The 4K reaction

The classical Koenigs-Knorr glycosidation of bromides or chlorides promoted with Ag2O or Ag2CO3 works only with reactive substrates (ideally both donor and acceptor). This reaction was found to be practically ineffective with unreactive donors such as per-O-benzoylated mannosyl bromide. Recently, it was discovered that the addition of catalytic (Lewis) acids to a silver salt-promoted reaction has a dramatic effect on the reaction rate and yield. A tentative mechanism for this cooperatively-catalyzed glycosylation reaction has been proposed, and the improved understanding of the reaction led to more efficient protocols and broader applications to a variety of glycosidic linkages. Since Ag2O-mediated activation was introduced by German chemists Koenigs and Knorr, and "cooperatively catalyzed" is Kooperativ Katalysiert in German, we refer to this new reaction as "the 4K reaction."

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

Chemical Synthesis of a Complex-Type N-Glycan Containing a Core Fucose

A chemical synthesis of a core fucose containing N-glycan was achieved. Asparagine was introduced at an early stage of the synthesis, and the sugar chain was convergently elongated. As for the fragment synthesis, we reinvestigated α-sialylation, β-mannosylation, and N-glycosylation to reveal that precise temperature control was essential for these glycosylations. Intermolecular hydrogen bonds involving acetamide groups were found to reduce the reactivity in glycosylations: the protection of NHAc as NAc₂ dramatically improved the reactivity. The dodecasaccharide-asparagine framework was constructed via the (4 + 4) glycosylation and the (4 + 8) glycosylation using the tetrasaccharide donor and the tetrasaccharide-asparagine acceptor. An ether-type solvent enhanced the yields of these key glycosylations between large substrates. After the whole deprotection of the dodecasaccharide, the target N-glycan was obtained.

O-Glycosylation methods in the total synthesis of complex natural glycosides

We highlight the total syntheses of 33 complex natural O-glycosides, with a particular focus on the O-glycosylation methods that enable the connection of the saccharides and aglycones.

Preparation and Properties of Gelatin-Immobilized β-Glucosidase fromPyrococcus furiosus

Hyperthermostable beta-glucosidase from Pyrococcus furiosus was enclosed in gelatin gel by cross-linking with transglutaminase. Gelatin-immobilized beta-glucosidase was considerably more thermostable than the native enzyme. Lyophilized immobilisate was stored at 90 degrees C for 1 month without loss of activity. The immobilized beta-glucosidase catalyzed transglucosylation of 5-phenylpentanol with 10.0 equivalent of cellobiose at pH 5.0 and 70 degrees C for 12 h to afford 5-phenylpentyl beta-D-glucopyranoside in 41% yield. The immobilized enzyme was more effective than the native one in transglucosylation. The gelatin-immobilized Pfu-beta-glucosidase recovered from the first run of the reaction was reusable on successive runs.

Rhamnosylation: diastereoselectivity of conformationally armed donors

The α/β-selectivity of super-armed rhamnosyl donors have been investigated in glycosylation reactions. The solvent was found to have a minor influence, whereas temperature was crucial for the diastereoselectivity. At very low temperature, a modest β-selectivity could be obtained, and increasing temperature gave excellent α-selectivity. The donors were highly reactive, and activation was observed at temperatures as low as -107 °C. Different promoter systems and leaving groups were investigated, and only activation with a heterogeneous catalyst increased the amount of the β-anomer significantly. By introducing an electron-withdrawing nonparticipating group, benzyl sulfonyl, on 2-O, an increase in β-product was observed.

Syntheses and Biological Activities of Glycoconjugates

This review describes the oligosaccharide syntheses and biological activities of glycosphingolipids, focusing especially on those found in invertebrates like millipedes, nematoda parasites, and cestoda parasites, and model compounds related to a major antigenic epitope against antibupleurum 2IIc/PG-1-IgG from antiulcer pectic polysaccharides. A novel and simple approach for the rational design of glycoclusters and glycodendrimers by coupling with a sugar unit and a cluster unit, was developed with beta-alanine derivatives used to construct the latter compounds.

