Chemical Synthesis Of The Repeating Units Of Type ? And Type ? Group B Streptococcus Capsular Polysaccharides

Group B Streptococcus(GBS)is the main pathogen causing neonatal meningitis and pneumonia.It can also cause amniotic membrane infection and premature rupture of membranes in pregnant women,as well as meningitis,pneumonia and other diseases in non-pregnant adults.Infections caused by GBS in newborns are divided into two types,early-onset(0-6 days)and late-onset(7-90 days)diseases.The overall neonatal infection rate associated with GBS was about 0.53 ‰,and the early-onset infections account for about 0.43 ‰.The reported data further showed that the incidence of GBS infection was even higher in the elderly over 65 years of age than in newborns.Thus,GBS infection represents a significant health problem.Unfortunately,there is no effective treatment to completely eliminate the threat of GBS to human life and health.In developed countries,the current strategy to reduce neonatal early-onset GBS infection is prenatal antibiotic treatment,but the method can not reduce the incidence of late-onset disease in newborns.In the meantime,overuse of antibiotics to control neonatal GBS infection has also led to increased antibiotic resistance.Furthermore,antibiotic treatment is expensive,which has limited its worldwide use,especially in developing countries.Therefore,to prevent GBS infection,a safe and effective GBS vaccine is highly desirable.As a Gram-positive bacterium,the surface of GBS is coated with capsular polysaccharides(CPSs),which covers the bacterial surface protein and simulates the polysaccharide structure on the surface of the host cell,allowing the bacteria to escape the host immune system.Furthermore,CPS is one of the major virulence factors of GBS.Consequently,CPSs are potent targets for the host immune system.Since CPS is T cell-independent antigen and possoess poor immunogenity,it needs to be conjugated with carrier protein to improve its immunogenety to result in T cell-dependent vaccine.According to this strategy,many glycoconjugate vaccines such as Haemophilus influenzae vaccine,Streptococcus pneumoniae vaccine,Neisseria meningitidis vaccine and so on,have been successfully developed and approved for clinical use.Vaccine development based on GBS CPS has been carrying out for nearly three decades.Among them,the ?-TT vaccine is the first glycoprotein conjugate vaccine that prepared and underwent clinical trial.Until now,the CPS-protein conjugate vaccines based on all of nine serotypes GBS CPSs(?a,?b,?-?)have been prepared,and phase ?-? clinical trails were already carried out for some of them,which revealed that the glycoconjugate vaccines based on GBS CPS were safe and well-tolerant,and,more importantly,they showed protective effects in animals and human.However,these glycoprotein conjugate vaccines consist of heterogeneous and easily contaminated natural CPSs that can bearly meet modern quality standards.In contrast,semi/fully synthetic glycoconjugate vaccines derived from coupling of synthetic carbohydrate antigens with immunological active carriers are a useful strategy to overcome these problems.In this thesis,we achieved the first chemical synthesis of the repeating units of? and ? GBS CPS.An aminoethyl group was designed to be at the reducing end of the target oligosaccharides,which would be used for coupling with carrier molecules to prepare glycoconjugate vaccines.This paper consists of three parts:In Chapter 1,we summarized the CPS structures of all ten serotypes of GBS and the chemical synthesis of various oligosaccharide gragments of GBS CPSs.For example,Jennings group synthesized an la type GBS CPS hexasaccharide and the pentasaccharide repeating unit of type ? GBS CPS,as well as its dimer by using chemoenzymatic strategy.Guo et al.finished the first synthesis of the repeating unit of type V GBS CPS based on preactivation-based one-pot gylcosylation.Whitfield et al.synthesized the repeating tetrasaccharide unit of type VIII GBS CPS tetrasaccharide and its dimer by chemical or chemoenzymatic method.Based upon previous research results and background,we proposed the research objectives of the current project.In Chapter 2,we successfully achieved the first-time synthesis of the repeating unit of type ? GBS CPS by using a preactivation-based one-pot two-step glycosylation.The synthetic strategy utilized thio-glycosides as glycosylation donors and TolSOTf as the promoter that was generated in situ from TolSCl and AgOTf.Thus,multiple steps of protecting group manipulation and intermediate purification were saved.All glycosylation reactions were carried out in one reaction flask,which enabled assembly of the fully protected hexasaccharide 2 within a few hours.In addition,the stereoselective construction of the glucosyl a-glycosidic bond in trisaccharide 17 was attributed to the use of 1.0 equivalent of AgOTf and diethyl ether as solvent,leading to double SN2 attacks at the anomeric carbon.Finally,the protecting groups including carboxylate,carbamate,amido,benzylidene,benzyl,azido and acyl groups in 2 was smoothly deprotected through an efficient five-step protocol to give the target molecule.In Chapter 3,after screening a series of glycosylation strategies,we finally achieved the construction of the fully protected heptasaccharide 28,the protected form of type II GBS CPS repeating unit,through a[1+3+2+1]strategy.It was demonstated that the trisaccharide donors 20 and 21 had low reactivity and maybe only suitable for the high reactive glycosyl acceptor such as primary hydroxyl group but not the second hydroxyl group in tetrasaccharide accepror 5 or 24.In this context,monosaccharide donor 32 was used to glycosylate 24 first to result in tetrasaccharide 33.Next,the glycosylation coupling of sialodisaccharide 3 and 33 afforded 29,which was followed by condensation with galactose donor to give the fully protected heptasaccharide 28 smoothly.The above synthetic approach could overcome the problem of the low reactivity of trisaccharide donor 20/21 and acceptor 5/24.