Large-Scale Synthesis of Man9GlcNAc2 High-Mannose Glycan and the Effect of the Glycan Core on Multivalent Recognition by HIV Antibody 2G12.

Access to homogeneous high-mannose glycans in high-mg quantities is necessary for carbohydrate-based HIV vaccine development research. We have used directed evolution to design highly antigenic oligomannose clusters that are recognized in low-nM affinity by HIV antibodies. Herein we report an optimized large-scale synthesis of Man9GlcNAc2 including improved building block synthesis and a fully stereoselective 5 + 6 coupling, yielding 290 mg of glycan. We then use this glycan to study the effect of the GlcNAc2 core on the antigenicity of an evolved 2G12-binding glycopeptide, 10F2.

Synthesis of Mannosidase-Stable Man3 and Man4 Glycans Containing S-linked Manα1→2Man Termini.

Oligomannose glycans are of interest as HIV vaccine components, but they are subject to mannosidase degradation in vivo. Herein, we report the synthesis of oligosaccharides containing a thio linkage at the nonreducing end. A thio-linked dimannose donor participates in highly stereoselective glycosylations to afford trimannose and tetramannose fragments. Saturation transfer difference nuclear magnetic resonance (STD NMR) studies show that these glycans are recognized by HIV antibody 2G12, and we confirm that the reducing terminal S-linkage confers complete stability against x. manihotis mannosidase.

Synthesis of the hyper-branched core tetrasaccharide motif of chloroviruses.

Chemical synthesis of complex oligosaccharides, especially those possessing hyper-branched structures with one or multiple 1,2-cis glycosidic bonds, is a challenging task. Complementary reactivity of glycosyl donors and acceptors and proper tuning of the solvent/temperature/activator coupled with compromised glycosylation yields for sterically congested glycosyl acceptors are among several factors that make such syntheses daunting. Herein, we report the synthesis of a semi-conserved hyper-branched core tetrasaccharide motif from chloroviruses which are associated with reduced cognitive function in humans as well as in mouse models. The target tetrasaccharide contains four different sugar residues in which l-fucose is connected to d-xylose and l-rhamnose via a 1,2-trans glycosidic bond, whereas with the d-galactose residue is connected through a 1,2-cis glycosidic bond. A thorough and comprehensive study of various accountable factors enabled us to install a 1,2-cis galactopyranosidic linkage in a stereoselective fashion under [Au]/[Ag]-catalyzed glycosidation conditions en route to the target tetrasaccharide motif in 14 steps.

N-Heterocyclic silylene/germylene ligands in Au(i) catalysis.

Cationic Au(i) complexes (2, 5 and 8) supported by N-heterocyclic carbene, silylene and germylene ligands were prepared and their potential as catalysts in glycosidation chemistry has been evaluated. Insights into the mechanism are provided using DFT studies. Practical application of them as catalysts was achieved by the synthesis of the branched pentamannan core of the HIV-gp120 envelope under mild conditions.

Stable Benzylic (1-Ethynylcyclohexanyl)carbonates Protect Hydroxyl Moieties by the Synergistic Action of [Au]/[Ag] Catalytic System.

Chemical syntheses of oligosaccharides and glycosides call utilization of many protecting groups that can be installed or deprotected without affecting other functional groups present. Benzyl ethers are routinely used in the synthesis of glycans as they can be subjected to hydrogenolysis under neutral conditions. However, installation of benzyl ethers is often carried out under strong basic conditions using benzyl halides. Many a times, strongly basic conditions will be detrimental for some of the other sensitive functionalities (e.g., esters). Later introduced reagents such as benzyl trichloroacetimidate and BnOTf are not shelf-stable, and hence, a new method is highly desirable. Taking a cue from the [Au]/[Ag]-catalyzed glycosidations, we have identified a method that enables protection of hydroxyl groups as benzyl, p-methoxybenzyl, or naphthylenemethyl ethers using easily accessible and stable carbonate reagent. A number of saccharide-derived alcohols were subjected to the benzylation successfully using a catalytic amount of gold phosphite and silver triflate. Furthermore, the protocol is suitable for even protecting menthol, cholesterol, serine, disaccharide OH, and furanosyl-derived alcohol easily. The often-utilized olefins and benzoates, as well as benzylidene-, silyl-, Troc-, and Fmoc-protecting groups do not get affected during the newly identified protocol. Regioselective protection and one-pot installation of benzyl and p-methoxybenzyl ethers are demonstrated.

