Synthesis of Urea-Tethered Neoglycoconjugates and Pseudooligosaccharides in Water

Allyl Cyanate/Isocyanate Rearrangement in Glycals: Stereoselective Synthesis of 1-Amino and Diamino Sugar Derivatives

The [3,3]-sigmatropic allyl cyanate/isocyanate rearrangement of glycals in the presence of O-, N-, and C-nucleophiles afforded β-N-glucosyl and galactosyl carbamates, ureas, and amides in good yields. The unsaturated products were elaborated to N-glycosides by dihydroxylation, to 1,3-diaminosugars by tethered aminohydroxylation, or to 1,2-diaminosugars by iteration of the sigmatropic rearrangement. This metal-free methodology represents an excellent and general method for the stereoselective synthesis of N-glycosides and diamino sugars with complete transmission of stereochemical information.

Facile synthesis of urea-and thiocarbamate-tethered glycosyl beta-amino acids

An efficient way to synthesize series of new urea- and thiocarbamate-tethered glycosyl β-amino acids under mild conditions was described.

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.

Carbon dioxide as a primary oxidant and a C1 building block

2,3-Dihydro-2,2,2-triphenylphenanthro[9,10-d]-1,3,2-λ⁵-oxazaphospholes react with carbon dioxide in an overall second order reaction at room temperature to give 3H-phenanthro[9,10-d]oxazol-2-ones and triphenylphosphine oxide in good yields.

Protecting group free synthesis of urea-linked glycoconjugates: efficient synthesis of β-urea glycosides in aqueous solution

The one step process, involving reactions between urea and protecting group free d-glucose, N-acetyl-d-glucosamine or d-xylose in acidic aqueous solution, furnishes the corresponding β-urea glycosides.

Recent developments in glycosyl urea synthesis

The area of sugar urea derivatives has received considerable attention in recent years because of the unique structural properties and activities that these compounds display. The urea-linkage at the anomeric center is a robust alternative to the naturally occurring O- and N-glycosidic linkages of oligosaccharides and glycoconjugates, and the natural products that have been identified to contain these structures show remarkable biological activity. While methods for installing the β-urea-linkage at the anomeric center have been around for decades, the first synthesis of α-urea glycosides has been much more recent. In either case, the selective synthesis of glycosyl ureas can be quite challenging, and a mixture of α- and β-isomers will often result. This paper will provide a comprehensive review of the synthetic approaches to α- and β-urea glycosides and examine the structure and activity of the natural products and their analogues that have been identified to contain them.

Protein glycoengineering enabled by the versatile synthesis of aminooxy glycans and the genetically encoded aldehyde tag

Homogeneously glycosylated proteins are important targets for fundamental research and for biopharmaceutical development. The use of unnatural protein–glycan linkages bearing structural similarity to their native counterparts can accelerate the synthesis of glycoengineered proteins. Here we report an approach toward generating homogeneously glycosylated proteins that involves chemical attachment of aminooxy glycans to recombinantly produced proteins via oxime linkages. We employed the recently introduced aldehyde tag method to obtain a recombinant protein with the aldehyde-bearing formylglycine residue at a specific site. Complex aminooxy glycans were synthesized using a new route that features N-pentenoyl hydroxamates as key intermediates that can be readily elaborated chemically and enzymatically. We demonstrated the method by constructing site-specifically glycosylated variants of the human growth hormone.

Recent progress in the field of neoglycoconjugate chemistry

Glycosylation is probably the most complex secondary gene event that affects the vast majority of proteins in nature resulting in the occurrence of a heterogeneous mixture of glycoforms for a single protein. Many functions are exerted by single monosaccharides, well-defined oligosaccharides, or larger glycans present in these glycoproteins. To unravel these functions it is of the utmost importance to prepare well-defined single glycans conjugated to the underlying aglycon. In this review, the most recent developments are described to address the preparation of carbohydrate-amino acid (glyco-conjugates). Naturally occurring N- and O-linked glycosylation are described and the preparation of non-natural sugar-amino acid linkages are also included.

(Pseudo)amide-linked oligosaccharide mimetics: molecular recognition and supramolecular properties

Oligosaccharides are currently recognised as having functions that influence the entire spectrum of cell activities. However, a distinct disadvantage of naturally occurring oligosaccharides is their metabolic instability in biological systems. Therefore, much effort has been spent in the past two decades on the development of feasible routes to carbohydrate mimetics which can compete with their O-glycosidic counterparts in cell surface adhesion, inhibit carbohydrate processing enzymes, and interfere in the biosynthesis of specific cell surface carbohydrates. Such oligosaccharide mimetics are potential therapeutic agents against HIV and other infections, against cancer, diabetes and other metabolic diseases. An efficient strategy to access this type of compounds is the replacement of the glycosidic linkage by amide or pseudoamide functions such as thiourea, urea and guanidine. In this review we summarise the advances over the last decade in the synthesis of oligosaccharide mimetics that possess amide and pseudoamide linkages, as well as studies focussing on their supramolecular and recognition properties.

