Synthesis Of The Oligosaccharide Antigen Of The Repeating Units Of Escherichia Coli O36 And Burkholderia Multivorans C1576

Carbohydrates, one kind of biopolymers with distinct structural features, mainly exist in the form of carbohydrate complexity such as glycolipids, glycoproteins and so on. The N-linked glycans and O-linked glycans are two forms in the nature. Currently, the low quantities and difficult isolation and purification of oligosacahride and/or glycoconjugates from nature sources unfufills the sufficient amount for further biological studies, thus the development of simple and effective synthetic methods are in need.Enzyme synthesis is one kind of effective approach to obtain oligosaccharide chains. Glycosidases and glycosyltransferases are two main participators. The glycosidases mainly promote the hydrolytic reaction, and, however, involve in the glycosylation reaction if the concentration of glycosyl donor is too high. While, the glycosyltransferases catalyze the specific transferation of a sugar nucleotide donor to glycosyl acceptor in good yield and high regioselectivity.Chemical synthesis is another kind of effective approach to obtain structural well-defined oligosaccharide chains, which has many advantages such as the repeatability and the large-scale preparation. However, the chemical synthesis involves in many protection-deprotection manipulations and careful designation of glycosylation strategies to generate the desired α/β glycosyl bonds.Usually, carbohydrate molecules are poorly immunogenic antigen and can only induce the T cell-independent immune response, which are weak in protection and lack of immune memory. When carbohydrate antigen is covalently attached to the immunogenic protein carrier (such as TT, CRM197, KLH), the resulting glycoconjugates can induce strong T cell-dependent antibodies. This vaccine strategy are widely applied to the commercial carbohydrate-based vaccines. The hetetopolysaccharide obtained from nature might affect the bioactivity efficiency of glycoconjugate vaccines. Thus, fully chemical-synthetic strategy will offer the structural well-defined carbohydrate antigens-based vaccines, which have recently attracted more attention in the development of carbohydrate vaccines.In this thesis, we described the synthesis of the pentasaccharide repeating unit of the O-antigen of Escherichia coli 036, and synthesis of the trisaccharide repeating unit and their oligomers of the O-antigen of Burkholderia multivorans C1576, and synthesis of the tetrasaccharide repeating unit of extracellular polysaccharides (EPS) of B. multivorans C1576. All these synthetic target compounds could be coupled with carrier proteins to generate the glycoconjugates for further biological and immunological studies. There are four chapters included:Chapter 1 includes a brief introduction of the bacterial lipopolysaccharides and extracellular polysaccharides and their applications to the development of carbohydrate based vaccine.In chapter 2, an efficient synthesis of the pentasaccharide repeating unit of the O-antigen from Escherichia coli 36 strain was described.The chemical structure of the pentasaccharide comprised of L-rhamnose, L-fucose, D-mannose and D-glucosamine. The fully protected target pentasaccharide analog was prepared via a "3+2" glycosylation manner. Thus, the trisaccharyl trichloroacetimidate 3 was assembled from rhamnosyl trichloroacetimidate donor 6, fucosyl thioglycoside 8 and para-methoxyphenyl mannoside acceptor 9, followed by removal of para-methoxyphenyl group on anomeric carbon and trichloroacetimidation. The disaccharide acceptor 2 was preapared from the coupling reaction of fucosyl imidate donor 4 and glucosaminyl acceptor 5. Finally, the trisaccharyl donor 3 was coupled with the disaccharide acceptor 2 in the presence of a catalytic amount of trimethylsily trifluoromethanesulfonate (TMSOTf) to give the fully protected target pentasaccharide 1.Charpter 3 presented the chemical synthesis of the trisaccharide repeating unit and its oligomers of the O-antigen from the gram-negative pathogen Burkholderia multivorans C1576. The retrosynthetic disconnection of target molecules 27-29 resulted in the trisaccharyl thioglycoside 30, which could be glycosylated with 3-azado-l-propanol 15 to introduce an amino group at the reducing end for further conjugation with carrier protein. Thioglycoside 30 could be used to elongate the carbohydrate chain to obtain its dimer and trimer in a "3+3" and "3+3+3" manner. The trisaccharide 30 could be obtained from assembly of rhamnosyl and mannosyl trichloroacetimidate donors 31 and 32 and thiomannoside acceptor 33. Finally, multi-step deprotecting group manipulations of compounds 50,52,54 completed the synthesis of the target molecucules 27-29.In chapter 4, the synthesis of the tretrasaccharide repeating unit of novel exopolysaccharide from Burkholderia multivorans C1576 was descrided. Retrosynthetic analysis of the target molecule 55 revealed that it could be prepared by the coupling reaction of the known mannosyl disaccharide donor 56 and the rhamnosyl disaccharide acceptor 57. The disaccharide acceptor was synthesized from the condenation of the 3-O-methyl rhamnosyl trichloroacetimidate donor 58 and rhamnosyl acceptor 59, followed by enlongation of 3-azadio-l-propanyl chain on anomeric carbon and selective deacetylation on C-2’ position. Finally, the desired tetrasaccharide 55 was obtained from the coupling reaction of 56 with 57 under the promotion of the catalytic NIS/TMSOTf, followed by the globle deprotection.