| Carbohydrates play essential roles in many biological processes.Hence,carbohydrates have been successfully used in the development of various biomedical products.The most prevalent procedures in the elucidation of the structure-activity relationships of carbohydrates are conjugation or immobilization of synthetic glycans.Notably,the synthetic glycans are usually designed to contain orthogonal linkers to ensure the chemoselective cross-coupling,which can improve the accuracy of activity estimates.Among them,amine linker is the most commonly used due to its good stability and reactivity.On the other hand,various polysaccharides containing free amino groups have been found on the surface of pathogenic bacteria.Inevitably,the chemoselectivity of amidation is potentially affected by the presence of additional free amino groups on the sugar ring,thereby limiting the application of amine linker in these important glycans.The traditional method for selective coupling of these glycans relies on a protective group strategy.Considering that the orthogonal protection of amino groups may reduce the synthesis efficiency,an understanding of the reactivity difference between amino groups on the sugar ring and linker can provide a basis for highly selective conjugations of glycans.In this study,eight amine-linked aminomonosaccharides were designed and prepared.3,3’-Dithiobis(sulfosuccinimipropionate)(DTSSP)was used as the amidation reagent.The chemoselectivity of N-acylation was investigated during the introduction of the thiol moiety into amine-linked aminosugars,aiming to explore the nucleophilic differences of each amino group.The main conclusions are as follows:(1)Syntheses of the eight aminosugars bearing an amine linker.The most common monosaccharides,including D-glucose,D-galactose,and D-mannose,with an alkyl amino linker were chosen as backbones,which resulted in the design of eight amine-linked aminomonosaccharides,including 2-aminoglucoside,2-aminogalactoside,2-aminomannoside,3-aminoglucoside,3-aminogalactoside,3-aminomannoside,4-aminoglucoside,and 4-aminogalactoside.(2)Chemoselectivity of DTSSP-based functionalization of aminoglycosides.To explore the chemoselectivity of amino groups,the eight amine-linked aminomonosaccharides were 3-mercaptopropionylated by DTSSP.For 2-aminoglucoside,3-aminoglucoside,4-aminoglucoside and 4-aminogalactoside,the 3-mercaptopropionyl group was selectively installed onto the amine of the linker to produce the corresponding thiol-linked aminoglycosides.For 2-aminomannoside and 3-aminogalactoside,the amino groups on both the linker and sugar ring were 3-mercaptopropionylated in 82% and 80% yields,respectively.The 2-aminogalactoside and the 3-aminomannoside were transformed into a mixture of mono-3-mercaptopropionylated compound and di-3-mercaptopropionylated compound.It’s found that the intermediates formed from the reactions of aminosugars with DTSSP,the mono-3-mercaptopropionylated products were generated through the reduction of the disulfide bond of the corresponding carboxylic acid intermediates,and the di-3-mercaptopropionylated products were formed from the intramolecular cyclization of the corresponding carboxylic acid intermediates and then reduction of the disulfide bond.(3)Computational analysis of the nucleophilicities of the eight aminoglycosides.The Fukui function was used to further quantify the nucleophilicity of amino groups,demonstrating that the amino group on the linker has higher nucleophilicity.These results will serve well for the preparation of conjugation-ready glycans containing free amino groups.Therefore,the highly chemically selective oligosaccharides bearing a free amino group on the sugar ring and the thiol moiety on the linker was obtained in this study.Instead of the amine linker,the thiol linker enabled the efficient conjugation of oligosaccharides to glycan array and carrier protein.In this study,chemoselectivity of the DTSSP-based amidation reaction of amine-linked aminomonosaccharides was elucidated.It would provide a reference for the selection of crosslinking agents in the preparation of glycoconjugates and lay a foundation for the subsequent development of glycoconjugate vaccines. |
