Stereospecificity, glycosidic bond

The cationic polymerization of monomer 25 occurs via the initial coordination the electrophilic initiator, PF5, at the indicated ring oxygen atom to produce the oxonium ion 26. The latter is attacked by another molecule of 25 at its electrophilic site, C-1, to form a new oxonium ion 27. Repetition of this process leads to the consecutive formation of a set of glycosidic bonds in a stereospecific fashion as defined by inversion of configuration in every single attack at C-1. It is clear that the nature of the reaction and the structure of the monomer ensures both the desired stereo- and regiospecificity. Debenzylation of the final polymer 28 readily affords the target product 23. 

A different approach was used in the synthesis of another biopolymer, 1 - 6-glucan 29, possessing the p, rather than a, configuration of the intermolecular glycosidic bonds. The basic reaction employed in this approach is shown in Scheme 3.3. Here the necessary stereospecificity of formation of the jS-glucosidic bond, as shown in a model glucoside 30, is achieved by utilizing a triphenyl-methyl cation catalysed reaction of cyclic ketals of type 31 with trityl ethers 32, and proceeds via initial formation of the cyclic ion 33. ... 

In spite of highly stereospecific coupling reactions, the outcome of coupling is not completely specific and anomeric products are formed. In general the stereospecificity of a reaction is somewhat lowered by use of polymer-supported reactants. It is perhaps for this reason that no synthesis of a disaccharide with an o-glycoside bond has been made. 

When the anomeric hydroxyl group of one monosaccharide is bound glycosidically with one of the OH groups of another, a disaccharide is formed. As in all glycosides, the glyco-sidic bond does not allow mutarotation. Since this type of bond is formed stereospecifically by enzymes in natural disaccharides, they are only found in one of the possible configurations (a or P). 

---