C-glucopyranoside with vinyl oxazolidine mediated by the second generation Grubbs catalyst, followed by further synthetic manipulations to afford C-glucopyranosyl amino acid (Scheme 16A) (Dondoni et al, 2001)
The above narration of the chemical methods points to the fact that glycoconjugation of chosen sugars on to proteins and other biomolecules has emerged as an important area of research
Summary
The roles of carbohydrates in a multitude of biological functions are at the forefront of research in glycobiology (Varki et al, 2015). The approach was to conduct (i) amidation of available amines of the carrier protein with an activated carboxylic acid containing a distal diphenylphosphanophenyl ester or Sdiphenylphosphanomethyl thioester and (ii) reaction of the resulting protein modified with the thioester moieties with azide tethered oligo- and polysaccharides (Scheme 9B). The mechanism of the reaction, described as early as in 1938 by Kharasch et al proceeds through the generation of thiyl radical II upon activation of thiol I, by a peroxide (ascaridole) as the radical initiator in this work, the addition of which across the ene moiety occurs, leading to the formation of a new C-S bond, along with free radical at β-carbon to thioether functionality (III) (Scheme 13A). Not used widely for glycoconjugations, the method has seen initial success and wider applicability can be expected further
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