Synthesis of the trisaccharide portion of soyasaponin beta g: evaluation of a new glucuronic acid acceptor

Expedient Synthesis of Superarmed Glycosyl Donors via Oxidative Thioglycosidation of Glycals

Superarmed glycosyl donors have higher reactivity compared to their perbenzylated armed counterparts. Generally, the 2-O- benzoyl-3,4,6-tri-O-benzyl protecting group pattern gives rise to increased reactivity due to an O-2/O-5 cooperative effect. Despite having a high reactivity profile and applicability in many expeditious strategies for glycan synthesis, regioselective introduction of the superarming protecting group pattern is tedious for most sugar series. Reported herein is a streamlined synthetic route to yield superarmed glycosyl donors of the d-gluco and d-galacto series equipped with an ethylthio, phenylthio, p-tolylthio, benzoxazol-2-ylthio, O-allyl, or O-pentenyl anomeric leaving group. This streamlined approach was made possible due to the refinement of the oxidative thioglycosylation reaction of the respective glucal and galactal precursors. The applicability of this approach to the direct formation of disaccharides is also showcased.

Chemical Synthesis of Human Milk Oligosaccharides: para-Lacto-N-hexaose and para-Lacto-N-neohexaose

Human milk oligosaccharides (HMO) have emerged as a very active area of research in glycoscience and nutrition. HMO are involved in the early development of infants and may help to prevent certain diseases. The development of chemical methods for obtaining individual HMO aids the global effort dedicated to understanding the roles of these biomolecules. Reported herein is the chemical synthesis of two common core hexasaccharides found in human milk, i. e. para-lacto-N-hexaose (pLNH) and para-lacto-N-neohexaose (pLNnH). After screening multiple leaving groups and temporary protecting group combinations, a 3+3 convergent coupling strategy was found to work best for obtaining these linear glycans.

Formation of Glycosyl Trichloroacetamides from Trichloroacetimidate Donors Occurs through an Intermolecular Aglycon Transfer Reaction

To probe the reaction mechanism, underlying the rearrangement of oft-used trichloroacetimidate glycosyl donors into the corresponding anomeric trichloroacetamides, we have used a combination of 13C- and 15N-labeled glycosyl trichloroacetimidate donors in a series of crossover experiments. These unambiguously show that trichloroacetamides are formed via an intermolecular aglycon transfer mechanism. This insight enables the design of more effective glycosylation protocols, preventing the formation of dead-end side products.

d-Glucuronate and d-Glucuronate Glycal Acceptors for the Scalable Synthesis of d-GlcN-α-1,4-d-GlcA Disaccharides and Modular Assembly of Heparan Sulfate

Reported herein is a scalable chemical synthesis of disaccharide building blocks for heparan sulfate (HS) oligosaccharide assembly. The use of d-glucuronate-based acceptors for dehydrative glycosylation with d-glucosamine partners is explored, enabling diastereoselective synthesis of appropriately protected HS disaccharide building blocks (d-GlcN-α-1,4-d-GlcA) on a multigram scale. Isolation and characterization of key donor (1,2 glycal)- and acceptor (ortho-ester, anhydro)-derived side products ensure methodology improvements to reduce their formation; protecting the d-glucuronate acceptor at the anomeric position with a para-methoxyphenyl unit proves optimal. We also introduce glycal uronate acceptors, showing them to be comparative in reactivity to their pyranuronate counterparts. Taken together, this gram-scale access offers the capability to explore the iterative assembly of defined HS sequences containing the d-GlcN-α-1,4-d-GlcA repeat, highlighted by completing this for two tetrasaccharide syntheses.

Synthesis of oligosaccharides to identify an immunologically active epitope against Candida auris infection

Candida auris (C. auris) is an emerging multidrug-resistant fungal pathogen that represents a significant public health challenge as it can spread rapidly and result in high mortality rates. The mannans on the C. auris cell surface are potent immunogens and attractive targets for developing a glycoconjugate vaccine. We synthesized the oligosaccharides resembling cell surface mannans of C. auris and printed them onto microarray slides that were used to screen plasma from mice infected with C. auris. IgM antibodies in mouse plasma recognize the β-1,2 linkage present in C. auris surface mannans. Disaccharide 19 emerged from glycan array screening as a lead for developing a vaccine against C. auris, as the majority of patient plasma samples showed antibodies against this glycan. The synthetic oligosaccharides can be used for the early detection of C. auris infections.

The NKT cell TCR repertoire can accommodate structural modifications to the lipid and orientation of the terminal carbohydrate of iGb3

Isoglobotrihexosylceramide (iGb3) is a known NKT cell agonist, however the specific interactions required to trigger NKT cell TCR activation in response to this mammalian glycolipid are not fully understood. Here we report the synthesis of 1,3-β-Gal-LacCer (βG-iGb3) that displays a β-linked terminal sugar. βG-iGb3 activated NKT cells to a similar extent as iGb3 with a terminal α-linkage, indicating that the conformation of the terminal sugar residue of iGb3 is not essential to facilitate NKT cell TCR recognition. In addition, the immunological activity of four recently described iGb3 analogues with modifications to their terminal sugar or lipid backbone were also investigated. These iGb3 analogues all induced NKT cell proliferation, with IL-13 the predominate cytokine detected. This highlights the ability of the NKT cell TCR to accommodate variations in iGb3-based glycolipids and suggests that undiscovered NKT cell ligands may exist within the lacto-series of mammalian glycosphingolipids.

The synthesised βG-iGb3 glycolipid, with a terminal 1,3-β linked galactose, induced NKT cell proliferation indicating that the α conformation of the terminal sugar residue of iGb3 is not essential for NKT cell TCR recognition.

Synthetic Investigation toward QS-21 Analogues

With glycosyl o-alkynylbenzotes as donors, a highly efficient protocol to construct the challenging glycosidic linkages at C3-OH of C23-oxo oleanane triterpenoids is disclosed, on the basis of which different strategies for the highly efficient synthesis of QS-21 analogues with the west-wing trisaccharide of QS-21 have been established.

A convenient synthesis of short-chain α-(1 → 2) mannopyranosyl oligosaccharides

Sugar 1,2-orthoesters are by-products of chemical glycosylation reactions that can be subsequently rearranged in situ to give trans glycosides. They have been used as donors in the synthesis of the latter glycosides with good regio- and stereo-selectivity. Alkyl α-(1 → 2) linked mannopyranosyl disaccharides have been reported as the major products from the rearrangement of mannopyranosyl orthoesters. Recent studies in this laboratory have shown that α-(1 → 2) linked mannopyranosyl di-, tri- and tetrasaccharides can be obtained in one step from mannopyranosyl allyl orthoester under optimized reaction conditions. In addition to the expected mono- and disaccharides (56%), allyl 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-tri-O-acetyl-α-d-mannopyranoside and allyl 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranoside were obtained in 23% and 6% isolated yields, respectively, from the oligomerization of a β-d-mannopyranosyl allyl 1,2-orthoester, along with small amounts of higher DP oligomers. Possible mechanisms for the oligomerization and side reactions are proposed based on NMR and mass spectrometric data.

The chemical synthesis of human milk oligosaccharides: Lacto-N-neotetraose (Galβ1→4GlcNAcβ1→3Galβ1→4Glc)

The discovery of innovative methods that offer new capabilities for obtaining individual oligosaccharides from human milk will help to improve understanding their roles and boost practical applications. The total chemical synthesis of lacto-N-neotetraose (LNnT) has been completed using both linear and convergent strategies. The donor and acceptor protecting and leaving group combinations were found to be of paramount significance to successful couplings.