Synthesis of neoglycoconjugates by the desulfurative rearrangement of allylic disulfides

Two series of neoglycosyl donors are prepared on the basis of connection of an allylic disulfide motif to the anomeric center via a simple O-glycosyl linkage or N-glycosyl amide unit. Conjugation of both sets of donors to cysteine in peptides is demonstrated through classical disulfide exchange followed by the phosphine-mediated desulfurative allylic rearrangement resulting in neoglycopeptides characterized by a simple thioether spacer. The conjugation reaction functions in the absence of protecting groups on both the neoglycosyl donor and peptide in aqueous media at room temperature.

Palladium(II)-catalyzed rearrangement of glycal trichloroacetimidates: application to the stereoselective synthesis of glycosyl ureas

The research on the area of glycosyl urea derivatives, in which the O- and N-glycosidic bonds are replaced with the urea-glycosidic linkages, has recently emerged with applications in the field of aminoglycoside antibiotics. We have developed a novel method for the stereoselective synthesis of alpha- and beta-glycosyl ureas via Pd(II)-catalyzed rearrangement of glycal trichloroacetimidates. In our approach, the alpha- and beta-selectivity at the anomeric carbon of N-glycosyl trichloroacetamides depends on the nature of the palladium-ligand catalyst. While the cationic Pd(II)-L-4 (2-trifluoroacetylphenol) complex promotes alpha-selectivity, the neutral Pd(II)-TTMPP-L-5 (4-chloro-2-trifluoroacetylphenol) complex favors beta-selectivity. The resulting alpha- and beta-N-glycosyl trichloroacetamides were further coupled with a diverse array of primary and hindered secondary nitrogen nucleophiles to provide the corresponding glycosyl ureas in moderate to good yields and with no loss of stereochemical integrity at the anomeric carbon. We have further demonstrated the utility of N-glycosyl trichloroacetamides as robust and versatile intermediates in the synthesis of unsymmetrical urea-linked disaccharides and trisaccharide.

Amino Acid and Peptide Synthesis and Functionalization by the Reaction of Thioacids with 2,4-Dinitrobenzenesulfonamides

Readily prepared amino thioacids react at room temperature in DMF in the presence of cesium carbonate with 2,4-dinitrobenzenesulfonamides to give amides. When the sulfonamide is derived from an amino acid the method results in peptide bond formation, whereas the use of carbohydrate derived sulfonamides gives neoglycoconjugates.

Palladium-catalyzed glycal imidate rearrangement: formation of alpha- and beta-N-glycosyl trichloroacetamides

A novel palladium(II)-catalyzed stereoselective synthesis of alpha- and beta-N-glycosyl trichloroacetamides has been developed. The alpha- and beta-selectivity at the anomeric carbon depends on the nature of the palladium-ligand catalyst. While the cationic palladium(II) promotes the alpha-selectivity, the neutral palladium(II) favors the beta-selectivity.

Sigmatropic rearrangements as tools for amino acid and peptide modification: application of the allylic sulfur ylide rearrangement to the preparation of neoglycoconjugates and other conjugates

Reaction of S-allyl cysteine derivatives, generated by the selenocysteine ligation, with rhodium carbenoids, stabilized and unstabilized, enables the attachment of diverse functionality onto cysteine residues. The reaction is successfully applied to the introduction of lipid-like residues, a fluorous alkyl chain, and mono- and disaccharides.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Synthesis of Urea-Tethered Disaccharides in Water

A new method for the synthesis of urea-linked disaccharides in aqueous media has been developed. The key feature of our approach is two strained Steyermark-type gluco- and galactopyranosyl oxazolidinones. Each oxazolidinone is attached to a pyranose ring in a di-equatorial trans-annulation framework. Reaction of these oxazolidinones with 4-aminohexopyranose in water proceeded smoothly to afford the urea-tethered cellobiose and lactose analogues. The galactose-type oxazolidinone proved to be more reactive than the glucose-type, which is explained by the presence of an axial hydroxy group at C4 in the former.

Enantiomeric impurities in chiral synthons, catalysts, and auxiliaries. Part 3

The enantiomeric excess of chiral reagents used in asymmetric syntheses directly affects the reaction selectivity and product purity. In this work, 84 of the more recently available chiral compounds were evaluated to determine their actual enantiomeric composition. These compounds are widely used in asymmetric syntheses as chiral synthons, catalysts, and auxiliaries. These include chiral alcohols, amines, amino alcohols, amides, carboxylic acids, epoxides, esters, ketones, and oxolanes among other classes of compounds. All enantiomeric test results were categorized within five purity levels (i.e. <0.01%, 0.01% to 0.1%, 0.1% to 1%, 1% to 10%, and >10%). The majority of the reagents tested were determined to have enantiomeric impurities over 0.01%, and two of them were found to contain enantiomeric impurities exceeding the 10% level. The most effective enantioselective analysis method was a GC approach using a Chiraldex GTA chiral stationary phase (CSP). This method worked exceedingly well with chiral amines and alcohols.

A facile stereoselective synthesis of α-glycosyl ureas

Alpha-glycosyl ureas can be synthesised directly from tetra-O-benzyl glycosyl azides and isocyanates, using a one-pot procedure that is simple and general in scope. The benzyl protecting groups are easily removed from the urea products by catalytic hydrogenation. The synthesised alpha-glycosyl ureas represent a new class of neo-glycoconjugates with the potential of being resistant towards carbohydrate processing enzymes.