Bausteine von Oligosacchariden, LVIII. Regioselektive Glycosylierung an 3′-OH oder 4′-OH der Lactose durch Einsatz unterschiedlicher Katalysator-Systeme

Controlling the stereoselectivity of glycosylation via solvent effects

This review covers a special topic in carbohydrate chemistry — solvent effects on the stereoselectivity of glycosylation reactions. Obtaining highly stereoselective glycosidic linkages is one of the most challenging tasks in organic synthesis, as it is affected by various controlling factors. One of the least understood factors is the effect of solvents. We have described the known solvent effects while providing both general rules and specific examples. We hope this review will not only help fellow researchers understand the known aspects of solvent effects and use that in their experiments, but moreover, we expect that more studies on this topic will be started and continued to expand our understanding of the mechanistic aspects of solvent effects in glycosylation reactions.

Lactotriaose-containing carbosilane dendrimers: Syntheses and lectin-binding activities

Carbosilane dendrimers periphery-functionalized with lactotriaose (GlcNAcbeta1-3Galbeta1-4Glc) with valencies of three, four, six, and twelve were prepared for use in a lectin-binding assay. A lactotriaose derivative was prepared from D-glucosamine and D-lactose derivatives. The N-Troc-protected glucosamine glycosyl donor and 3'-O-unprotected lactose glycosyl acceptor were condensed in the presence of silver trifluoromethanesulfonate and methylsulfenyl bromide to provide corresponding trisaccharide with new beta-1-3 linkages in 92% yield. The protection group of the trisaccharide was transformed into an acetyl group. The 4-pentenyl glycoside was prepared from the acetate via glycosyl bromide. The alkene was converted into acetyl sulfide by addition of thioacetic acid under radical conditions. The lactotriaose unit was linked with carbosilane dendrimers to afford acetyl-protected glycodendrimers. De-O-acetylation of the dendrimers was carried out in the presence of sodium methoxide and then aq NaOH to give the desired lactotriaose clusters using a carbosilane dendrimer backbone. Their biological activities toward WGA were evaluated by fluorescence methods. The binding constants of free lactotriaose and trivalent, tetravalent, hexavalent, and dodecavalent glycodendrimers to WGA were determined to be 1.1x10(3), 4.4x10(4), 5.1x10(4), 2.8x10(6), and 1.3x10(6) M-1, respectively. The hexavalent glycodendrimer showed a 2500-fold larger binding effect than that of free lactotriaose.

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

Neoglycoproteins bearing key glycosyl substituents of several glycopeptidolipid antigens of pathogenic Mycobacterium species have been synthesized. Allyl glycosides of the terminal 6-deoxyhexose-containing units of the antigens were prepared, with appropriate ether and ester substituents in place. Ozonolysis of the allyl glycosides was then followed by reductive coupling with epsilon-amino groups of lysine residues in bovine serum albumin, using sodium cyanoborohydride at pH 7.8. The resulting neoglycoproteins emulated the antigenicity of the native molecule in several serological tests.

Total synthesis of X hapten, III3 Fuc alpha-nLc4 Cer

Total synthesis of O-beta-D-galactopyranosyl-(1----4)-O-[alpha-L- fucopyranosyl- (1----3)]-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----3)-O-beta- D- galactopyranosyl- (1----4)-O-beta-D-glucopyranosyl-(1----1)-2-N-tetracosanoyl-(2S,3R ,4E)- sphingenine was achieved by use of the key glycosyl donors O-(2,3,4,6-tetra-O-acetyl-beta-D- galactopyranosyl)-(1----4)-O-[(2,3,4-tri-O-acetyl-alpha-L-fucopyranosyl) - (1----3)]-O-(2- acetamido-6-O-acetyl-2-deoxy-beta-D-glucopyranosyl)-(1----3)-O-(2,4,6-tr i-O- acetyl- beta-D-galactopyranosyl)-(1----4)-2,3,6-tri-O-acetyl-alpha-D-glucopyrano syl trichloroacetimidate and fluoride, as well as key glycosyl acceptor 3-O-benzoyl-2-N-tetracosanoyl- (2S,3R,4E)-sphingenine, in an unambiguous manner.