Glycosidation, Fischer

An aldose or kelose is simply maKd in lf in the presence of an acid (protonic or Lewis) with ihe appropriate alcohol. 

This produces a mixture of ol- and (3-furanosides and pyranosides. Analysis of the progress of methanolysis has shown that competing equilibria are set up between. 

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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). 

Preparation of NPGs may be carried out by standard procedures for making alkyl glycosides (Scheme 3) [6]. The most direct method is the Fischer glycosidation (5 —> 1, Scheme 3), in which the aldose is treated with n-pentenyl alcohol [8] and an acid catalyst... 

P. Konradsson, C. Roberts, and B. Fraser-Reid, Conditions for modified Fischer glycosidation with n-pentenol and other alcohols, Reel. Trav. Chim. Pays-Bas. 7/0 23 (1991). 

Interestingly, the original Fischer glycosidation satisfies the first four conditions admirably well using the simplest of glycosyl donors namely, the free sugar itself. 

Ballou and Fischer393 treated D-m<2 o-hexodialdose and its 2,3 4,5-di-O-isopropylidene derivative with methanol under Fischer glycosidation conditions, and obtained not only glycosides, but also some dimethyl acetals, and a crystalline difuranoside (probably the a,a anomer) was obtained in 20% yield. 

Arasappan and Fraser-Reid described the preparation of w-pentenyl galactofuranosides and evaluated their prospects as glycosyl donors (O Scheme 21) [81]. Fischer glycosidation of D-galactose under kinetic conditions using -pentenyl alcohol and DMSO as co-solvent [82]... 

The hydrochloride catalyzed Fischer glycosidation with alcohols... 

In the Fischer glycosidation reaction, we can imagine that the role of the acid catalyst is to protonate the anomeric hydroxyl, thus transforming it into the leaving group (Fig. 3.2), which would facilitate nucleophilic substitution. In the... 

In summary, alkyl glycosides with an axial aglycon are easily obtained using the Fischer glycosidation, whether they are 1,2-cis or 1,2-trans. The 1,2-trans diequatorial alkyl glycosides result from participating reactions. This leaves the... 

The reader will have perhaps wondered why, in the Fischer glycosidation reaction, there is no acetalation by two alcohol functions of the sugar molecule, leading to a completely internal and bicyclic mixed acetal. As a general rule, an intramolecular reaction is faster than the intermolecular analogue and this type... 

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