Synthesis of allyl glycosides for conversion into neoglycoproteins bearing epitopes of mycobacterial glycolipid antigens

Synthesis of the tetrasaccharide outer core fragment of Burkholderia multivorans lipooligosaccharide

The first synthesis of the outer core fragment of Burkholderia multivorans lipooligosaccharide [β-D-Glc-(1→3)-α-D-GalNAc-(1→3)-β-D-GalNAc-(1→3)-L-Rha] as α-allyl tetrasaccharide was accomplished. The glycosylations involving GalNAc units were studied in depth testing them under several conditions. This allowed the building of both the α- and the β-configured glycosidic bonds by employing the same GalNAc glycosyl donor, thus considerably shortening the total number of synthetic steps. The target tetrasaccharide was synthesized with an allyl aglycone to allow its future conjugation with an immunogenic protein en route to the development of a synthetic neoglycoconjugate vaccine against the Burkholderia cepacia pathogens.

Synthetic neoglycoconjugates of cell-surface phosphoglycans of Leishmania as potential anti-parasite carbohydrate vaccines

Leishmania are a genus of sandfly-transmitted protozoan parasites that cause a spectrum of debilitating and often fatal diseases in humans throughout the tropics and subtropics. During the parasite life cycle, Leishmania survive and proliferate in highly hostile environments. Their survival strategies involve the formation of an elaborate and dense cell-surface glycocalyx composed of diverse stage-specific glycoconjugates that form a protective barrier. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan and secreted proteophosphoglycans. In this paper, we discuss structural aspects of various phosphoglycans from Leishmania with the major emphasis on the chemical preparation of neoglycoconjugates (neoglycoproteins and neoglycolipids) based on Leishmania lipophosphoglycan structures as well as the immunological evaluation for some of them as potential anti-leishmaniasis vaccines.

Separation of O- and C-allyl glycoside anomeric mixtures by capillary electrophoresis and high-performance liquid chromatography

Rapid and reliable methods for the analysis of O- and C-allyl galactopyranosides and glucopyranosides are presented, based on capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MEKC). In MEKC, the formation of chromophoric and charged complexes between the saccharides and borate as well as the hydrophobic interactions with micelles jointly contributed to the selective separation and sensitive detection of all the investigated anomeric couples. Some non-purified synthesis mixtures of C-allyl glycosides were successfully characterised without pre-treatment. MEKC buffer conditions for which glycosides separation was successfully achieved were then exported and applied to reverse-phase liquid chromatography (RP-HPLC), for the quantitative isolation of each allyl glycoside anomer. Identification of the obtained anomeric products was performed by electrospray mass spectrometry and (13)C NMR spectroscopy. Glycoside-solvent interactions driving the selective anomeric separation were shortly addressed and discussed on the basis of sugar derivatives structural differences.

The preparation of neoglycoconjugates containing inter-saccharide phosphodiester linkages as potential anti-Leishmania vaccines

The Leishmania express complex glycoconjugates containing phosphosaccharide repeat units at all stages of their life-cycle. One of these molecules, lipophosphoglycan (LPG) has been suggested to be a vaccine candidate. To assess the immunological properties of Leishmania phosphosaccharides, we have prepared neoglycoproteins and neoglycolipids containing synthetic Leishmania phosphosaccharide repeats. The coupling procedure uses the dec-9-enyl spacer of previously synthesised phosphosaccharides for linkage to protein and phospholipid. This alkene moiety is converted by ozonolysis to an aldehyde which is then attached to protein and phospholipid amino groups by reductive amination. The procedure produces neoglycoconjugates in good yield and without compromising the labile phosphodiester linkages within the phosphosaccharide chains.

