Prevention anomerization

Mannosides are difficult to obtain since here a 2-O-acyl group blocks the -position. 2-O-Benzyl-a-mannosyl bromides give, however, high yields of pure -glycosides with a heterogeneous silver silicate catalyst preventing anomerization and SnI reaction of the bromide H. Paulsen, 1981 B, Q. 

The main problem of these protocols is (3-elimination which occurs if a leaving group such as an alkoxy is present in the adjacent position. The elimination is prevented if the anomeric anion is strongly stabilized, for example by a nitro group,31 or in presence of a bad leaving group, such as the anions -CC32 or -NAc (Fig. 13).33... 

The gauche effect — which has the same electronic origin as that of the anomeric effect—is able to stabilize a conformation that should, a priori, be disfavored by steric factors. Thus, the gauche effect prevents the trans antiperiplanar conformation of two electronegative substituents borne by two vicinal carbon atoms (Figure 3.2). ° ... 

D-Glucose may be benzoylated with benzoyl chloride in pyridine/chloroform solution (Expt 5.108). Here temperature control is essential to prevent the formation of an anomeric mixture. 

Although Koenigs and Knorr isolated a small yield of methyl /3-n-gluco-pyranoside (as such) from a solution of tetra-O-acetyl-a-D-glucopyranosyl bromide in methanol that had stood at room temperature for several days, it is customary to add an acid acceptor to speed up the reaction and to prevent deacetylation of the product. Silver, in the form of the oxide or a salt, was the first acid acceptor to be employed, and is still the one in most common use. Unless the aglycon is a simple alcohol, it is usual to dissolve the reactants in a solvent, which is often an organic base to act as an additional acid acceptor. Walden inversion at Cl is almost the invariable rule when the reaction is done in the presence of silver ion. Under special circumstances, however, both anomeric glycoside acetates may be obtained. ... 

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