A-Glycoside synthesis

In conclusion, it appears unlikely that a general method for stereoselective a-glycoside synthesis will be found. 

A Glycoside synthesis via reverse hydrolysis (thermodynamic control... 

The application of these observations has also led to many excellent results in A -glycoside synthesis. (The discussion of the special aspects of N- and C-glycoside synthesis is beyond the scope of this chapter.) However, severe, partly inherent disadvantages of foe Koenigs-Knorr method could not be overcome. These include foe low thermal stability and the sensitivity to hydrolysis of many glycosyl halides, and the use of at least equimolar amounts of heavy metal salts as catalysts , which are partly toxic or even explosive, to mention only a few aspects. This had led to an increased search for improved methods applicable also to large scale preparations. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Stereoselective inverse-demand hetero (4 + 2) cycloadditions. A Chiral Template for C-Aryl Glycoside Synthesis. Chiral allenamides2 4 had been used in highly stereoselective inverse-demand hetero (4 + 2) cycloaddition reactions with heterodienes.5 These reactions lead to stereoselective synthesis of highly functionalized pyranyl heterocycles. Further elaboration of these cycloadducts provides a unique entry to C-aryl-glycosides and pyranyl structures that are common in other natural products (Scheme 1). 

Deslongschamps P (1993) In Thatcher GJ (ed), The Anomeric Effect and Associated Stereoelectronic Effects. American Chemical Society, Washington, DC, p 26 Recent reviews on C-glycoside synthesis (a) Du Y, Linhardt RJ, Vlahov IR (1998) Tetrahedron 54 9913 (b) Levy DE, Tang C (1995) The Chemistry of C-Glycosides. Elsevier Science, Oxford... 

After his doctoral graduation, Helferich became Emil Fischer s personal assistant for two years, and, from 1913 onwards, a teaching assistant. Together with Fischer, he published a series of papers on glycoside synthesis during this period. His scientific career was then interrupted by the First World War, in which he served as an officer throughout. 

For conversion of amide to other acid derivatives, a novel synthesis of urea glycosides in aqueous media has been reported via the reaction of Steyermark s glucosyl carbamate with amines in good yields (Eq. 9.17).38 This method was successfully applied to develop a new route to the synthesis of urea-tethered neo-glycoconjugates and pseudooligosaccharides. 

J.-C. Florent and C. Monneret, Stereocontrolled route to 3-amino-2,3,6-trideoxy-hexopyranoses. K-10 Montmorillonite as a glycosidation reagent for acosaminide synthesis, J. Chem. Soc., Chem. Commun. (1987) 1171-1172. 

Scheme 8.7. Nucleoside synthesis, glycoside synthesis, and aromatic alkylation with a dielectrophilic donor.

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