Amino Sugars and Glycosylamines as Tools in Stereoselective Synthesis

Carbohydrates contain a high density of functional and stereochemical information in one molecular unit. They have been exploited as enantiopure starting materials in numerous ex-chiral pool syntheses. During the past decade, the potential of carbohydrates in terms of stereodifferentiating auxiliaries has been recognised. Glycosylamines proved to be efficient auxiliaries in stereoselective Strecker syntheses of α-amino nitriles and Ugi reactions yielding α-amino acid amides. Mannich-reactions of glycosyl imines with silyl ketene acetals resulted in the formation of β-amino acid derivatives with high diastereoselectivity. N-Glycosyl imines unfold their high potential in stereoselective reactions, in particular, in Mannich-Michael reaction sequences with 2-silyloxy-butadiene derivatives furnishing 2-substituted piperidinone derivatives with high asymmetric induction. On the basis of this efficient stereodifferentiation on glycosyl imines a number of alkaloids of the piperidine, indolizine, quinazoline, and decahydro-quinoline series have been synthesised in enantiomerically pure form. The N-glycosyl auxiliary has been demonstrated to control stereoselective protonation of enolates, for example, in alternative formation of cis-annulated perhydro-quinoline alkaloids typical for toxins of Dendrobates pumilio or trans-annulated analogues characteristic for toxins from Dendrobates histrionicus. Derivatives of amino sugars have been also investigated as components of enantioselective catalysts, e.g. palladium(0)-catalysed allylation reactions, allylic oxidation processes and Strecker syntheses.