Total Synthesis of Tricolorin A

Tricolorin A (1) is a novel tetrasaccharide macrolactone that is a natural herbicide. In this paper is reported a total synthesis of 1. Coupling of hydroxy ester 18 with D-fucosyl trichloroacetimidate 23 gave fucoside 24. Removal of the C-2 pivaloyl group of 24 followed by coupling with D-glucosyl trichloroacetimidate 29 resulted in isolation of disaccharide 30. Saponification of the ester groups of 30 and subsequent selective macrolactonization of the acid diol 31 by the Yonemitsu protocol gave only the desired lactone 32. The key step in the assembly of disaccharide glycosyl trichloroacetimidate 52 was coupling L-rhamnoside 47 with L-rhamnosyl trichloroacetimidate 43. Attempts to couple lactone disaccharide 32 with disaccharide 52 were unsuccessful. Using an alternate plan for assembly of the tetrasaccharide, reaction of disaccharide glycosyl trichloroacetimidate 58 with disaccharide 37 gave tetrasaccharide 59. Diester lactone 63 was generated by selective macrolactonization of tetrasaccharide acid triol 60, again using the Yonemitsu protocol, followed by addition of the chiral side chain acid to the reaction vessel. Synthetic tricolorin A (1) was obtained by deprotection of 63. Starting from fucose, glucose, rhamnose, and (S)-1-octyn-3-ol, the synthesis required 39 steps overall. The longest linear sequence was 14 steps, with an overall yield for this longest linear sequence of 6%.

Synthesis of 5-aminopentyl mono- to tri-saccharide haptens related to the species-specific glycopeptidolipids of Mycobacterium avium-intracellulare serovars 8 and 21

The preparation of pyruvate acetal-containing 5-aminopentyl mono-, di-, and tri-saccharide fragments related to serovars 8 and 21 of the species-specific glycopeptidolipid of Mycobacterium avium-intracellulare is described. The saccharides were constructed by sequential coupling of the suitably protected 4,6-O-[(S)-1-methoxycarbonylethylidene]-D-glucopyranosyl trichloroacetimidates 6 and 7 to Z-protected 5-aminopentanol, to give the serovar 21 monosaccharide fragment 9 upon deblocking; and to ethyl 2-O-benzoyl-4-O-benzyl-1-thio-alpha-L-rhamnopyranoside (18), to give the corresponding ethyl 1-thio-disaccharides 21 and 23, respectively. Subsequent N-iodosuccinimide-promoted coupling of the latter with Z-protected 5-aminopentanol followed by deblocking of the products afforded the corresponding disaccharide fragments 26 (serovar 8) and 27 (serovar 21), respectively. Condensation of 21 with Z-protected 5-aminopentyl 3,4-di-O-benzyl-6-deoxy-alpha-L-talopyranoside (28) and subsequent deblocking of the resulting trisaccharide gave the serovar 8 fragment 5-aminopentyl O-(4,6-O-[(S)-1-carboxyethylidene]-3-O-methyl-beta-D-glucopyranosyl)- (1-->3)-O-alpha-L-rhamnopyranosyl-(1-->2)-6-deoxy-alpha-L-talopyranos ide (30).

Synthesis of p-trifluoroacetamidophenyl 6-deoxy-2-O-(3-O-[2-O-methyl-3-O- (2-O-methyl-alpha-D-rhamnopyranosyl)-alpha-L-fucopyranosyl]-alpha-L- rhamnopyranosyl)-alpha-L-talopyranoside: a spacer armed tetrasaccharide glycopeptidolipid antigen of Mycobacterium avium serovar 20

The synthesis of the title tetrasaccharide glycoside 38 is reported. p-Nitrophenyl endo-3,4-O-benzylidene-6-deoxy-alpha-L-talopyranoside (4), 3-O-acetyl-2,4-di-O-benzyl-alpha-L-rhamnopyranosyl trichloroacetimidate (7), methyl 3-O-acetyl-4-O-benzyl-2-O-methyl-1-thio-beta-L-fucopyranoside (15), 3-O-acetyl-4-O-benzyl-2-O-methyl-alpha-L-fucopyranosyl bromide (16), and ethyl 3-O-acetyl-4-O-benzyl-2-O-methyl-1-thio-alpha-D-rhamnopyranoside (33) were prepared as intermediates. Compound 4 was glycosylated with imidate 7 as well as with methyl 3-O-acetyl-2,4-di-O-benzyl-1-thio-alpha-L-rhamnopyranoside (9), affording the same disaccharide derivative 8. Deacetylation of 8 gave crystalline 17. Condensation of 17 with both fucosyl donors 15 and 16 yielded the same trisaccharide derivative 18 stereoselectively. Compound 18 was also prepared by the coupling of 4 with disaccharide glycosyl donor 20. After deacetylation of 18 (-->34), methyl triflate-promoted glycosylation with compound 33 resulted in tetrasaccharide 35. Conversion of the p-nitrophenyl group of 35 into the p-trifluoroacetamidophenyl group (-->36) and removal of the protecting groups gave the title tetrasaccharide glycoside 38.

