CHEMICAL SYNTHESIS AND STRUCTURE OF BACTERIAL POLYSACCHARIDES

这项工作的目的是发展一种固相法

This work has been directed towards the development of a solid phase method for the synthesis of an oligosaccharide related to the capsule of Neisseria meningitidis group A. The long-term aim is to provide a tool for the study several structural aspects of saccharide-protein conjugate antigenicity. Toward this end, a scheme has been devised which allows the synthesis of defined length oligosaccharides bearing, at the reducing end, a linker arm terminating in a thiol that could be used for coupling. The synthesis of the solid phase support and the required reactive monomers has been completed. In the past year synthesis of a protected hexasaccharide has been completed, and several outstanding problems have been studied. To maximize efficiency of synthesis it is desirable that the only solid phase- bound nucleophilic sites be those at the end of a disulfide linked spacer, however in a large number of experiments it has been found that 30-50% of the sites are cleaved by oxidizing reagents. A fraction of the sites cannot be cleaved by reducing reagents. Analytical methods have therefore been developed to study these problems. The chemistry of the solid phase has also been reexamined. Separation of the four closely related diastereomers of the reactive monomer has been completed and they have been characterized by 1H NOE NMR. These studies confirm the assignments made earlier on the basis of chemical shift similarities and confirm the superiority of the benzyl group for protecting the phosphite in this class of compounds. Removal of the benzyl protecting group from the phosphate and the 4-position of the sugar has been difficult. Model compounds have been synthesized and used to test several published deprotection strategies. To date, catalytic hydrogenation, transfer hydrogenation, electrochemical oxidation, indirect electrochemical reduction, free electron reduction, and cleavage with trimethylsilyl iodide have been found to be unusable. The chemistry required to replace the disulfide bond in the linker is being considered.