Although anomerisation reactions have received considerable attention in carbohydrate chemistry, the anomerisation of glycosyl fluorides has not been reported hitherto. It is well known that treatment of acylated sugars with liquid hydrogen fluoride' gives the thermodynamically stable a-acylglycosyl fluorides. Under the strongly acidic conditions employed, epimerisation of the neighbouring substituents often takes place2. We now report on the anomerisation of glycopyranosyl fluorides under mild conditions.
In contrast to the easy SN2 anomerisation of glycosyl chlorides and bromides in the presence of the corresponding tetrabutylammonium halides3, treatment of 2,3,4,6-tetra-O-acetyl-P-D-glucopyranosyl fluoride (la) with tetrabutylammonium fluoride did not give the cY-anomer (lj?), whereas the reaction of lcu with classic anomerising agents (TiCI,, SnC14, BF3 -EtzO) resulted in the rapid formation of a mixture of D-glUCOSC tetra-acetates. However, by using g.1.c. and t.l.c., it was possible to detect, in the former experiment, traces of l/3, evidently arising from cations 5 and 6 (&l), but variations of solvent, temperature, and concentration of Lewis acid did not increase the yield of 18.
An increase in the concentration of F-in the reaction mixture would be expected to shift the equilibrium towards 18. Recently4, it has been shown that, in reactions with acylglycosyl chlorides, AgBF4 can be used as an organic solventsoluble F-donor. When solutions of la and AgBF, in aromatic solvents were boiled under reflux, 63% of l/3 was obtained. The reaction proceeded smoothly, but there was a considerable induction period which could be attributed to the need for the formation (AgBF 4 8 AgF + BF3) of microquantities of BF3 to initiate anomerisation.
The conversion of lay into l/3 took place easily at room temperature (50% yield) if traces of anhydrous HCl or BF3 . Et,0 were added to a solution of la and AgBF4 in benzene, which confirms the mechanism mentioned above. The reaction rate depends on the concentration of BF3 and solvation decreases it considerably.