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 polysaccharide and oligosaccharide-protein conjugate antigenicity. A scheme was devised which allows the synthesis of oligosaccharides of defined length. Since the polysaccharide is a repeat of a(1-6)N-acetylmannosamine phosphodiesters, solid phase phosphoramidite chemistry was adapted. During the previous reporting period a fully protected hexasaccharide was completed and several problems were encountered in the cleavage of the disulfied at the solid phase and removal of the benzyl protecting groups. These problems have been solved. Triphenylphosphine very efficiently cleaves all disulfide linkage sites. Iodine which is used in oligonucleotide synthesis to oxidize phosphite to phosphate was not effective in the current oligosaccharide scheme as it resulted in cleavage of the oligosaccharide from the solid phase. Tert-butyl peroxide and hydrogen are mild and were found to be the preferred reagents. Removal of the benzyl protecting group was difficult. Fresh palladium black in ethanol using cyclohexene as hydrogen donor worked well. Thus, now in principle the entire scheme could be completed to yield deprotected oligosaccharides of desired structure. We would like to execute this scheme to produce oligosaccharides for studying oligosaccharide interactions with antibodies to meningococcal group A vaccines.
