Glycosides Synthesis of Oligosaccharides

As we saw in Section 23.14, the preparation of glycosides by acid-catalyzed condensation of a carbohydrate with a simple alcohol—the Fischer glycosidation— is thermodynamically controlled and favors the formation of pyranose over furanose rings. The anomeric effect causes the a stereoisomer to predominate over the p. 

When the desired glycoside is a disaccharide, however, the alcohol is no longer simple it is a carbohydrate with more than one OH group capable of bonding to the anomeric carbon of the other carbohydrate. Thus, constitutionally isomeric as well as stereoisomeric pyranosides are possible. 

Formation of the glycosidic C—O bond by a nucleophilic substitution in which an OH group of the glycosyl acceptor acts as the nucleophile toward the anomeric carbon of the glycosyl donor. 

Removal of the protecting groups from the protected disaccharide formed in stage 2. 

6-Tetra-O-benzoyl-a-D-glucopyranosyl bromide Gfycosyl Donor 

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Synthesis of Oligosaccharides on Solid Support Using Thioglycosides and Pentenyl Glycosides... 

Both transferases from the Leloir and non-Leloir pathways have been employed for the in vitro synthesis of oligosaccharides and glycoconjugates. Glycosidases have also been exploited for synthesis. The function of glycosidases in vivo is to cleave glycosidic bonds,... 

In early attempts directed towards the solid-phase synthesis of oligosaccharides, glycosidic bonds were formed by treating pyranosyl bromides with partially protected carbohydrates (Figure 16.16). These syntheses could be performed with either the pyranosyl bromide (glycosyl donor) [184,185] or the alcohol (glycosyl acceptor) [186— 188] linked to the support. 

The success of the mercuric acetate-glycoside syntheses led to the synthesis of oligosaccharides. He selected, for the alcoholic component, methyl... 

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