Anomeric alkylations and acylations of unprotected mono- and disaccharides mediated by pyridoneimine in aqueous solutions

Trivalent dialkylaminopyridine-catalyzed site-selective mono-O-acylation of partially-protected pyranosides

This work demonstrates trivalent tris-(3-N-methyl-N-pyridyl propyl)amine (1) catalyzing the site-selective mono-O-acylation of glycopyranosides. Different acid anhydrides were used for the acylation of monosaccharides, mediated by catalyst 1, at a loading of 1.5 mol%; the extent of site-selectivity and the yields of mono-O-acylation products were assessed. The reactions were performed between 2 and 10 h, depending on the nature of the acid anhydride, where the bulkier pivalic anhydride required a longer duration for acylation. The glycopyranosides are maintained as diols and triols, and from a set of experiments, the site-selectivity of acylations was observed to follow the intrinsic reactivities and stereochemistry of hydroxy functionalities. The trivalent catalyst 1 mediates the reactions with excellent site-selectivities for mono-O-acylation product formation in the studied glycopyranosides, in comparison to the monovalent N,N-dimethylamino pyridine (DMAP) catalyst. This study illustrates the benefits of the multivalency of catalytic moieties in catalysis.

Stereoselective protecting-group-free synthesis of alkyl glycosides using dibenzyloxy triazine type glycosyl donors

Chemical O-glycosylation is a key step for the synthesis of sugar-containing molecules such as glycolipids. However, traditional carbohydrate chemistry is characterized by extensive use of protective groups, resulting in laborious manipulations and poor atom economy. Here, we present a protecting-group-free glycosylation strategy employing dibenzyloxy-1,3,5-triazin-2-yl glycosides (DBT-glycosides) as glycosyl donors. The DBT-glycosyl donors could be prepared directly through an alkaline nucleophilic substitution from unprotected sugars in aqueous media. The O-glycosylation of alcohols by using DBT-glycosyl donors has been carried out under mild hydrogenolytic conditions, affording the corresponding alkyl glycosides stereo-selectively in good yields.

Synthesis and Studies of Pyridoneimine-Functionalized PETIM Dendrimers

Pyridinoimine-functionalized poly(ether imine) (PETIM) dendrimers of 1-3 generations, possessing 4-16 moieties at the peripheries, are synthesized. Chloride-functionalized dendrimers are reacted with N-methylamino pyridine, under basic conditions, which led to functionalization of the peripheries of a dendrimer with pyridoneimine moieties. Variable-temperature 1H NMR studies are performed to assess the contributing resonance forms of pyridoneimine in the dendrimers. Solvatochromism and 15N NMR studies aid further the assessment of the contributing resonance forms. Comparison with derivatives that possess 1 and 2 pyridoneimines illustrates the contributing resonance forms between nonaromatic pyridoneimine and zwitter ionic aromatic imidopyridinium species.

Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Pyridoneimine-catalyzed anomeric aqueous oxa-Michael additions of native mono- and disaccharides

A pyridoneimine-catalyzed oxa-Michael addition of protecting groups-free, native mono- and disaccharides with Michael acceptors in aq. solution is reported. Several mono- and disaccharides are reacted with acceptors, namely, methylvinyl ketone, acrylonitrile and tert-butyl acrylate in aq. solution, the addition catalyzed by n-pentylpyridone imine. The addition occurs site-selectively at the anomeric lactol and the remaining hydroxy functionalities are un-affected. The resulting keto-glycopyranoside products are explored in aldol, allylation and oxime product formation, occurring at either α-methyl moiety or at the keto-moiety, with appropriate synthons. In another direction, the keto-glycopyranoside is functionalized further with amino acids through reductive amination in aq. methanol solution. Formation of hemiacetal anion occurs in the presence of pyridoneimine in aq. Solution, enabling subsequent addition to occur with acceptors. Facile reductive amination of the resulting keto-glycoside provides an avenue for conjugations with amino acids in the present work.