Ein neuer Ansatz zur stereoselektiven Synthese von β-Mannosiden

Synthetic neoglycoconjugates of hepta- and nonamannoside ligands for eliciting oligomannose-specific HIV-1-neutralizing antibodies

Oligomannose-type glycans on the spike protein of HIV-1 constitute relevant epitopes to elicit broadly neutralizing antibodies (bnAbs). Herein we describe an improved synthesis of α- and β-linked hepta- and nonamannosyl ligands that, subsequently, were converted into BSA and CRM 197 neoglycoconjugates. We assembled the ligands from anomeric 3-azidopropyl spacer glycosides from select 3-O-protected thiocresyl mannoside donors. Chain extensions were achieved using 4+3 or 4+5 block synthesis of thiocresyl and trichloroacetimidate glycosyl donors. Subsequent global deprotection generated the 3-aminopropyl oligosaccharide ligands. ELISA binding data obtained with the β-anomeric hepta- and nonamannosyl conjugates with a selection of HIV-1 bnAbs showed comparable binding of both mannosyl ligands by Fab fragments yet lesser binding of the nonasaccharide conjugate by the corresponding IgG antibodies. These results support previous observations that a complete Man 9 structure might not be the preferred antigenic binding motif for some oligomannose-specific antibodies and have implications for glycoside designs to elicit oligomannose-targeted HIV-1-neutralizing antibodies.

A Mechanistic Probe into 1,2-cis Glycoside Formation Catalyzed by Phenanthroline and Further Expansion of Scope

Phenanthroline, a rigid and planar compound with two fused pyridine rings, has been used as a powerful ligand for metals and a binding agent for DNA/RNA. We discovered that phenanthroline could be used as a nucleophilic catalyst to efficiently access high yielding and diastereoselective α-1,2-cis glycosides through the coupling of hydroxyl acceptors with α-glycosyl bromide donors. We have conducted an extensive investigation into the reaction mechanism, wherein the two glycosyl phenanthrolinium ion intermediates, a 4C1 chair-liked β-conformer and a B2,5 boat-like α-conformer, have been detected in a ratio of 2:1 (β:α) using variable temperature NMR experiments. Furthermore, NMR studies illustrate that a hydrogen bonding is formed between the second nitrogen atom of phenanthroline and the C1-anomeric hydrogen of sugar moiety to stabilize the phenanthrolinium ion intermediates. To obtain high α-1,2-cis stereoselectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the 4C1 chair-like β-conformer and B2,5 boat-like α-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the α-face of the more reactive 4C1 β-phenanthrolinium intermediate to give an α-anomeric product. The utility of the phenanthroline catalysis is expanded to sterically hindered hydroxyl nucleophiles and chemoselective coupling of an alkyl hydroxyl group in the presence of a free C1-hemiacetal. In addition, the phenanthroline-based catalyst has a pronounced effect on site-selective couplings of triol motifs and orthogonally activates the anomeric bromide leaving group over the anomeric fluoride and sulfide counterparts.

Synthetic preparation and immunological evaluation of β-mannosylceramide and related N-acyl analogues

The synthesis of the invariant natural killer (iNK) T cell agonist β-mannosylceramide along with a series of fatty amide analogues is reported. Of the six β-glycosylation protocols investigated, the sulfoxide methodology developed by Crich and co-workers proved to be the most effective where the reaction of a mannosyl sulfoxide and phytosphingosine derivative gave a key glycolipid intermediate as a 95 : 5 mixture of β- to α-anomers in high yield. A series of mannosyl ceramides were evaluated for their ability to activate D32.D3 NKT cells and induce antitumour activity.

Stereoselective β-Mannosylation with 2,6-Lactone-bridged Thiomannosyl Donor by Remote Acyl Group Participation

Stereoselective β-mannosylation has been recognized as one of the greatest challenges of carbohydrate chemistry. Herein, we described a practical method for stereoselective construction of β-mannosides by using a 2,6-lactone-bridged thiomannosyl donor through the remote acyl-group participation as well as the steric effect of O-4 substituent. The two effects are enabled through the conversion of a regular mannopyranosyl ⁴ C₁ conformation into a 2,6-lactone bridged conformation. The lactone donor could be readily prepared in three steps on a gram scale and the β-mannosylation proceeded smoothly with high stereoselectivity for primary, secondary and tertiary alcohol acceptors. In addition, this strategy was successfully applied to the synthesis of a naturally occurring trisaccharide.

Stereoselective Construction of β-Mannopyranosides by Anomeric O-Alkylation: Synthesis of the Trisaccharide Core of N-linked Glycans

A new and efficient approach for direct and stereoselective synthesis of β-mannopyranosides by anomeric O-alkylation has been developed. This anomeric O-alkylation of mannopyranose-derived lactols is proposed to occur under synergistic control of a kinetic anomeric effect and metal chelation. The presence of a conformationally flexible C6 oxygen atom in the sugar-derived lactol donors is required for this anomeric O-alkylation to be efficient, probably because of its chelation with cesium ion. In contrast, the presence of a C2 oxygen atom plays a minor role. This glycosylation method has been successfully utilized for the synthesis of the trisaccharide core of complex N-linked glycans.

