Glycoside Helferich reaction

The application of diazo coupling is somewhat limited by the availability of the p-aminophenyl glycosides, particularly of those of the oligosaccharides. Their precursors, the p-nitrophenyl glycosides, are usually obtained by the condensation of a per-O-acetylated glycosyl halide with p-nitrophenol in ethanol (Michael reaction)19,20 or by the reaction of a per-O-acetylated sugar with p-nitrophenol in the presence of a Lewis acid catalyst (Helferich reaction).21 p-Nitrobenzyl 1-thioglycosides have also been prepared by the condensation of the... 

The other two general reactions for the synthesis of glycosides require the fully acetylated sugar as starting-materiaL In the Koenigs-Knorr reaction, this is next converted into the acetylglyco l halide, whilst in the Helferich reaction it is used directly. 

Knorr and Helferich reactions, the intermediates are prepared from the methyl ester of the uronic acid. / -Glycosides of D-glucuronic acid have been prepared by both reactions. Decomposition of the methyl ester occurs during the deacetylation. No great success has attended attempts to prepare the a-glycosides of D-glucuronic acid by the Helferich reaction. 

The Fischer- Helferich method, as a direct anomeric-oxygen replacement reaction (Scheme 1, path A), has been very successfully applied for syntheses of simple alkyl glycosides. However, because of its reversibility, it has not gained general importance in the synthesis of complex oligosaccharides and glycoconjugates (1). 

However, another application of the Fischer-Helferich method clearly exhibits its limitations. Long chain alkyl glycosides have gained interest as nonionic surfactants. Their technical synthesis is based on acid catalysis of the reaction between glucose and alcohol. Due to the inherent solubility problems, typical drawbacks of the Fischer-Helferich method are encountered the technical product consists of a mixture of different anomers and, due to the fact that excess alcohol cannot be used for solubilization of the reaction mixture, reaction of glucose with itself is also found. 

Reaction, Purification Conditions, and Yield for the Preparation of Fatty Acid Methyl Ester Acetylated Glycoside Amines [37a-h] According to the Helferich Method... 

Zinc chloride Aryl glycosides via aryl polyacylglycosides Helferich-Schmitz-Hillebrecht reaction... 

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