Glycoside Fischer reaction

Although the final equilibrium in the Fischer reaction favors the pyrano-sides, the furanosides appear to be formed first, and they can sometimes be isolated by performing the reaction under mild conditions and arresting it at an early stage. In the same way, advantage can sometimes be taken, at the expense of the yield, of the fact that the a and /3 anomers (furanose or pyranose) may be formed at different rates. If an alcoholic glycoside is refluxed with the alcohol and an acid catalyst, equihbrium between the different forms is re-established. 

With the exception of the Fischer reaction, the general methods employed for the formation of the glycoside bond result in the formation of glycoside acetates (or benzoates). Deacylation is accomplished by treatment with alkali, to which the glycoside bond is stable, unless the bond approximates to the ester type. The original method of deacetylation was by quantitative saponification with sodium hydroxide, potassium hydroxide, or barium hydroxide. It was noted, however, that, in anhydrous media, much less than the theoretical amount of alkali was required, and the reaction has since been made entirely catalyt.ic, with considerable gain in efficiency and convenience. The same methods have been employed for debenzoylation. 

Fischer glycosidation A reaction in which glycosides are formed by treating a carbohydrate with an alcohol in the presence of an acid catalyst. 

Preparation of Methyl o-Glucopyranosides by Fischer Glycosidation THE REACTION ... 

Emil Fischer discovered the synthesis of alkyl glycosides by reaction of glucose and alcohol in the presence of an acidic catalyst. During the course of the further development of alkyl polyglycosides, several laboratory methods have been developed for synthesis of a variety of substances and to facilitate study of their physicochemical properties [3,12-15]. The various syntheses... 

In the Fischer glycosidation, the reaction parameters temperature and pressure are closely related. To produce an alkyl polyglycoside low in secondary products, pressure and temperature have to be adapted to one another and carefully controlled. Low reaction temperatures (<100°C) in the acetalization lead to aUcyl polyglycosides low in secondary products. However, low temperatures result in relatively long reaction times (depending on the chain length of the alcohol) and low specific reactor efficiencies. 

Fischer projections and, 977-978 glycosides of, 989-990 hemiacetals of, 984-986 osazones from, 1013 oxidation of, 992-994 phosphorylation of, 991 reaction with acetic anhydride, 988... 

A number of glycosides were available to Fischer by way of the Koenigs-Knorr reaction and his own glycoside synthesis, which involves treatment of... 

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). 

The Fischer glycosidation of A-O-deoxy-D-fructos-l-yOaminomethy-lenemalonate (1448) was carried out in a 1.25% solution of hydrogen chloride in methanol at ambient temperature to produce a mixture of fructofuranosides (1449 and 1450) and fructopyranoside (1451). The reaction was complete after 5 hr, but the glycoside equilibrium was attained only after 24 hr (86MI10). 

It had been known since Emil Fischer s time that the glycosidation reaction was complex and that different products were obtainable imder... 

In the formation of glycosides by the Fischer process (equilibration in the presence of an acid catalyst), fucopyranosides are apparently formed more rapidly than galactopyranosides. In the presence of mineral acids, fucose attains equilibrium in boiling methanol within 6-8 h, whereas a reaction time of 20 h is customary for D-galactose. In reactions catalyzed by cation-exchange resins, maximum pyranoside formation is attained by 8 h for L-fucose,35a whereas D-galactose requires35612-24 h. 

---