A streamlined strategy for aglycone assembly and glycosylation

Multipurpose sugars: Carbohydrate-derived silane reagents are utilized as the reductant for nickel-catalyzed aldehyde-alkyne reductive coupling reactions and as the glycosyl donor for subsequent intramolecular glycosylation. The approach enables the assembly of the carbon-carbon framework and stereochemical features of an aglycone while simultaneously establishing the site of glycosylation.

Ketone hydrosilylation with sugar silanes followed by intramolecular aglycone delivery: an orthogonal glycosylation strategy

Gettin' a little sugar-no alcohol required: A procedure for the direct glycosylation of ketones without a hydroxy intermediate enables the site-selective glycosylation of hydroxyketones at the ketone or the alcohol functionality without the use of protecting groups on the aglycone (see scheme). Site selectivity is controlled by the catalyst structure in hydrosilylation and dehydrogenative silylation reactions with sugar silanes. Bn=benzyl.

Automated solid-phase synthesis of protected oligosaccharides containing beta-mannosidic linkages

For automated oligosaccharide synthesis to impact glycobiology, synthetic access to most carbohydrates has to become efficient and routine. Methods to install "difficult" glycosidic linkages have to be established and incorporated into the overall synthetic concept. Described here is the first automated solid-phase synthesis of oligosaccharides containing the challenging beta-mannosidic linkage. Carboxybenzyl mannoside building blocks proved effective beta-mannosylation agents and resulted in excellent conversion and good to moderate selectivities. [(Triisopropylsilyl)oxy]-methyl ether (Tom), served as an orthogonal, minimally intrusive, and readily cleavable protecting group for the elongation of the C3 position of mannose. The desired oligosaccharide products were readily separated from by-products containing unwanted stereoisomers using reverse-phase HPLC. The methods described here expand the scope of carbohydrates currently accessible by automation as many oligosaccharides of biological interest contain beta-mannosidic linkages.

Efficient intramolecular beta-mannoside formation using m-xylylene and isophthaloyl derivatives as rigid spacers

A series of mannosyl donors linked via position 2 to an m-xylylene or an isophthaloyl spacer which was connected to the position 6 of a glucoside acceptor afforded, via intramolecular glycosylation, the corresponding disaccharides with high beta anomeric ratio.

Total synthesis of everninomicin 13,384-1--Part 4: explorations of methodology; stereocontrolled synthesis of 1,1'-disaccharides, 1,2-seleno migrations in carbohydrates, and solution- and solid-phase synthesis of 2-deoxy glycosides and orthoesters

Methods for the stereocontrolled construction of 1,1'-disaccharides, 2-deoxy glycosides, and orthoesters are reported. Specifically, a tin-acetal moiety was utilized to fix the anomeric stereochemistry of a carbohydrate acceptor leading to an efficient and stereoselective synthesis of 1,1'-disaccharides, while a newly discovered 1,2-phenylseleno migration reaction in carbohydrates opened entries to 2-deoxy glycosides and orthoesters. Thus, reaction of 2-hydroxy phenylselenoglycosides with DAST led to 2-phenylselenoglycosyl fluorides which reacted with carbohydrate acceptors to afford, stereoselectively, 2-phenylselenoglycosides. The latter compounds could be reductively deselenated to 2-deoxy glycosides or oxidatively converted to orthoesters via the corresponding ketene acetals.

4,6-di-O-benzoyl-3-O-benzyl-alpha-D-arabino-hexo-pyranos-2-ulosyl bromide: a conveniently accessible glycosyl donor for the expedient construction of diantennary beta-D-mannosides branched at O-3 and O-6

A concise practical, large scale-adaptable six-step sequence has been developed for the transformation of diacetone-glucose into 4,6-di-O-benzoyl-3-O-benzyl-alpha-D-arabino-hexopyranos-2-ulosy l bromide (7), a most useful indirect beta-D-mannosyl donor as its blocking group pattern allows the construction of biologically relevant beta-D-mannosides branched at O-3 and O-6. The broad utility of this new ulosyl bromide 7 resides in its high anomeric reactivity, and in the ease and uniformity with which beta-stereocontrol can be achieved over both, glycosidations and carbonyl reduction of the beta-ulosides formed: Koenigs-Knorr conditions exclusively provide beta-glycosiduloses, hydride reduction of their carbonyl functions proceeds with high stereoselectivities (> 20:1) in favor of the beta-D-mannosides. These preparatively auspicious properties are materialized in an efficient, straightforward synthesis of alpha-D-Manp-(1-->6)-[alpha-D-Manp-(1-->3)]-beta-D-Manp++ +-(1-->O)-Octyl, the 3,6-O-branched core-mannotrioside carrying an octyl spacer instead of the chitobiosyl unit.