Synthesis of oligosaccharides corresponding to Vibrio cholerae O139 polysaccharide structures containing dideoxy sugars and a cyclic phosphate

A spacer-equipped tetrasaccharide, p-aminocyclohexylethyl alpha-l-Colp-(1-->2)-beta-d-Galp-(1-->3)-[alpha-l-Colp-(1-->4)]-beta-D-GlcpNAc, containing a 4,6-cyclic phosphate in the galactose residue, has been synthesised. The structure corresponds to a part of the repeating unit of the capsular (and lipo-) polysaccharide of the endemic bacteria Vibrio cholerae type O139 synonym Bengal. The synthetic strategy allows continuous syntheses of the complete O139 hexasaccharide repeating unit as well as of the structurally related repeating unit of serotype O22. Starting from ethyl 2-azido-4,6-O-benzylidene-2-deoxy-1-thio-beta-D-glucopyranoside, a thioglycoside tetrasaccharide donor block was constructed through two orthogonal glycosylations with glycosyl bromide donors. First, a properly protected galactose moiety was introduced using silver triflate as promoter and subsequently the two colitose residues, carrying electron-withdrawing protecting groups for stability reasons, under halide-assisted conditions. The tetrasaccharide block was then linked to the spacer in a NIS-TMSOTf-promoted coupling. Transformation of the azido group into an acetamido group using H2S followed by removal of temporary protecting acetyl groups gave a 4',6'-diol, which was next phosphorylated with methyl dichlorophosphate and deprotected to yield the 4,6-cyclic phosphate tetrasaccharide target structure.

Studies in polyphenol chemistry and bioactivity. 4.(1) Synthesis of trimeric, tetrameric, pentameric, and higher oligomeric epicatechin-derived procyanidins having all-4beta,8-interflavan connectivity and their inhibition of cancer cell growth through cell cycle arrest

We report an improved synthesis of bis(5,7,3',4'-tetra-O-benzyl)epicatechin 4beta,8-dimer (3) from 5,7,3',4'-tetra-O-benzylepicatechin (1) and 5,7,3',4'-tetra-O-benzyl-4-(2-hydroxyethoxy)epicatechin (2) by replacing the previously employed Lewis acid, titanium tetrachloride, with the clay mineral Bentonite K-10. Under the same conditions, the benzyl-protected all-4beta,8-trimer, -tetramer, and -pentamer were obtained regioselectively from their lower homologues, albeit in rapidly decreasing yields. Reaction of 2 with an organoaluminum thiolate generated from 2-mercaptobenzothiazole and trimethylaluminum followed by acetylation produced 3-O-acetyl-4-[(2-benzothiazolyl)thio]-5,7,3',4'-tetra-O-benzylepicatechin (12). Medium-sized protected oligomers with 4beta,8-interflavan linkages are obtained in improved yields by using this compound as the electrophile and silver tetrafluoroborate as activator and are isolated by reversed-phase HPLC. Their deprotection by ester saponification followed by hydrogenolysis yielded the free procyanidins, which were characterized as their peracetates. The synthetic procyanidins are identical by normal-phase HPLC with fractions isolated from cocoa. The principle of chain extension by two members was demonstrated using a dimeric electrophile obtained by self-condensation of compound 12. Both the synthetic and natural pentamer 32 inhibit the growth of several breast cancer cell lines. Using the MDA MB 231 line, it was established that this outcome is based on the induction of cell cycle arrest in the G0/G1 phase. Subsequent cell death is more likely necrotic rather than apoptotic. Control experiments demonstrate that the polyphenol itself, rather than hydrogen peroxide potentially formed by its autoxidation, is the causative agent.

