Synthesis Of The Oligosaccharide Fragments Of The Repeating Units Of Burkholderia Polysaccharide Antigens

Carbohydrates, one of the three most important biopolymers, are critical cell surface components of microorganisms, animals, plants, and human being and play vital roles in growth, development, and metabolic processes. Capsular polysaccharides (CPSs) and lipopolysaccharides (LPSs) as the major components of the bacterial cell surface are important virulence factors for bacterial pathogenicity. Due to their unique and diverse structures, high abundacy, exposure on the cell surface, and conserved presence as compared to proteins, CPSs and LPSs are attractive candidates as antigens for the development of carbohydrate-based antibacterial vaccines.The genus of Burkholderia bacteria comprises more than 60 species, isolated from diverse ecological niches, such as river, soil, fresh water sediments, and plant rhizosphere. Several species of Burkholderia bacteria including B. cepacia complex (Bcc), B. pseudomallei and B. mallei, are human, animal and plant pathogens. The Bcc species can cause fatal lung infections in cystic fibrosis patients, resulting in rapid and clinically uncontrollable "cepacia syndrome" such as necrotizing pneumonia and septicaemia, which usually leads to high mortality. B. pseudomallei and B. mallei are two highly pathogenic bacteria, responsible for melioidosis and glanders, respectively. Melioidosis, varying in clinical manifestations and presenting as flu-like symptoms including benign pneumonitis, acute and chronic pneumonia or fulminating septicemia, is often difficult to diagnose. Therefore, the development of safe and effective therapeutic protocols for the prevention and treatment of Burkholderia infections is in urgent need.It has been well established that the unique polysaccharides on bacterial surfaces such as CPSs and LPSs could induce immune responses against the bacteria. Unfortunately, polysaccharides are typically T-cell independent antigens and elicit poor antibody responses in infants, elderly and immunocompromised patients. To overcome these disadvantages of polysaccharide vaccines, it has been proved that neoglycoconjugate vaccines, i.e., polysaccharide-protein conjugates, are efficient in inducing substantial T-cell dependent and memorable immune responses, which makes it possible for the development of new and advanced polysaccharide-based vaccines. Currently, a number of polysaccharide or neoglycoconjugate vaccines are available on the market. An important issue about vaccine derived from natural polysaccharide is that they are heterogeneous and can be easily contaminated. On the other hand, when fully synthetic oligosaccharides of bacterial polysaccharides are used for coupling with proteins to form neoglycoconjugate vaccines, the above problems can be overcome. Therefore, oligosaccharide-based neoglycoconjugate vaccines have become potentially new, efficient, and safe weapons against bacteria.This project is focused on synthetic studies of repeating units of a lipopolysaccharide of B. anthina. These synthetic oligosaccharides can be used for regioselective conjugation with carrier molecules to generate glycoconjugate vaccines and for the studys of their structure-immunological activity relationships.Chapter 1 provides a brief introduction to the history and development of vaccines, polysaccharide vaccines, and neoglycoconjugate vaccines, as well as the taxonomy and pathogenicity of Burkholderia species.Chapter 2 describes the chemical synthesis of the repeating unit and its oligomers of a lipopolysaccharide from B. anthina. The repeating unit of this lipopolysaccharide is a trisaccharide composed of D-galactose and two L-rhamnoses, with the following structure: α-L-Rha-(1â†'2)-α-D-Rha-(1â†'2)-α-D-Gal-(1â†'3). Our first synthetic approach for this trisaccahride was to introduce a large protecting group in the glycosyl donor, hoping to construct the cis a-galactopyranosyl bond based on steric hindrance, which did not afford the desired results. The second synthetic plan was to use dimethyl formamide or diethyl ether to control the stereochemistry of glycosylation through solvent participation for the formation of the difficult cis glycosides. This strategy was eventually successfully employed to prepare the trisaccharide and hexasaccharide by’1+2’and’2+2+2’ glycosylation strategies, respectively. First, monosaccharide building blocks 7,8,9 and 34 were synthesized from D-galactose and L-rhamnose as glycosyl donors and acceptors. Next, they were coupled under proper glycosylation conditions to generate disaccharides 4, 32 and 33. Then, disaccharide donor 32 and acceptor 39 were coupled in the presence of N-iodosuccinimide and a catalytic amount of silver trifluoromethanesulfonate to give fully protected tetrasccharide 41. After deprotection, the resultant tetrasccharide 42 as a glycosyl acceptor was coupled with disaccharide donor 4 through a similar glycosylation reaction to get hexasaccharide 31. Finally debenzylation followed by debenzoylation of compound 31 gave the desired hexasaccharide 2.