Revisiting Wittig Olefination and Aza-Wittig Reaction for Carbohydrate Transformations and Stereocontrol in Sugar Chemistry

Recent advances in stereocontrolled Wittig olefination of sugars with stabilized and semi-stabilized phosphoranes are reviewed. Transformations of free and protected sugars for chain elongation at both reducing and terminal ends, and branched chain construction at the carbon atoms 2, 3 and 5 demonstrate the usefulness of this reaction in the preparation of a number of biologically active molecules and their precursors, starting from readily available sugars. Higher sugars, bicyclic structures, and C-C linked oligosaccharides lie amongst the diversity of compounds shown in this review that are synthesized by this reaction, easily carried out and efficient in terms of reaction yield and stereoselectivity, depending on reagents and reaction conditions. The analogy of aza-Wittig reaction, based on reaction of N-alkyl-triphenyl-λ5-phosphanimines with aldehydes or ketones, encouraged us to consider its contribution for the synthesis of N-linked sugars and imino sugars, which have biological and - some of them - industrial application as inhibitors of glycoprocessing enzymes. The use of this reaction for the preparation of pseudonucleosides with the N-linked appendages, N-linked and thiazolidinone-linked saccharides, and a diversity of imino sugars will be highlighted and the reaction stereocontrol discussed.