Synthesis of Urea-Tethered Disaccharides in Water

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.

(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.

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.