Eva Eichler

The design and synthesis of antibody binding site probes: three pentasaccharide analogues of the Brucella A antigen prepared by activation in situ of thioglycosides with bromine☆

Three pentasaccharide analogues of the Brucella A antigen [----2)-alpha-D-Rhap4NFo-(1----], each with one formamido group replaced by a hydroxyl group, have been prepared as their methyl glycosides. Mono- and di-saccharide thioglycosides of D-rhamnose and 4-azido-4,6-dideoxy-D-mannose were used as glycosyl donors for the preparation of protected pentasaccharide derivatives with trisaccharides as intermediates. Glycosylations were performed by activation in situ of the thioglycosides with bromine in the presence of a glycosyl acceptor and silver triflate as promoter. Reduction of the azido groups with hydrogen sulfide. N-formylation with ethyl formate, and hydrogenolysis then gave the target pentasaccharides.

1-Methyl 1′-cyclopropylmethyl: an acid labile O-protecting group for polymer-supported oligosaccharide synthesis

Abstract R , S -1-Methyl 1′-cyclopropylmethanol can be converted to its trichloroacetimidate derivative. Lewis acid catalyzed etherification can then be used to prepare methyl cyclopropylmethyl (MCPM) ethers. Thus, ethyl 2,3,4-tri- O -benzyl-6- O -( R , S -1-methyl 1′-cyclopropylmethyl)-1-thio-α/β- d -glucopyranoside was prepared and the linker polymer combination (MPEG)(DOX)OH glycosylated. The MCPM group was cleaved with 10% trifluoroacetic acid in CH 2 Cl 2 and the resulting alcohol glycosylated. After protecting group manipulations and cleavage the peracetylated disaccharide GlcNAc(β1→6)GlcOAc was isolated. Similarly the 4,6- O -phenylboronate diester of ethyl 1-thio-β- d -galactopyranoside was regioselectively etherified at O-3 and after boronate cleavage the sugar benzoylated to give ethyl 2,4,6-tri- O -benzoyl-3- O -( R , S -1-methyl 1′-cyclopropylmethyl)-1-thio-β- d -galactopyranoside. Through a similar sequence of glycosylation of (MPEG)(DOX)OH, MCPM deprotection, glycosylation, functional group manipulation and cleavage the peracylated disaccharide GlcNAc(β1→3)Gal(β1→)DOXOAc was prepared. Finally, the donor 2,6-di- O -benzoyl-4- O -levulinoyl-3- O -( R , S -1-methyl 1′-cyclopropylmethyl)-1-thio-β- d -galactopyranoside was prepared and elaborated into the trisaccharide GlcNAc(β1→3)[Glc(β1→4)]Gal(β1→)DOXOH. This trisaccharide was elaborated into the pentasaccharide Neu5Ac(α2→3)Gal(β1→4)GlcNAc(β1→3) [Glc(β1→4)]Gal(β1→)DOXOH using GalE-LgtB fusion and CMP-Neu5Ac synthetase/sialyltransferase fusion enzymes. This pentasaccharide is a single repeat unit of the capsular polysaccharide of Group B Streptococcus type 1A.

Synthesis of archaeal glycolipid adjuvants—what is the optimum number of sugars?

As part of a programme to optimize the use of archaeal-lipid liposomes (archaeosomes) as vaccine adjuvants, we present the synthesis and immunological testing of an oligomeric series of mannose glycolipids (Manp(1-5)). To generate the parent archaeol alcohol precursor, the polar lipids extracted from the archaeon Halobacterium salinarum were hydrolyzed to remove polar head groups, and the archaeol so generated partitioned into diethyl ether. This alcohol was then iteratively glycosylated with the donor 2-O-acetyl-3,4,6-tri-O-benzyl-alpha/beta-d-mannopyranosyl trichloroacetimidate to yield alpha-Manp-(1-->2) oligomers. A starch-derived trimer was also synthesized as a control. To promote hydration and form stable archaeosomes, an archaeal anionic lipid archaetidylglycerol (AG) was included in a 4:1 molar ratio. Archaeosomes prepared from Manp(1-2)-AG were recovered at only 34-37%, whereas Manp(3-4)-AG recoveries were 72-77%. Lipid recovery following hydration of Manp(5)-AG archaeosomes declined to 34%, indicating an optimum of 3-4 Manp units for bilayer formation. The CD8(+) T cell response in mice immunized with Manp(3-5) archaeosomes containing ovalbumin was highest for Manp(4) and declined for Manp(3) and Manp(5), revealing an optimum length of four unbranched units. The starch-derived trimer was more active than the Manp oligomers, suggesting the involvement of either a general binding lectin on antigen-presenting cells with highest affinity for triglucose or multiple lectin receptors.

Access to fluorescent probes via allyl glycosides : the synthesis of a Brucella trisaccharide epitope linked to a coumarin

Oligosaccharide allyl glycosides are demonstrated to provide a route to fluorescent probes and simple inhibitors. Ethyl 2-O-acetyl-4-azido-3-O-benzoyl-4,6-dideoxy-1-thio-α-d-mannopyranoside (6) was used as glycosyl donor in the preparation of the trisaccharide [α-d-Rhap4NFo(1 → 2)-]2-α-d-Rhap4NFo-O-allyl (16). Thioglycoside6 was activated withN-iodosuccinimide and triflic acid or by bromine in the glycosylations and the inhibitor16 was obtained after deprotection by transesterification, reduction of the azido groups with hydrogen sulfide, andN-formylation with ethyl formate. Ozonolysis of the allyl glycoside in16 and reductive amination with 7-amino-4-methylcoumarin then gave the target fluorescent trisaccharide conjugate.

