Stereoselective α-glycosylation of C(6)-hydroxyl myo-inositols via nickel catalysis-application to the synthesis of GPI anchor pseudo-oligosaccharides

A True Reverse Anomeric Effect Does Exist After All: A Hydrogen Bonding Stereocontrolling Effect in 2-Iminoaldoses

The reverse anomeric effect is usually associated with the equatorial preference of nitrogen substituents at the anomeric center. Once postulated as another anomeric effect with explanations ranging from electrostatic interactions to delocalization effects, it is now firmly considered to be essentially steric in nature. Through an extensive research on aryl imines from 2-amino-2-deoxyaldoses, spanning nearly two decades, we realized that such substances often show an anomalous anomeric behavior that cannot easily be rationalized on the basis of purely steric grounds. The apparent preference, or stabilization, of the β-anomer takes place to an extent that not only neutralizes but also overcomes the normal anomeric effect. Calculations indicate that there is no stereoelectronic effect opposing the anomeric effect, resulting from the repulsion between electron lone pairs on the imine nitrogen and the endocyclic oxygen. Such data and compelling structural evidence unravel why the exoanomeric effect is largely inhibited. We are now confident, as witnessed by 2-iminoaldoses, that elimination of the exo-anomeric effect in the α-anomer is due to the formation of an intramolecular hydrogen bond between the anomeric hydroxyl and the iminic nitrogen, thereby accounting for a true electronic effect. In addition, discrete solvation may help justify the observed preference for the β-anomer.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.

Utilization of bench-stable and readily available nickel(II) triflate for access to 1,2-cis-2-aminoglycosides

The utilization of substoichiometric amounts of commercially available nickel(II) triflate as an activator in the reagent-controlled glycosylation reaction for the stereoselective construction of biologically relevant targets containing 1,2-cis-2-amino glycosidic linkages is reported. This straightforward and accessible methodology is mild, operationally simple and safe through catalytic activation by readily available Ni(OTf)₂ in comparison to systems employing our previously in-house prepared Ni(4-F-PhCN)₄(OTf)₂. We anticipate that the bench-stable and inexpensive Ni(OTf)₂, coupled with little to no extra laboratory training to set up the glycosylation reaction and no requirement of specialized equipment, should make this methodology be readily adopted by non-carbohydrate specialists. This report further highlights the efficacy of Ni(OTf)₂ to prepare several bioactive motifs, such as blood type A-type V and VI antigens, heparin sulfate disaccharide repeating unit, aminooxy glycosides, and α-GalNAc-Serine conjugate, which cannot be achieved in high yield and α-selectivity utilizing in-house prepared Ni(4-F-PhCN)₄(OTf)₂ catalyst. The newly-developed protocol eliminates the need for the synthesis of Ni(4-F-PhCN)₄(OTf)₂ and is scalable and reproducible. Furthermore, computational simulations in combination with ¹H NMR studies analyzed the effects of various solvents on the intramolecular hydrogen bonding network of tumor-associated mucin Fmoc-protected GalNAc-threonine amino acid antigen derivative, verifying discrepancies found that were previously unreported.

Synthesis of a polymerizable, bivalent glycan mimetic of the HIV envelope spike gp120

A synthetic study on the creation of a bivalent, ROMP capable monomer has the ability to be polymerized into the corresponding neo-glycopolymer mimetic of the surface glycans on gp120 envelope spike of the HIV virus. In our approach, we have developed a new strategy for orthogonally attaching both the terminal Manα1-2Man disaccharide unit of the D1 arm of Man9GlcNAc2 of HIV gp120 and the terminal Manα1-2 unit of its D2 arm to a bivalent scaffold to produce the corresponding polymerizable monomer. The Manα1-2 saccharide moieties were assembled using a nickel catalyst, Ni(4-F-PhCN)₄(OTf)₂, to activate trihaloacetimidate donors under mild and operationally simple procedure.

Scalable synthesis of Fmoc-protected GalNAc-threonine amino acid and T(N) antigen via nickel catalysis

The highly α-selective and scalable synthesis of the Fmoc-protected GalNAc-threonine amino acid and TN antigen in gram scale (0.5-1 g) is described. The challenging 1,2-cis-2-amino glycosidic bond is addressed through a coupling of threonine residues with C(2)-N-ortho-(trifluoromethyl)benzylidenamino trihaloacetimidate donors mediated by Ni(4-F-PhCN)4(OTf)2. The desired 1,2-cis-2-amino glycoside was obtained in 66% yield (3.77 g) with α-only selectivity and subsequently transformed into the Fmoc-protected GalNAc-threonine and TN antigen. This operationally simple procedure no longer requires utilization of the commonly used C(2)-azido donors and overcomes many of the limitations associated with the synthesis of 1,2-cis linkage.

Investigations of scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent, atom-economical, one-step conversion to α-urea-glycosides

The development and mechanistic investigation of a highly stereoselective methodology for preparing α-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the selective nickel-catalyzed conversion of glycosyl trichloroacetimidates to the corresponding α-trichloroacetamides. The α-selective nature of the conversion is controlled with a cationic nickel(II) catalyst, [Ni(dppe)(OTf)2 ] (dppe=1,2-bis(diphenylphosphino)ethane, OTf=triflate). Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2 -ether functionality of the α-glycosyl trichloroacetimidate to be paramount for achieving an α-stereoselective transformation. A cross-over experiment has indicated that the reaction does not proceed in an exclusively intramolecular fashion. The second step in this sequence is the direct conversion of α-glycosyl trichloroacetamide products into the corresponding α-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only α-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric CN bond.