Studies towards a Conjugate Vaccine for Anthrax: Synthesis of the Tetrasaccharide Side Chain of the Bacillus anthracis Exosporium

Abstract

The first synthesis of b-L-glycoside 17 of the tetrasaccharide b-Ant-(1 ! 3)-a-L-Rhap-(1 ! 3)-a-LRhap-(1 ! 2)-L-Rhap is described (Schemes 1 -3). Its spacer can be functionalized to make it amenable to conjugation to proteins by different conjugation methods. The synthesis was performed in a stepwise manner starting from the aglycon-bearing terminal saccharide with thioglycosides as glycosyl donors. To attach the upstream terminal anthrose residue, the assembled linker-equipped trisaccharide was glycosylated with ethyl 4-azido-3-O-benzyl-2-O-(bromoacetyl)-4,6-dideoxy-1-thio-b-D-glucopyranoside (11). Further functionalization of the tetrasaccharide thus obtained, followed by deprotection gave the target substance 17. Synthesis of substructures of 17 equipped with the same spacer, namely b- (23), is also described (Scheme 4).Introduction. -Bacillus anthracis, the causative agent of anthrax, is a spore-forming pathogen. One of the components of its exosporium is a collagen-like protein [1] whose carbohydrate portion is formed by the tetrasaccharide Fig.) [2]. Anthrose (= 4,6-dideoxy-4-[(3-hydroxy-3-methyl-1-oxobutyl)amino]-2-O-methyl-D-glucopyranose), which forms the upstream terminal residue of the tetrasaccharide, was first discovered as a component of the exosporium glycoprotein. In connection with our attempts to develop a conjugate vaccine for anthrax, we have recently synthesized this rare sugar, as well as its methyl glycosides [3]. Our approach to the vaccine is based on the premise that spores could be destroyed by antibodies specific for components on their surface. Such antibodies should be possible to elicit by immunization with, for example, immunogens prepared from the above-mentioned tetrasaccharide. Within a project aimed at immunogenic neoglycoconjugates, we have prepared tetrasaccharide glycoside 17, whose aglycone makes it amenable to conjugation with suitable carriers. Here we report full details of the synthesis of 17, together with complete assignment of 1 H-and

Glycan surface antigens fromBacillus anthracisas vaccine targets: current status and future perspectives

“…Syntheses of different series of fragments from the tetrasaccharide structure including oligosaccharides with the terminal end in both b-and a-configuration [64,65], or trisaccharide analogues with variations at position 2 or 4 [66] have also been reported with the aim of identifying minimal structural requirements for the immunogenicity of the tetrasaccharide.…”

Immunogens related to the synthetic tetrasaccharide side chain of the Bacillus anthracis exosporium

“…After concentration, chromatography gave 14 (5.6 g, 86%), mp 73-74°C (from EtOH), [α] ,4.87;N,10.14. Found: C,46.25;H,4.82;N,10.09.…”

Recent Developments in Synthetic Oligosaccharide-Based Bacterial Vaccines

“…became a target for vaccine development: two reports have been published in quick succession towards the synthesis of 43 in spacer-linked forms [80,81].…”

“…The second route used a stepwise approach in which the first unit was linked to the spacer in 1,2-cis (beta) stereochemistry ( 49) followed by sequential elongation with monosaccharide units to afford the tetrasaccharide 48 suitable for bioconjugation after modification of its aglyconic part [81].…”