Nucleofuge Generating Glycosidations by the Remote Activation of Hydroxybenzotriazolyl Glycosides.

Hydroxybenzotriazole is routinely used in peptide chemistry for reducing racemization due to the increased reactivity. In this article, very stable hydroxybenzotriazolyl glucosides were identified to undergo glycosidation. The reaction was hypothesized to go through the remote activation by the Tf2O at the N3-site of HOBt followed by the extrusion of the oxocarbenium ion that was attacked by the glycosyl acceptor. Further, equilibration of the zwitterionic benzotriazolyl species makes the leaving group noncompetitive and generates the nucleofuge that has been reconverted to the glycosyl donor. The reaction is mild, high yielding, fast and suitable for donors containing both C2-ethers and C2-esters as well. The regenerative-donor glycosidation strategy is promising as it enables us to regenerate the glycosyl donor for further utilization. The utility of the methodology for the oligosaccharide synthesis was demonstrated by the successful synthesis of the branched pentamannan core of the HIV1-gp120 complex.

[Au]/[Ag]-catalysed expedient synthesis of branched heneicosafuranosyl arabinogalactan motif of Mycobacterium tuberculosis cell wall.

Emergence of multidrug-resistant and extreme-drug-resistant strains of Mycobacterium tuberculosis (MTb) can cause serious socioeconomic burdens. Arabinogalactan present on the cellular envelope of MTb is unique and is required for its survival; access to arabinogalactan is essential for understanding the biosynthetic machinery that assembles it. Isolation from Nature is a herculean task and, as a result, chemical synthesis is the most sought after technique. Here we report a convergent synthesis of branched heneicosafuranosyl arabinogalactan (HAG) of MTb. Key furanosylations are performed using [Au]/[Ag] catalysts. The synthesis of HAG is achieved by the repetitive use of three reactions namely 1,2-trans furanoside synthesis by propargyl 1,2-orthoester donors, unmasking of silyl ether, and conversion of n-pentenyl furanosides into 1,2-orthoesters. Synthesis of HAG is achieved in 47 steps (with an overall yield of 0.09%) of which 21 are installation of furanosidic linkages in a stereoselective manner.

Facile synthesis of aminooxy glycosides by gold(III)-catalyzed glycosidation.

The O-glycosidation of hydroxysuccinimides and hydroxyphthalimides with a variety of aldose derived propargyl 1,2-orthoesters under the gold(III)-catalyzed glycosidation conditions is reported. A wide range of hydroxysuccinimidyl and hydroxyphthalimidyl glycosides were synthesized from corresponding glycosyl orthoesters including glucosyl, mannosyl, galactosyl, ribofuranosyl, arabinofuranosyl, lyxofuranosyl and xylofuranosyl using gold catalysis repertoire. The protocol is identified to be compatible for the synthesis of aminooxy glycosides of higher oligosaccharides as well.

Stable Alkynyl Glycosyl Carbonates: Catalytic Anomeric Activation and Synthesis of a Tridecasaccharide Reminiscent of Mycobacterium tuberculosis Cell Wall Lipoarabinomannan.

Oligosaccharide synthesis is still a challenging task despite the advent of modern glycosidation techniques. Herein, alkynyl glycosyl carbonates are shown to be stable glycosyl donors that can be activated catalytically by gold and silver salts at 25 °C in just 15 min to produce glycosides in excellent yields. Benzoyl glycosyl carbonate donors are solid compounds with a long shelf life. This operationally simple protocol was found to be highly efficient for the synthesis of nucleosides, amino acids, and phenolic and azido glycoconjugates. Repeated use of the carbonate glycosidation method enabled the highly convergent synthesis of tridecaarabinomannan in a rapid manner.