A highly stereoselective construction of beta-glycosyl linkages by reductive cleavage of cyclic sugar ortho esters

The preparation of beta-glycosides by the reductive cleavage of spiro sugar ortho esters is described in this report. This procedure is based on a concept completely different from those of other methods for glycosylation. Twelve sugar ortho esters that commonly possess perhydrospiro[2H-pyran-2,2'-pyrano[3,2-d][1,3]dioxin] ring systems in their molecules were reduced by LiAlH(4)/AlCl(3) or NaCNBH(3)/AlCl(3). Among these ortho esters, those (9a-12a) prepared from the D-sugar lactones (1-4) and 2, 3-di-O-benzyl-alpha-D-glucopyranoside (7) or those (19a, 20a) prepared from the L-sugar lactones (5, 6) and 2, 3-di-O-benzyl-alpha-D-galactopyranoside (8) were selectively converted into beta-(1 --> 4)-glycosides (9b-12b or 19b, 20b) in excellent yields by the treatment of LiAlH(4)/AlCl(3). In contrast, the ortho esters (13a-16a or 17a, 18a) that were prepared from combinations of the D-sugar lactones and 8 or those of the L-sugar lactones and 7 were efficiently reduced with NaBH(3)CN/AlCl(3) to afford beta-(1 --> 6)-glycosides (13b-16b or 17b, 18b) selectively. It was remarkable that the resulting disaccharides were obtained with extremely high beta-selectivity even in the cases with mannosyl or rhamnosyl glycosides. Moreover, these products would be useful units for the construction of branched saccharides, because the newly formed hydroxy groups could be again glycosylated without further deprotection procedures. The high regio- and stereoselectivity was totally explained by considering the structures and the conformations of these ortho ester molecules and the stereoelectoronic effects of their spiro ring systems. In addition, the preparation of the sugar ortho esters with glucosamine derivatives and the reactivity of these ortho esters are described in this report. N-Phthaloyl glucosamine derivatives (21, 22) were efficiently reacted with the benzyl-protected gluconolactone (1) in the presence of TMSOMe and TMSOTf to afford ortho esters (23a-c). After the conversion of the phthalimido functionality to the dibenzyl amino group, glucosylideneglucosamine (25) was reduced with LiAlH(4)/AlCl(3) to afford beta-(1 --> 4)-glycoside (26) selectively.

Synthesis of alpha- and beta-linked tyvelose epitopes of the Trichinella spiralis glycan: 2-acetamido-2-deoxy-3-O-(3,6-dideoxy-D-arabino-hexopyranosyl)-beta -D-galactopyranosides

The anomeric configuration of tyvelose, 3,6-dideoxy-D-arabino-hexopyranose, in the recently discovered glycan epitopes of the parasite Trichinella spiralis has not been established. Two 2-(trimethylsilyl)ethyl disaccharide glycosides, alpha- and beta-Tyv-(1-->3)-beta-D-GalNAc (4 and 5), have been synthesized to provide model compounds that, together with the methyl 3,6-dideoxy-alpha- and beta-D-arabino-hexopyranosides (2 and 3), aid the determination of the anomeric configuration of tyvelose residues in the parasite glycan, either indirectly by immunochemical inhibition data or directly by the technique of 1H NMR spectroscopy. Methyl 3,6-dideoxy-beta-D-arabino-hexopyranoside (3) was synthesized from methyl 2,3-anhydro-4,6-O-benzylidene-beta-D-mannopyranoside (9) by a method previously used for the alpha anomer 2. Benzylation of 2 provided a route to the glycosyl donor, 2,4-di-O-benzyl-3,6-dideoxy-alpha-D-arabino-hexopyranosyl chloride (30), that reacted with the selectively protected 2-acetamido-2-deoxy-D-galactopyranoside alcohol 18 in the presence of an insoluble silver zeolite catalyst to give the alpha- and beta-linked disaccharides 31 and 32. Glycosylation of the related 2-acetamido-2-deoxy-D-galactopyranoside alcohol 27 by 30 under similar conditions provided disaccharides 33 and 34 containing a tether. Deprotection of the saccharide and derivatization of the tether with 1,2-diaminoethane provided amide derivatives 35 and 36 suitable for the preparation of neoglycoconjugate antigens. Complete 1H and 13C NMR chemical shifts of the deprotected disaccharides and monosaccharides are reported.

