The development and mechanistic investigation of a highly stereoselective methodology for preparing alpha-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the stereoselective nickel-catalyzed transformations of glycosyl trichloroacetimidates to the corresponding alpha-trichloroacetamides. The alpha-selective nature of the conversion is controlled with a cationic nickel(II) catalyst. Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2-ether functionality of the alpha-glycosyl trichloroacetimidate to be paramount for achieving an alpha-selective 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 alpha-glycosyl trichloroacetamide products into the corresponding alpha-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only alpha-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric center

Scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent conversion to alpha-urea-glycosides

248th National Meeting (2014)

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