Attribution des signaux de résonance magnétique nucléaire-13c de disaccharides peracétylés dans la série du D-glucose

Lipo-oligosaccharidic antigen from Mycobacterium gastri. Complete structure of a novel C4-branched 3,6-dideoxy-alpha-xylo-hexopyranose

The following incomplete structure, alpha-X-(1-->3)-[beta-L-Xylp-(1-->4)]6-3-O-Me-alpha-Rhap-(1- ->3)- beta-D-Galp-(1-->3)-beta-D-Glcp-(1-->4)-2-O-acyl-alpha-D-Glcp-(1<-->1)-4 ,6-di-O-acyl-alpha-D-Glcp, was previously established for the antigenic lipo-oligosaccharide typifying Mycobacterium gastri, namely, LOS-III. The partial structure of the distal monosaccharide (X) was assigned as 3,6-dideoxy-4-C-(1,3-di-O-methylpropyl)-alpha-hexopyranose, which corresponds to a new-found monosaccharide in nature [Gilleron M., Vercauteren J., & Puzo G. (1993) J. Biol. Chem. 268, 3168-3179]. This article reports the complete structure of X, which was determined from the FAB-MS and 2D NMR analysis of the peracetylated LOS-III. The comparative analyses of the native and per-O-acetylated LOS-III FAB-MS spectra revealed, for the monosaccharide X, a molecular mass of 370 Da and five hydroxyl groups that could be acetylated. Additionally, the 1D 1H NMR spectrum of the per-O-acetylated LOS-III showed a dramatically increased dispersion of the protons, which resonated between 3 and 4 ppm in the spectrum of the underivatized LOS-III. Thus, thanks to 2D NMR sequences (COSY, HOHAHA, HMQC, HMQC-HOHAHA, and HMBC), the complete assignment of the 1H and 13C signals was achieved. Starting from the quaternary C4 resonance, the spin system of the C-alkyl chain was assigned, allowing us to propose the following structure, 3,6-dideoxy-4-C-(1,3-dimethoxy-4,5,6,7-tetrahydroxyheptyl)-alpha-x ylo- hexopyranose. The xylo configuration was established from the ROESY spectrum.

Chemical synthesis and NMR spectra of a protected branched-tetrasaccharide thioglycoside, a useful intermediate for the synthesis of branched oligosaccharides

Acid-catalyzed thiophenolysis of per-O-acetylated 1,6-anhydromaltose (3) gave phenyl 2,3-di-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-alpha-D- glucopyranosyl)-1-thio-beta-D-glucopyranoside (4) in quantitative yield. Phenyl 4-O-alpha-D-glucopyranosyl-1-thio-beta-D-glucopyranoside (5) was obtained by acid-catalyzed thiophenolysis of maltose octaacetate (2), using trimethylsilyl triflate as catalyst, and subsequent deacetylation. Standard benzylation of 5 gave phenyl 2,3-di-O-benzyl-4-O- (2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-1-thio-beta-D-glucopy ran oside (6) which upon treatment with N-bromosuccinimide in aqueous acetone gave 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D- glucopyranosyl)-D-glucopyranose (8). Compound 8 was treated with trichloroacetonitrile in the presence of anhydrous potassium carbonate to give 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl- alpha-D-glucopyranosyl) -alpha,beta-D-glucopyranosyl trichloroacetimidate (9), which was effectively used as the glycosyl donor in the condensation reaction with compound 4, using trimethylsilyl triflate as catalyst, to obtain the branched tetrasaccharides phenyl O-[2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)- (1-->4)]-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->6)-O-(2,3,4, 6- tetra-O-acetyl-alpha-D-glucopyranosyl)-(1-->4)-2,3-di-O-acetyl-1-thio-be ta-D- glucopyranoside (10) and phenyl O-[(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)]- O-(2,3,4-tri-O-benzyl-beta-D-glucopyranosyl)-(1-->6)-O-(2,3,4,6-tetra-O- acetyl- alpha-D-glucopyranosyl)-(1-->4)-2,3-di-O-acetyl-1-thio-beta-D-glucopy ran oside (11) in 67 and 21% yield, respectively. A complete NMR interpretation of 10 is presented. Alternative methodologies for the synthesis of the branched tetrasaccharides were investigated. Chemical synthesis of the phenyl thioglycoside 5 was achieved by deacetylation of 4. Reaction of 6 with diethylaminosulfur trifluoride in the presence of N-bromosuccinimide gave 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D- glucopyranosyl)-alpha,beta-D-glucopyranosyl fluoride (7) in 78% yield. Subsequent condensation of 7 and 4, using the combination silver perchlorate-stannous chloride as catalyst, gave the corresponding branched tetrasaccharides 10 and 11 in 55 and 10% yield, respectively.