Synthesis and conformational studies of the tyvelose capped, Lewis-x like tetrasaccharide epitope of Trichinella spiralis

Chemical synthesis and high resolution NMR studies are reported for the tetrasaccharide epitope and constituent structures that occur as terminal elements of Trichinella spiralis cell surface glycans. The 2-(trimethylsilyl)ethyl and methyl glycosides of Lewis-x type trisaccharides, in which beta GalNAc replaces beta Gal were synthesized from monosaccharide synthons utilizing thioglycoside and halide-ion glycosylation methods. The unique 3,6-dideoxy-beta-D-arabinohexopyranosyl residue that caps the structure was introduced to selectively protected 2-(trimethylsilyl)ethyl and methyl trisaccharide glycosides by utilizing an insoluble silver zeolite catalyst and a glycosyl halide. In separate reactions not only were the principal targets obtained but also the corresponding alpha-linked tetrasaccharides. A comparison of the NMR spectra of the methyl tetrasaccharide glycosides showed that at the site of the alpha-linked tyvelose structure, specific GalNAc resonances (C-1, C-2, C-3) possess uncharacteristically wide 13C (8-21 Hz) and 1H lines. The beta-linked tetrasaccharide glycoside, that represents the native parasite epitope, exhibited only narrow line widths (3 Hz, 13C). Since NOE derived distance constraints for the alpha-linked tyvelose structure were not consistent with the existence of unusual glycosidic conformers, the origin of the limited number of wide lines was attributed to local rigidity in the GalNAc residue, at the site of tyvelose glycosylation.

Chemistry of 1-Alkoxy-1-glycosyl Radicals: The Manno- and Rhamnopyranosyl Series. Inversion of alpha- to beta-Pyranosides and the Fragmentation of Anomeric Radicals

The formation and stereoselective quenching of 1-mannopyranosyl radicals by a tributyltin hydride-mediated intramolecular 1,5-hydrogen abstraction sequence is described. A competing process is 1,4-hydrogen atom abstraction leading principally to glucopyran-2-ulosides. Fragmentation of the anomeric radical resulting in the formation of ring opened products is a problem in certain series. The chemistry is dictated to a considerable extent by the nature of the protecting groups employed with the 4,6-benzylidene series and, for rhamnose, the Ley 3,4-dispiroketal, being particularly susceptible to the 1,4-hydrogen atom abstraction but less to the fragmentation. Photochemical conditions are described, in which these side reactions are practically eliminated, and applied to the inversion of an alpha- to a beta-mannoside in a disaccharide.

Synthesis of a glycopeptide carrying a N-linked core pentasaccharide

A glycopeptide carrying a pentasaccharide core structure of asparagine-linked glycoproteins was synthesized. The synthesis of the carbohydrate part was performed starting from monosaccharide components in an unambiguous manner. The resultant pentaglycosyl azide was reduced into corresponding glycosyl amine and coupled with an aspartic acid derivative to furnish an Asn-linked oligosaccharide in a protected form. Subsequent coupling with a dipeptide, followed by deprotection gave the target compound.

Cross-reactivity between the mannan of Candida species, Klebsiella K24 polysaccharide and Salmonella C1 and E O-antigens is mediated by a terminal non-reducing beta-mannosyl residue

Rat monoclonal antibody MASC1-MR9 (MR9) binds to a mannan of Candida species and the O-antigenic polysaccharides of Salmonella bacteria of serogroups C1 (CO) and E (EO). Mannan and glycoconjugates comprising BSA and O-antigen polysaccharides, decasaccharide-BSA (CO-BSA) or trisaccharide-BSA (EO-BSA), inhibited each other's reactivity with MR9. The saccharides beta-D-Manp-(1-->6)-alpha-D-Manp-1-OMe, beta-D-Manp(1-->3)-alpha-D-Manp-1-OMe, beta-D-Manp(1-->2)-alpha-D-Manp-1-OMe (corresponds to the terminal non-reducing end of Salmonella serogroup C1 O-antigen) and beta-D-Manp(1-->4)-alpha-L-Rhap(1-->3)-alpha-D-Galp-1-O-p-++ +trifluoroacetamido aniline (corresponds to the backbone of Salmonella serogroup E O-antigen) inhibited the binding of MR9 to these antigens whereas alpha-D-Manp(1-->3)-alpha-D-Manp-1-OMe and alpha-D-Manp(1-->4)-alpha-L- Rhap-1-O-p-nitrophenyl did not. Saccharides (3-10 residues) of mammalian origin with terminal and internal Manp alpha-1-->2, Manp alpha-1-->3 and Manp alpha-1-->6 residues also failed to inhibit at any concentration. None of the saccharides with internal beta-mannosyl residue was able to inhibit the MR9 antibody. Monosaccharides D-mannose, beta-D-Manp-1-OMe and 1,5 anhydro-D-mannitol inhibited the MR9 monoclonal antibody whereas alpha-D-Manp-1-OMe, beta-D-Glcp-1-OMe, and beta-D-Galp-1-OMe did not. In addition a Klebsiella K24 capsular polysaccharide containing a beta-D-Manp(1-->4)-alpha-D- GlcA (GlcA, glucuronic acid) as a structural element possessed an inhibitory activity. MR9 therefore recognizes an epitope within beta-mannose monosaccharide residues at the terminal non-reducing ends of carbohydrate chains in mannan, and polysaccharides in Salmonella serogroups CO and EO and Klebsiella K24.