Synthesis of a disialylated hexasaccharide of type VIII group B Streptococcus capsular polysaccharide.

As part of our program to design, develop and prepare protective vaccines against the bacterial pathogens Group B Streptococcus, we report the synthesis of a disialylated hexasaccharide. This hexasaccharide represents a portion of the serotype-specific capsular polysaccharide of Type VIII that has the tetrasaccharide repeat unit [beta-L-Rhap-(1-->4)-beta-D-Glcp-(1-->4)-[alpha-Neu5Ac-(2--> 3)]-beta-D- Galp-(1-->4)]n. A tetrasaccharide corresponding to this repeat unit has been synthesized by us [E. Eichler, H.J. Jennings, D.M. Whitfield, J. Carbohydr. Chem., 16 (1997) 385-411]. Since the protective epitopes are believed to involve several repeat units, methods to extend this tetrasaccharide were examined. This objective requires a glycosylation of the unreactive OH-4 of the beta-L-Rhap, which was accomplished by coupling a D-Galp glycosyl trichloroacetimidate donor with a beta-L-Rhap-(1-->4)-D-Glcp acceptor. Subsequent coupling of this trisaccharide as a donor to an alpha-Neu5Ac-(2-->3)-D-Galp disaccharide acceptor gave a pentasaccharide. The pentasaccharide was deprotected and enzymatically sialylated using an alpha-(2-->3)-sialyltransferase from Campylobacter jejuni to give the title hexasaccahride alpha-Neu5Ac-(2-->3)- beta-D-Galp-(1-->4)-beta-L-Rhap-(1-->4)-beta-D-Glcp-(1-->4)-[alpha -Neu5Ac- (2-->3)]-beta-D-Galp-(1-->O)-(CH2)3N3.

Modulation of antibody affinity by synthetic modifications of the most exposed pyranose residue of a trisaccharide epitope

When the Salmonella trisaccharide epitope, methyl 3-O-(3,6-dideoxy-alpha-D-xylo-hexopyranosyl)-2-O-(alpha-D-galactopyra nos yl)- alpha-D-mannopyranoside 12 is bound by a monoclonal antibody Se155.4, the 3,6-dideoxy-alpha-D-hexose is completely buried, while the galactopyranosyl mannopyranosyl units lay across the protein surface. Crystallography of an antibody complexed with 12 also shows that the galactose residue is the most exposed saccharide. A simplified strategy to synthesize 12 and analogues modified at the galactose residue is described. Monosaccharide building blocks containing benzyl ether and ester protecting groups were used for efficient assembly of trisaccharides that can be deprotected by a single hydrogenolysis step, or occasionally preceded by a transesterification stage. Glycosylation of methyl 2-O-benzoyl-4,6-di-O-benzyl-alpha-D- mannopyranoside 4 by 2,4-di-O-benzyl-3,6-dideoxy-D-xylo-hexopyranosyl chloride 8 affords after transesterification the disaccharide acceptor 10. This disaccharide serves as a universal acceptor for glycosylation by glycosyl donors that lead, following facile deprotection, to the alpha- and beta-D-galacto, alpha- and beta-D-gluco, and 2-amino-2-deoxy-alpha-D-galacto trisaccharides 12, 14, 17, 18 and 21. Only a small change in binding energy delta (delta G) occurs when the alpha-D-galactopyranosyl residue of 12 is replaced by either an alpha-D-glucopyranosyl 17 or a 2-amino-2-deoxy alpha-D-galactopyranosyl unit 21. Whereas binding of the beta-D-glucopyranose congener 18 was tolerated by the antibody, the beta-D-galactopyranose analogue 14, showed a 250 fold loss of affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of a Single Repeat Unit of Type VIII Group B Streptococcus Capsular Polysaccharide 1

Abstract We have synthesized a single repeat unit of type VIII Group B Streptococcus capsular polysaccharide, the structure of which is {L-Rhap(β1→4)-D-Glcp(β1→4)[Neu5Ac(α2→3)]-D-Galp(β→4)}n. The synthesis presented three significant synthetic challenges namely: the L-Rhap(β→4)-D-Glcp bond, the Neu5Ac(α2→3)-D-Galp bond and 3,4-D-Galp branching. The L-Rhap bond was constructed in 60% yield (α:β 1:1.2) using 4-O-acetyl-2,3-di-O-benzoyl-α-L-rhamnopyranosyl bromide 6 as donor, silver silicate as promotor and 6-O-benzyl-2,3-di-O-benzoyl-1-thio-β-D-glucopyranoside as acceptor to yield disaccharide 18. The Neu5Ac(α2→3) linkage was synthesized in 66% yield using methyl [phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-nonulopyranosid]onate as donor and triol 2-(trimethylsilyl) ethyl 6-O-benzyl-β-D-galactopyranoside as acceptor to give disaccharide 21. The 3,4-D-Galp branching was achieved by regioselective glycosylation of disaccharide diol 21 by disaccharide 18 in 28% yield to giv...