A convenient preparation of 2-acetamido-2,6-dideoxy-d-glucose, some of its alkyl glycosides, and allyl 2-acetamido-2,6-dideoxy-α-d-galactopyranoside

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Synthesis of C6-substituted UDP-GlcNAc derivatives

UDP-sugar analogs are useful for the study of glycosyltransferases and the production of unnatural glycans. The preparation of five UDP-GlcNAc derivatives is reported with 6-deoxy, 6-azido, 6-amino, 6-mercapto, or 6-fluoro substitutions. A concise chemoenzymatic synthesis was developed using the kinase NahK (B. longum JCM1217) and the uridyl transferase GlmU (E. coli K12).

Recent advances in synthesis of bacterial rare sugar building blocks and their applications

This Highlight describes recent advances in the synthesis of the bacterial deoxy amino hexopyranoside building blocks and their application in constructing various biologically important bacterial O-glycans.

UDP-Gal: GlcNAc-R beta1,4-galactosyltransferase--a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine beta1,4-Gal-transferase T1 (beta4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for beta4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric beta-configuration were more active than those having the alpha-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNbeta-(2-naphthyl) with a K(i) of 0.01 mM. These studies help to delineate beta4GalT-substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

An improved synthesis of 4-azido-4-deoxy- and 4-amino-4-deoxy-alpha,alpha-trehalose and their epimers

The order of esterification of the eight hydroxyl groups of alpha,alpha-trehalose is HO-6.6' > HO-2.2' > HO-3.3' > HO-4.4'. Under the appropriate conditions of benzoylation, the heptabenzoate with HO-4' free was obtained in good yield (58%), along with the octabenzoate and the hexabenzoate having HO-4.4' free. The readily isolated heptabenzoate was a convenient starting material for the synthesis of 4-azido-4-deoxy- (84%) and 4-amino-4-deoxy-alpha-D-galactopyranosyl alpha-D-glucopyranoside, and the heptabenzoate of alpha-D-galactopyranosyl alpha-D-glucopyranoside with HO-4' free, which was used as a synthetic precursor of 4-azido-4-deoxy-alpha-D-glucopyranosyl alpha-D-glucopyranoside and its amino analogue.

Synthesis of 3,6-dideoxy-3-(methylamino)hexoses for g.l.c.-m.s. identification of Rhizobium lipopolysaccharide components

A direct synthetic route from methyl alpha-D-glucopyranoside to 3,6-dideoxy-3-(methylamino)hexoses having the D-gluco, D-galacto, and D-manno configurations has been developed. Methyl alpha-D-glucoside was converted into the 4,6-O-benzylidene-2,3-di-O-tosyl derivative, which was then transformed into the 4-O-benzyl-6-deoxy 2,3-ditosylate (5) by successive reductive cleavage of the acetal ring, iodination, and reduction. The intermediate 5 was readily converted into the allo 2,3-epoxide, which yielded the pivotal intermediate methyl 4-O-benzyl-3,6-dideoxy-3-(methylamino)-alpha-D-glucopyranoside (7) by cleavage of the oxirane ring with methylamine. The amino compound 7 can be directly converted into the derivatized galacto and manno derivatives for mass-spectrometric identification by selective inversion at C-4 and C-2, respectively, followed by hydrolysis, reduction, and acetylation.

Protected glycosides and disaccharides of 2-amino-2-deoxy-D-glucopyranose by ferric chloride-catalyzed coupling

The ferric chloride-catalyzed glycosylation of hydroxy compounds by protected 2-acylamino-2-deoxy-beta-D-glucopyranose 1-acetates is described. In addition to known glycosides from the reaction of alcohols with 2-acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-beta-D-glucopyranose (3), ally (and other alkyl) beta-glycosides were obtained from the N-benzoyl, N-phenoxyacetyl, N-methoxyacetyl, N-chloroacetyl, and N-phthaloyl congeners of 3. The latter compounds, except for the N-phthaloyl derivative, gave oxazolines in the absence of an alcoholic reactant. Compound 3 and the related N-benzoyl, N-chloroacetyl, N-acetyl-3,4,6-tri-O-benzyl, and N-acetyl-4-O-acetyl-3,6-di-O-benzyl derivatives were coupled to one or more protected sugars to form protected, beta-linked disaccharides. Coupling at the 6-positions of acceptors proceeded smoothly and gave 67-80% yields. For successful coupling at positions 3 and 4, long reaction times and multiple additions of glycosyl donor were required, and yields ranged from 60% to as low as 30%. 1,3,4,6-Tetra-O-acetyl-2-(chloroacetamido)-2-deoxy-beta-D- glucopyranose appeared to be the most reactive glycosyl donor in this series. The reaction of 2-methyl-(3,4,6-tri-O-acetyl-1,2-dideoxy-alpha-D-glucopyrano)[2,1- d]-2-oxazoline (derived from 3) with allyl alcohol was catalyzed by ferric chloride, and oxazolines were detected as intermediates in some of the glycosylations of protected sugars.