Synthesis of a selectively protected trisaccharide building block that is part of xylose-containing carbohydrate chains from N-glycoproteins

The synthesis is reported of ethyl 4-O-[3-O-allyl-4,6-O-isopropylidene-2-O-(2,3,4-tri-O-acetyl-beta-D- xylopyranosyl)-beta-D-mannopyranosyl]-3,6-di-O-benzyl-2-deoxy-2-phthalim ido-1 - thio-beta-D-glucopyranoside (16), a key intermediate in the synthesis of xylose-containing carbohydrate chains from N-glycoproteins. Condensation of ethyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-beta-D- glucopyranoside (5) with 2,4,6-tri-O-acetyl-3-O-allyl-alpha-D-glucopyranosyl bromide, using silver triflate as a promoter, gave the beta-linked disaccharide derivative 8 (84%). O-Deacetylation of 8 and then isopropylidenation afforded 10, which was converted via oxidation-reduction into ethyl 4-O-(3-O-allyl-4,6-O-isopropylidene-beta-D-mannopyranosyl)-3,6-di-O-benz yl-2- deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (12). Silver triflate-promoted condensation of 12 with 2,3,4-tri-O-acetyl-alpha-D-xylopyranosyl bromide gave 16 (71%). The Xylp unit in 16 and in de-isopropylidenated 16 (17) existed in the 1C4(D) conformation, but that in O-deacetylated 17 (18) existed in the 4C1(D) conformation.

Synthesis of beta-D-mannosides from beta-D-glucosides via an intramolecular SN2 reaction at C-2

The selective synthesis of beta-D-mannosides was achieved by first synthesizing beta-D-glucosides that carry a N-phenylcarbamoyl protecting group at O-3. These derivatives were transformed into the corresponding beta-D-mannosides by intramolecular nucleophilic substitution with inversion of configuration at C-2, the O-trifyl group being the leaving group. Subsequent intramolecular attack of the neighboring carbamoyl group resulted in the formation of the 2,3-carbonate of the desired beta-D-mannoside.

Synthesis of a tetrasaccharide acceptor for use in the assay of UDP-GlcpNAc:beta-D-Galp-(1----4)-beta-D-GlcpNAc (GlcNAc to Gal) beta(1----3)-N-acetylglucosaminyltransferase activity and the pentasaccharide product that would be formed by its enzymic glycosylation

The tetrasaccharide beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)-alpha-D- Manp-(1----6)-beta-D-Manp-OR (2) and the pentasaccharide beta-D-GlcpNAc-(1----3)-beta-D-Galp-(1----4)-beta-D- GlcpNAc-(1----6)-alpha-D-Manp-(1----6)-beta-D-Manp-OR (3), where R = (CH2)8COOMe, have been prepared by using combined chemical and enzymic procedures. Structure 2 is a substrate for UDP-GlcpNAc:beta-D-Galp-(1----4)- beta-D-GlcpNAc (GlcNAc to Gal) beta(1----3)-N-acetylglucosaminyltransferase, and 3 would be the product of its action. Antibodies raised against 3 are intended for use in an ELISA assay that would quantitate the enzymic conversion of immobilized 2 into 3.