P-mannoside bond

Acetonitrile is another participating solvent, which in many cases leads to the formation of an equatorially linked glycoside [125-131], It has been proposed that these reactions proceed via an a-nitrilium ion intermediate. It is not well understood why the nitrilium ion adopts an axial orientation however, spectroscopic studies support the proposed anomeric configuration [130,131], It is known that nucleophilic substitution of the a-nitrilium ion by an alcohol leads to P-glycosidic bonds and the best P-selectivities are obtained when reactive alcohols at low reaction temperatures are employed. Unfortunately, mannosides give poor anomeric selec-tivities under these conditions. 

Xanthan, the best studied of the exopolysaccharides, provides a good example of the complexity and specificity of enzymatic attack. Xanthan has a P-l,4-glucosidic (Glu) backbone containing branch points at the 3-positions of every fourth unit. The branches are trisaccharides with a-1,2-mannose acetate, P-l,4-glucuronic acid and P-mannoside (Man) units. Each of the different main chain, branch and side chain bonds can be hydrolyzed by only a specific enzyme, as illustrated below ... 

In order to access only the required stereoisomer, it is essential that the geometries of the glycosidic bonds are controlled. Some linkages are particularly difficult to form, for example p-mannosidic linkages, even though they are abundant within... 

These substitutions proceed in moderate yields because of repulsive electrostatic interaction between the incoming nucleophile and the lone pair of the ring oxygen atom [152] and non-bonded steric interactions in the transition state [150]. Extension of the repertoire of leaving groups has alleviated these problems and made the direct displacement a powerful approach to P-mannoside synthesis. 

The exo and endo isomers of benzylidene acetals of the carbohydrate series can be obtained separately. Their behavior on reduction with HzAlCl accords with the analysis outlined above frequently only one product is obtained. Thus, the 1,2,4,6-di-O-benzylidene derivatives of glucose (42) and (43 R = H or Bn) cleave rapidly with 1 equiv. of H2AICI at 0 °C (30 min for 42 and 10 min for 43) at the marked bonds." Here, at least, it is clear that steric effects outweigh polar ones. The 2,3-endo and -exo isomers of the a-D-mannoside series (44) and (45) likewise cleave mainly equatorial (78 22) and axial (96 4), respectively, when treated with 1 equiv. of H2AICI at room temperature. The same behavior is seen with the methyl 2,3-benzylidene-a-L-rhamnosides," the methyl 3,4-0-benzylidene-P-L-arabinosides" and... 

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