Four into One: Organocatalyzed Stereoselective Conjugate Addition of Unprotected and Unactivated Carbohydrates

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.

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

A site-specific deprotonation followed by alkylations and acylations of sugar hemiacetals to the corresponding alkyl glycosides and acylated sugars in aqueous solutions is disclosed herein. Pyridoneimine as a new base is developed to mediate the deprotonation of readily available sugar hemiacetals and further reactions with alkylation and acylation agents.

Organocatalysis applied to carbohydrates: from roots to current developments

Organocatalysis emerged in the last decade as a powerful tool for the synthesis of complex molecules. In the field of carbohydrates, it found widespread use in the synthesis of rare and non-natural carbohydrate derivatives. Additionally, it has also found important application in the stereoselective functionalization of the anomeric carbon in glycosylation reactions. These efforts culminated in the development of different types of catalysts operating through distinct activation modes that allow the selective synthesis of α- or β-glycosides even on daunting substrates. All these advances starting from its first examples in carbohydrate synthesis to the current developments in glycosylation reactions are reviewed.

Racemic hemiacetals as oxygen-centered pronucleophiles triggering cascade 1,4-addition/Michael reaction through dynamic kinetic resolution under iminium catalysis. Development and mechanistic insights

2-Hydroxydihydropyran-5-ones behave as excellent polyfunctional reagents able to react with enals through oxa-Michael/Michael process cascade under the combination of iminium and enamine catalysis. These racemic hemiacetalic compounds are used as unconventional O-pronucleophiles in the initial oxa-Michael reaction, also leading to the formation of a single stereoisomer under a dynamic kinetic resolution (DKR) process. Importantly, by using β-aryl or β-alkyl substituted α,β-unsaturated substrates as initial Michael acceptors either kinetically or thermodynamically controlled diastereoisomers were formed with high stereoselection through the careful selection of the reaction conditions. Finally, a complete experimental and computational study confirmed the initially proposed DKR process during the catalytic oxa-Michael/Michael cascade reaction and also explained the kinetic/thermodynamic pathway operating in each case.