Synthesis Of The Oligosaccharide Fragments Of The Repeating Units Of Two Bacterial Polysaccharide Antigens

Currently,microbial infections are still one of the greatest threats to the human health and life.Although antibiotics are primarily utilized and are generally effective for treating bacterial infections,their overuse results in the occurrence and rapid growth of resistant bacterial strains.This demands the development of new antibacterial drugs,which is very challenging.Therefore,antibacterial vaccines have become an alternative and powerful strategy to deal with bacterial infections.Carbohydrates,one of the three most important biopolymers,are the most abundant organic molecules in the world and widely found in animals,plants,and microorganisms.Glycosylation modifications of proteins and lipids are a common phenomenon in nature and have been playing a critical role in various physiological and pathological processes,such as fertilization,growth,aging,cancer,infection,and so on.Most carbohydrate molecules are exposed on the cell surface and highly conserved,thus they have become excellent molecular targets in the development of various diagnostic and therapeutic strategies for diseases,including antibacterial infections.For example,the capsular polysaccharides and lipopolysaccharides expressed on the bacterial cell surfaces have been widely utilized as potential antigens for the development of carbohydrate-based antibacterial vaccines.However,typically,carbohydrate molecules are poorly immunogenic and only induce T cell-independent immune responses that are rather weak in protection and lack immune memory.This propery of carbohydrates has significantly hindered their direct application as functional vaccines.A common strategy to overcome this problem is to covalently couple carbohydrate antigens with immunologically active carrier proteins to form neoglycoprotein conjugates,which have been proved to possess much improved immunogenicity and,more importantly,induce T cell-dependent immunities.This strategy has resulted in numerous carbohydrate-based vaccines that have been widely used in clinic.The glycoconjugate vaccines currently used in clinic are prepared by coupling natural polysaccharides isolated from bacteria with carrier molecules.Natural polysaccharides are heterogeneous,and it is impossible to obtain them in homogenous forms.As a result,their protein conjugates are very complex,making their quality control difficult and the study of their structure-activity relationships to further optimize the vaccines impossible.Moreover,during their isolation processes,the polysaccharides can be easily contaminated by various bacterial products,which causes safety concerns.One strategy to deal with this issue is to use fully synthetic oligosaccharides,such as the repeating units and their oligomers of bacterial polysaccharides,to form glycoconjugate vaccines with structurally well-defined antigens.These vaccines are not only potentially safer and more effective but also make structure-activity relationship and many other detailed immunological studies feasible.The project is focused on synthetic studies of the repeating units and their derivatives of the polysaccharide antigens discovered on the cell surface of Streptococcus pneumoniae type 3 and Cronobacter turicensis.The resulting oligosaccharides can be used to couple with proteins for vaccine development and for various other biological and immunological studies.This thesis is composed three chapters as listed below.Chapter 1 provides a simple introduction to the bacterial capsular polysaccharides and lipopolysaccharides,including their structures and functions,as well as their application to carbohydrate-based development.Chapter 2 describes the chemical synthesis of the repeating unit and its oligomers of the capsular polysaccharide from S.pneumoniae type 3.The disaccharide repeating unit of this capsular polysaccharide is composed of D-glucose and D-glucouronic acid residues.A pentasaccharide and a heptasaccharide analogs of the polysaccharide were prepared by the'3 + 2' and '3 + 4' glycosylation strategies,respectively.First,monosaccharide building blocks 5,8,and 9 were synthesized from D-glucose as glycosyl donors and acceptors.Next,they were coupled under proper glycosylation conditions and manipulations of protecting groups to generate trichloroacetimidate donor 3,disaccharide acceptor 7 and tetrasaccharide acceptor 4.Then,donor 3 was coupled with disaccharide acceptor 7 and tetrasaccharide acceptor 4 in the presence of a catalytic amount of trimethylsily trifluoromethanesulfonate to give fully protected pentasaccharide 19 and heptasaccharide 20.Finally,multi-step protecting group manipulations of 19 and 20,together with the selective oxidation of the hydroxyl group at their C-6 position to carboxylic acid groups,completed the synthesis of the target molecule 1 and 2.Chapter 3 presents the chemical synthesis of the para-methoxyphenyl glycoside of a tetrasaccharide repeating unit of the O-antigen derived from the gram-negative pathogen C.turicensis.This synthesis was accomplished by an efficient linear assembly strategy.First,an L-rhamnopyranoside derivative 29 with free 2,3-hydroxyl groups was prepared.Then,29 was consecutively and regioselectively glycosylated at the 3-O-and 2-O-positions with a D-glucosaminyl donor 26 and a D-glucosyl donor 28,respectively,to furnish the branched trisaccharide 33 rapidly.Selective 3'-O-deacetylation of the trisaccharide 33,followed by the introduction of another D-glucosaminyl residue,gave the fully protected tetrasaccharide 34.Finally,multi-step protecting group manipulations of 34 completed the synthesis of the target molecule 25.