Glycosylations participating solvent

Some solvents may form complexes with the oxocarbenium ion intermediates, thereby affecting the anomeric outcome of a glycosylation. For example, diethyl ether is known to increase the a anomeric selectivity. Probably, diethyl ether participates by the formation of diethyl oxonium ion (Scheme 4.9a). The (3 configuration of this intermediate is favoured because of the operation of the reverse anomeric effect (see Chapter 1). Nucleophilic displacement with inversion of configuration will give an a glycoside. Recently, it was shown that a mixture of toluene and dioxane provides a more efficient participating solvent mixture. 

If one can find ways to effect SN2 reactions of glycosyl halides with oxygen nucleophiles (e.g., sugar alcohols), selective access to both sets of anomeric halides could prove invaluable. Other studies which employ SN2 chemistry at glycosidic centers include the halide-assisted glycosylation developed by Lemieux et al.,45 the use of a participating solvent such as acetonitrile,46 and exploitation of torsional control of anomeric reactivity.47 This latter approach has been very successfully exploited by Crich and Sun in the synthesis of P-mannosides (Fig. 19).48... 

Because of the absence of a participating group at C-2 of donor 57, and in order to obtain the f>-( 6) linkage, glycosylation was performed in a participating solvent such as acetonitrile (nitrile effect).116 The condensation of benzyl (2,3,5,6-... 

A participating solvent can also control the stereochemical outcome of a glycosylation. A marked example is the use of acetonitrile, which in many cases leads to the formation of mainly equatorial glycosidic bonds (e.g. P-glucosides and p-galactosides). Several groups have independently proposed that this... 

The participation of solvents plays an important role. The use of non-polar solvents, favours the glycosylation reaction to follow a S -like... 

The influence of solvents in glycosylation reactions has been observed and discussed extensively already (1,4,74). For instance, the participation of ethers, when anomeric leaving-groups are removed under SNl-type conditions, results [because of the reverse anomeric effect (75,76)] in the genera-... 

---