Synthesis of ethyl 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-beta-D- galactofuranoside present in motif E of the Mycobacterium tuberculosis cell wall

(13)C-NMR glycosylation effects in (1→3)-linked furanosyl-pyranosides

Synthesis, theoretical conformational analysis (molecular mechanics and DFT calculations) and NMR spectral data including the (13)C-NMR glycosylation effects for six pairs of isomeric furanosyl-(1→3)-pyranosides with different anomeric and absolute configurations of furanosyl units as well as configurations of C2 and C4 in the pyranoside units are described. The determined (13)C-NMR glycosylation effects were shown to correlate with the pattern of intramolecular interactions around the inter-unit bonds.

Facile synthesis of α-D-Araf-(1→5)-D-Galf, the linker unit of the arabinan to the galactan in Mycobacterium tuberculosis

The arabinogalactan is a crucial constituent of the cell wall of mycobacteria. Both monosaccharides (arabinose and galactose) are found in the furanose configuration, absent in mammals. An efficient synthesis of α-D-Araf-(1→5)-D-Galf, the linker unit of the arabinan to the galactan, is described. The strategy relies on the use of a conveniently substituted D-galactono-1,4-lactone as a precursor of the reducing furanose ring. The glycosylation step was performed by the tin(IV) chloride promoted method using 1,2,3,5-tetra-O-benzoyl-α,β-D-arabinofuranose. The arabinose donor was obtained in a crystalline state in one step by benzoylation of arabinose in hot pyridine. Selective glycosylation of the exocyclic OH-5 was obtained in 75% yield to give 2,3,5-tri-O-benzoyl-α-D-arabinofuranosyl-(1→5)-2,6-di-O-pivaloyl-D-galactono-1,4-lactone. Reduction with disiamylborane gave the disaccharide synthon, useful for further glycosylations. Dec-9-enyl α-D-Araf-(1→5)-β-D-Galf, a convenient substrate for arabinofuranosyl transferases studies, was obtained by the trichloroacetimidate method of glycosylation.Key words: arabinofuranose, galactofuranose, Mycobacterium arabinogalactan, trichloroacetimidate, tin(IV) chloride.

Analogues of the mycobacterial arabinogalactan linkage disaccharide as cell wall biosynthesis inhibitors

The mycobacterial arabinogalactan linkage disaccharide [alpha-L-Rha-(1-->3)-alpha-D-GlcNAc] provides a basis for the design of new antitubercular drugs, since it supports a key skeletal structure in the bacterial cell wall. A series of analogues of the linker was synthesized by coupling appropriate thiorhamnosyl donors modified at their 4-positions, with an N-acetyl glucosamine acceptor. In a cell-free enzyme inhibition assay, three analogues inhibited the activity of the galactosyltransferase that adds a Galf residue to the linkage disaccharide. Although the compounds were modest inhibitors, these data confirm the viability of this approach to anti-mycobacterial agents. It is especially significant that the three effective compounds are modified at the site of the acceptor atom in the natural substrate.

A one-pot strategy for synthesis of 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-D-galactofuranose present in motif E of the Mycobacterium tuberculosis cell wall

5-O-(Alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-D-galactofuranose 6 present in motif E of the Macobacterium tuberculosis cell wall has been regio- and stereospecifically synthesized using 3-O-benzoyl-1,2-O-isopropylidine-alpha-D-galactofuranose (10) as the glycosyl acceptor by the trichloroacetamidate method in a one-pot manner. The diol glycosyl acceptor 10 was smoothly derived from 1,2:5,6-di-O-isopropylidene-alpha-D-galactofuranose (8) by 3-O-benzoylation and then selective 5,6-O-deacetonation. The preparation of 8 was greatly improved by increasing the ratio of DMF to acetone and using a solid-supported catalyst.

Synthesis of oligosaccharides of motifs D and E of arabinogalactan present in Mycobacterium tuberculosis

Syntheses of the ethyl glycosides of 5-O-(beta-D-galactofuranosyl)-beta-D-galactofuranose and 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-beta-D-galactofuranose present in motifs D and E of Mycobacterium tuberculosis arabinogalactan, respectively, have been presented. The pentenyl-mediated O-glycosylation reaction was utilized to obtain the disaccharide of motif D. The first coupling reaction to prepare the inner disaccharide portion of motif E was accomplished by trichloroacetamidate method while the installation of the terminal sugar by pentenyl glycosylation approach was successful.