Synthesis of triantennary blood group I antigens: Neolacto-glycopentadecaosyl ceramide

Application of a Janus aglycon with dual function in benzyl-free synthesis of spacer-armed oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7

Both protective and pre-spacer features of 4-(2-chloroethoxy)phenyl (CEP) aglycon, which belong to the class of Janus aglycons, were engaged in a benzyl-free synthesis of oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7. Introduction of α-1,4-linked L-fucose residue was performed using 3,4-di-O-benzoyl-2-O-triisopropylsilyl-α-L-fucopyranosyl N-phenyltrifluoroacetimidate in excellent stereoselectivity and high yields. The obtained deprotected di-, tri- and tetrasaccharides contain 4-(2-azidoethoxy)phenyl (AEP) spacer aglycon, which allows straightforward preparation of neoglycoconjugates that will be used for the study of the role of lipopolysaccharide of rhizobacterium A. brasilense sp7 in plant-microbe symbiosis. The intermediate protected oligosaccharide building blocks with cleavable CEP/AEP aglycons have a strong potential for further application in the synthesis of more complex oligosaccharides.

Chemo-enzymatic modification of poly-N-acetyllactosamine (LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) based on galactose oxidase treatment

The importance of glycans in biological systems is highlighted by their various functions in physiological and pathological processes. Many glycan epitopes on glycoproteins and glycolipids are based on N-acetyllactosamine units (LacNAc; Galβ1,4GlcNAc) and often present on extended poly-LacNAc glycans ([Galβ1,4GlcNAc](n)). Poly-LacNAc itself has been identified as a binding motif of galectins, an important class of lectins with functions in immune response and tumorigenesis. Therefore, the synthesis of natural and modified poly-LacNAc glycans is of specific interest for binding studies with galectins as well as for studies of their possible therapeutic applications. We present the oxidation by galactose oxidase and subsequent chemical or enzymatic modification of terminal galactose and N-acetylgalactosamine residues of poly-N-acetyllactosamine (poly-LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) by galactose oxidase. Product formation starting from different poly-LacNAc oligomers was characterised and optimised regarding formation of the C6-aldo product. Further modification of the aldehyde containing glycans, either by chemical conversion or enzymatic elongation, was established. Base-catalysed β-elimination, coupling of biotin-hydrazide with subsequent reduction to the corresponding hydrazine linkage, and coupling by reductive amination to an amino-functionalised poly-LacNAc oligomer were performed and the products characterised by LC-MS and NMR analysis. Remarkably, elongation of terminally oxidised poly-LacNAc glycans by β3GlcNAc- and β4Gal-transferase was also successful. In this way, a set of novel, modified poly-LacNAc oligomers containing terminally and/or internally modified galactose residues were obtained, which can be used for binding studies and various other applications.

Synthesis of lacto-N-neotetraose and lacto-N-tetraose using the dimethylmaleoyl group as amino protective group

The disaccharide donor O-[2,3,4,6-tetra-O-acetyl-beta-D- galactopyranosyl)-(1-->4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido - alpha,beta-D-glucopyranosyl] trichloroacetimidate (7) was prepared by reacting O-(2,3,4,6-tetra-O-acetyl- alpha-D-galactopyranosyl) trichloroacetimidate with tert-butyldimethylsilyl 3,6-di-O-benzyl-2-deoxy-2- dimethylmaleoylamido-glucopyranoside to give the corresponding disaccharide 5. Deprotection of the anomeric center and then reaction with trichloroacetonitrile afforded 7. Reaction of 7 with 3'-O-unprotected benzyl (2,4,6-tri-O-benzyl-beta-D-galactopyranosyl)- (1-->4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside (8) as acceptor afforded the desired tetrasaccharide benzyl (2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->4)-(3,6-di-O- benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl)-(1-->3)- (2,4,6- tri-O-benzyl-beta-D-galactopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-beta-D- glucopyranoside. Replacement of the N-dimethylmaleoyl group by the acetyl group, O-debenzylation and finally O-deacetylation gave lacto-N-neotetraose. Similarly, reaction of O-[(2,3,4,6-tetra-O-acetyl-beta- D-galactopyranosyl)-(1-->3)-4,6-O-benzylidene-2-deoxy-2-dimethylmalei mido- alpha,beta-D-glycopyranosyl] trichloroacetimidate as donor with 8 as acceptor afforded the desired tetrasaccharide benzyl (2,3,4,6-tetra-O-acetyl-beta-D- galactopyranosyl)-(1-->3)-(4,6-benzylidene-2-deoxy-2-dimethylmaleimid o- beta-D-glucopyranosyl)-(1-->3)-(2,4,6-tri-O-benzyl-beta-D-galactopyranos yl)- (1-->4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside. Removal of the benzylidene group, replacement of the N-dimethylmaleoyl group by the acetyl group and then O-acetylation afforded tetrasaccharide intermediate 15, which carries only O-benzyl and O-acetyl protective groups. O-Debenzylation and O-deacetylation gave lacto-N-tetraose (1). Additionally, known tertbutyldimethylsilyl (2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->3)-4,6-O-benzylide ne- 2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside was transformed into O-[2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)- (1-->3)-4,6-di-O-acetyl-2-deoxy-2-dimethylmaleimido-alpha,beta-D- glucopyranosyl] trichloroacetimidate as glycosyl donor, to afford with 8 as acceptor the corresponding tetrasaccharide 22, which is transformed into 15, thus giving an alternative approach to 1.

Rationally designed syntheses of high-mannose and complex type undecasaccharides

Synthetic routes are described to a high-mannose type triantenary undecasaccharide 1 and a completely protected form 39 of the complex type biantenary undecasaccharide 2 carrying alpha-(2-->3)-linked sialic acid residues. Starting from a previously reported trisaccharide 4, the core pentasaccharides 15 and 37 were synthesized through regio- and/or stereo-selective mannosylation with a suitably protected mannosyl donor. Chain elongation of 15 by stepwise addition of a mannose residue afforded an undecasaccharide 20 that was eventually deprotected to give 1. On the other hand, coupling of 38 and a trisaccharide glycosyl donor 36 afforded 39.

Enzyme-assisted synthesis of a bivalent high-affinity dodecasaccharide inhibitor of mouse gamete adhesion. The length of the chains carrying distal alpha 1,3-bonded galactose residues is critical

Proposing to study the molecular mechanisms of mouse gamete adhesion with the aid of high affinity adhesion inhibitors of saccharide nature, we report here the enzymatic synthesis of a bivalent oligosaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (4), consisting of two long arms that link together two distal alpha 1,3-galactose residues. Binding data reported elsewhere (E. Litscher et al., Biochemistry, 1995, 34, 4662-4669) show that 4 is a high affinity inhibitor of mouse gamete adhesion in vitro (IC50 = 9 microM), while a related octasaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc, consisting of two short arms is of very low inhibitory activity. The data highlight the importance of the two alpha-galactose residues of 4, and the length of the sugar chains joining them.