Structures of the glycopeptidolipid antigens of serovars 25 and 26 of the Mycobacterium avium serocomplex, synthesis of allyl glycosides of the outer disaccharide units and serology of the derived neoglycoproteins

The pentasaccharide hapten released from the glycopeptidolipid (GPL) antigen of M. avium serovar 26 has been characterized as O-(2,4-di-O-methyl-alpha-L-fucopyranosyl)-(1-->4)- O-beta-D-glucopyranosyluronic acid-(1-->4)-O-(2-O-methyl-alpha-L-fucopyranosyl)-(1-->3)-alpha-L- rhamnopyranosyl-(1-->2)-6-deoxy-L-talose. The allyl glycosides of the outer glycosyl and glycobiosyl units of this hapten have been synthesized, the latter by a route involving oxidation of the corresponding D-glucopyranose derivative. Conjugation of allyl glycosides to protein by ozonolysis and reductive coupling afforded neoantigens (neo 26-1 and 26-2), both of which interacted with antibodies to M. avium serovar 26. The terminal sugar residue of the pentasaccharide hapten of the serovar 25 GPL had been shown to have the galacto configuration on the basis of 1H-13C NMR correlation spectroscopy, but absolute configurational assignment for the sugar awaited the synthesis, as for neo 26, of two glycobiosyl NGPs bearing the terminal sugar in the D and L enantiomeric forms, respectively. Only the glycobiosyl NGP bearing the terminal sugar as the D-enantiomer interacted with antibodies to M. avium serovar 25, thus providing evidence for the absolute configuration of the sugar, and showing that the complete oligosaccharide hapten has the structure, O-(4-acetamido-4,6-dideoxy-2-O-methyl-alpha-D- galactopyranosyl)-(1-->4)-O-beta-D-glucopyranosyluronic acid-(1-->4)-O-(2-O-methyl-alpha-L-fucopyranosyl)-(1-->3)-O-alpha-L- rhamnopyranosyl-(1-->2)-6-deoxy-L-talose.

Structure of the glycopeptidolipid antigen of serovar 20 of the Mycobacterium avium serocomplex, synthesis of allyl glycosides of the outer di- and tri-saccharide units of the antigens of serovars 14 and 20, and serology of the derived neoglycoproteins

The tetrasaccharide hapten released from the glycopeptidolipid (GPL) antigen of Mycobacterium avium serovar 20 has been characterized as O-(2-O-methyl-alpha-D-rhamnopyranosyl)-(1----3)-O-(2-O-methyl-alpha-L- fucopyranosyl)-(1----3)-alpha-L-rhamnopyranosyl-(1----2)-6-deoxy-L-talos e. Syntheses are reported of allyl glycosides of the outer disaccharide unit of this hapten, O-(2-O-methyl-alpha-D-rhamnopyranosyl)-(1----3)-2-O-methyl-alpha-L-fu copyranose , and also of the outer di- and tri-saccharide units of the GPL antigen of M. avium serovar 14, O-(N-formyl-alpha-L-kansosaminyl)-(1----3)-2-O-methyl-alpha-D-rham nopyranose and O-(N-formyl-alpha-L-kansosaminyl)-(1----3)-O-(2-O-methyl-alpha-D- rhamnopyranosyl)-(1----3)-2-O-methyl-alpha-L-fucopyranose. The key steps in the latter synthesis involve the preparation of allyl 4-azido-4,6-dideoxy-3-C-methyl-2-O-methyl-alpha-L-mannopyranoside as a precursor for the N-formylkansosamine unit, followed sequentially by conversion into and use of a trichloroacetimidate as glycosyl donor for di- and tri-saccharide formation, O-deacylation, reduction, and N-formylation. The allyl glycosides, representative of the haptens from both serovars, have been converted into neoglycoproteins (NGPs) and their serological activities have been compared in the light of the structural relationship between them.