The structure of an antigenic glycolipid (SL-IV) from Mycobacterium tuberculosis

The structure of an antigenic glycolipid isolated recently from cell walls of various strains of Mycobacterium tuberculosis and believed to be a sulfolipid consisting chiefly of 2,3-di-O-(hexadecanoyl/octadecanoyl)-alpha,alpha-trehalose 2'-sulfate (designated as SL-IV), was reinvestigated by mass spectrometry and nuclear magnetic resonance spectroscopy. The material proved to be a complex mixture of closely related components which are indeed 2,3-di-O-acyltrehaloses. However, no sulfate group was found in the antigen, and revision of its designation as a sulfolipid is therefore required. The lipid substituents comprise an estimated 20% of C14-C19 fatty acids, including tetradecanoic (myristic), 9-tetradecenoic (myristoleic), 9-hexadecenoic (palmitoleic), 9-octadecenoic (oleic), 10-methylhexadecanoic (tuberculopalmitic), and 10-methyloctadecanoic (tuberculostearic) acids in addition to hexadecanoic (palmitic) and octadecanoic (stearic) acids, and approximately 80% of higher, methyl-branched acids. Among the latter are, principally, 2,4-dimethyldocosanoic and 3-hydroxy-2,4,6- trimethyltetracosanoic acids, and a smaller proportion of 2,4,6-trimethyltetracos-2-enoic acid. Numerous further acids, related to those mentioned by methylene homology, are also present in small proportions.

Selective protonic activation of isomeric glycosylfructoses with pyridinium poly(hydrogen fluoride) and synthesis of spirodioxanyl oligosaccharides

Selective activation of the ketose unit in the isomeric glycosylfructoses, palatinose, leucrose, maltulose, turanose and lactulose, with pyridinium poly(hydrogen fluoride) resulted in the almost quantitative formation of glycosylated difructose dianhydrides. The reaction preferentially involves a reactive fructofuranosyl oxocarbenium ion and is subject to stereoelectronic control. The relative amounts of isomeric spirodioxanyl oligosaccharides obtained within a series was shown to depend on the reaction conditions, especially on the hydrogen fluoride-pyridine ratio. Using suitable concentrations of hydrogen fluoride in pyridine, the reaction was easily directed to the formation of the kinetic difuranosyl or thermodynamic pyranosyl derivatives. More rigorous conditions resulted in the specific hydrolysis of one glycosidic bond in the tetrasaccharides derived from palatinose, leucrose and turanose, to yield spirodioxanyl trisaccharides.

Polyphthienoyl trehalose, glycolipids specific for virulent strains of the tubercle bacillus

Phthienoic acids constitute a family of dextro-rotary odd-numbered unsaturated fatty acids isolated exclusively from virulent strains of human and bovine tubercle bacilli. In the bacterial cell they are not free and a search for their linked form in complex wall lipids of Mycobacterium tuberculosis (strain Canetti) showed that they esterified trehalose. Structural elucidation of the major phthienoyl trehalose showed the occurrence of five acyl residues located at 2, 2', 3', 4 and 6' positions of trehalose. The acyl substituents were mainly 2,4,6-trimethyl tetracos-2-enoic acid (C27 phthienoic acid) accompanied by its homologs. In addition to these branched fatty acids, straight-chain C16 and C18 acyls composed about 20% of the substituents. The proposed structure is a new one, both for the mycobacterial-specific glycolipid and for the substituted positions on trehalose. Other minor acyl trehaloses were detected in M. tuberculosis (strain Canetti), differing from the major component by the occurrence of an additional hydroxy fatty acid (3-hydroxy-2,4,6-trimethyl tetracosanoic acid) or by the number of acyl substituents. The major glycolipid presented a weak activity in vitro on mitochondrial oxidative phosphorylation. These glycolipids and phthienoic acids could serve as virulence indicators.

The assignment of carbonyl resonances in 13C-n.m.r. spectra of peracetylated mono- and oligo-saccharides containing D-glucose and D-mannose: an alternative method for structural determination of complex carbohydrates

In the present study, proton homonuclear (COSY) and 13C-1H heteronuclear shift-correlation, n.m.r. spectroscopies have been used to assign the carbonyl carbon resonances of peracetylated D-gluco- and D-mannopyranose monosaccharides and oligosaccharides containing residues of parent D-glucopyranose monomers. Chemical shifts of these assigned resonances, particularly those arising from acetyl groups near to aglycon substitution sites, were found to be sensitive to the position and configuration of glycosidic linkages present. In addition, evidence is presented that indicates that the shifts of these carbonyl carbon resonances depend on long-range interactions with other peracetylated pyranose monomers resulting from folding of the oligosaccharide chain. These results suggest that carbonyl carbon resonances of peracetylated carbohydrates may be useful probes of oligosaccharide structure.

[13C and 1H NMR of (1 goes to 6)-beta-D-glucan, linear oligosaccharides and their corresponding cyclic forms]

The results of 1H-n.m.r. and 13C-n.m.r. studies of linear and cyclic oligosaccharides in the series of gentiodextrins, both in their hydroxylated and acetylated form, were compared to those obtained for the corresponding natural or synthetic polysaccharide. The 13C-signals of each D-glucopyranose unit of acetylated oligosaccharides are more distinct than those of the parent hydroxylated compounds. In order to relate the change of the various signals with the degree of polymerization, gentiotriose undecaacetate, enriched in 13C at C-1'', was prepared, as well as gentiotetraose tetradecaacetate selectively labeled at C-1'' and C-1'''. A (1 leads to 6)-beta-D-glucan having a D.P. of approximately 10 was chemically prepared. During the course of the polycondensation, the 2,3,4,2',3',4'-hexa-O-acetyl-di-beta-D-glucopyranosyl 1,6':6,1'-dianhydride, and the 2,3,4,2',3',4',2'',3'',4'',2''',3''',4'''-dodeca-O-acetyl-tetra-beta-D-glucopyranosyl 1,6''':6,1'''-tetraanhydride, respectively, were formed.