A facile stereoselective synthesis of α-glycosyl ureas

Direct formation and isolation of unprotected α-and β-d-ribopyranosyl urea, α-and β-d-ribofuranosyl urea, and a ribosyl-1,2-cyclic carbamate in carbohydrate melts

Solvent-free melts of unprotected d-ribose and urea generated mainly C₁- substituted ribosyl products. The remarkable resolving power of a graphitised-carbon HPLC column allowed products of the reaction formed over a range of heating times and temperatures to be monitored. Heating an uncatalysed mixture of d-ribose and urea at temperatures between 75 °C and 90 °C resulted in complex mixtures of compounds; after 19 h heating at 90 °C, up to ten components could be resolved. At shorter heating times and lower temperatures, the composition and distribution of products varied. By manipulation of the reaction time and temperature, and with the addition of an acid catalyst, it was possible to optimise the yields of selected products. Thus, the acid-catalysed reaction after 1-2 h at 80 °C gave optimal yields of α- and β-d-ribopyranosyl urea, whereas the uncatalysed reaction after 22 h at 75-78 °C in addition produced significant amounts of α-d-ribofuranosyl-1,2- cyclic carbamate [glyco-1,2-oxazolidin-2-one] plus the α- and β-ribofuranosyl ureas. The five compounds were isolated and characterised, demonstrating the significant advantages of this approach; its simplicity, and the ability to produce multiple compounds of biological interest in a single step. LC/MS was used to identify tentatively several other components of the reaction mixture. The unprotected title compounds were prepared, isolated and characterised with water as the only solvent.

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.

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.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

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.