A versatile, three-component-reaction route to N-glycosylamines

A Stereoselective Glycosylation Approach to the Construction of 1,2-trans-β-d-Glycosidic Linkages and Convergent Synthesis of Saponins

Conventional syntheses of 1,2-trans-β-d- or α-l-glycosidic linkages rely mainly on neighboring group participation in the glycosylation reactions. The requirement for a neighboring participation group (NPG) excludes direct glycosylation with (1→2)-linked glycan donors, thus only allowing stepwise assembly of glycans and glycoconjugates containing this type of common motif. Here, a robust glycosylation protocol for the synthesis of 1,2-trans-β-d- or α-l-glycosidic linkages without resorting to NPG is disclosed; it employs an optimal combination of glycosyl N-phenyltrifluroacetimidates as donors, FeCl₃ as promoter, and CH₂ Cl₂ /nitrile as solvent. A broad substrate scope has been demonstrated by glycosylations with 12 (1→2)-linked di- and trisaccharide donors and 13 alcoholic acceptors including eight complex triterpene derivatives. Most of the glycosylation reactions are high yielding and exclusively 1,2-trans selective. Ten representative, naturally occurring triterpene saponins were thus synthesized in a convergent manner after deprotection of the coupled glycosides. Intensive mechanistic studies indicated that this glycosylation proceeds by S_N 2-type substitution of the glycosyl α-nitrilium intermediates. Importantly, FeCl₃ dissociates and coordinates with nitrile into [Fe(RCN)_n Cl₂ ]⁺ and [FeCl₄ ]^- , and the ferric cationic species coordinates with the alcoholic acceptor to provide a protic species that activates the imidate, meanwhile the poor nucleophilicity of [FeCl₄ ]^- ensures an uninterruptive role for the glycosidation.

In situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement: copper-catalyzed synthesis of unsymmetrical diacylglycine esters

A novel in situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement of carboxylic acid, nitrile, and diazo compounds gave various unsymmetrical diacylglycine esters.

Recent advances in multicomponent reactions involving carbohydrates

Being an attractive class of naturally occurring molecules with fascinating properties, sugars and their derivatives have wide applications in different fields especially in drug discovery.

Ritter-type reaction of C-(1-bromo-1-deoxy-D-glycopyranosyl)formamides and its application for the synthesis of oligopeptides incorporating anomeric α-amino acids

O-Peracetylated or -perbenzoylated C-(1-bromo-1-deoxy-D-glycopyranosyl)formamides of D-gluco, D-galacto, and D-arabino configuration were reacted with Ag(I)-salts or HgO in nitrile solvents to give N-acyl-1-cyano-D-glycopyranosylamines with an axial C-N bond at the anomeric centre. In the presence of HgBr(2), Hg(CN)(2), or InCl(3) the anomer of the above glycosylamine with an equatorial C-N bond was also isolated or detected. In CH(3)NO(2) solutions as few as 5-10 equiv of the nitrile were sufficient to get acceptable yields for the products. Under similar conditions N-substituted C-(2,3,4,6-tetra-O-acetyl-1-bromo-1-deoxy-β-D-galactopyranosyl)formamides gave anomeric spiro-oxazoline derivatives which, upon mild acidic hydrolysis, opened up to di- and tripeptides of anomeric α-amino acids.

On the stereoselectivity of glycosidation of thiocyanates, thioimidates, and thioglycosides

Comparative side-by-side glycosylation studies demonstrated that glycosyl thiocyanates, thioimidates, and thioglycosides provide comparative stereoselectivities in glycosylations. Very high α-stereoselectivity that was previously recorded for glycosyl thiocyanates can be achieved, but only if glycosyl acceptors are equipped with electron-withdrawing acyl substituents. Partially benzylated glycosyl acceptors provided relatively modest stereoselectivity, which was on a par with other common glycosyl donors. Accordingly, thioimidates and thioglycosides showed high stereoselectivity similarly to that of thiocyanates with different classes of acylated primary and secondary glycosyl acceptors.

A Tuneable Method forN-Debenzylation of Benzylamino Alcohols

[reactions: see text] N-Iodosuccinimide provides a mild, convenient, and tuneable reagent for the selective mono- or didebenzylation in representative, multifunctionalized carbohydrate and amino acid derived N-dibenzylamines with neighboring O-functionality.

A novel neoglycopeptide building block

The synthesis of a neoglycopeptide building block is described. The key step is a cycloaddition where the chemistry is orthogonal to standard glycosyl transfer methodology. Also described is some exploratory chemistry of the building block.