Formation of Glycosides

Levene and coworkers, 1 using the ease of hydrolysis (10 minutes in 0.1 N hydrochloric acid at 100°) of the aldofur anosides as an analytical tool, studied the rate of glycoside formation (see Fig. 1) and confirmed the preliminary formation of aldofuranosides and subsequent conversion into aldopyranosides. 

These polymerizations hold great interest, because of the pronounced tendency for steric control of glycoside formation. 

The mechanism of glycoside formation is the same as the second part of the mechanism for acetal formation. Propose a mechanism for the formation of methyl /3-D-glucopyranoside. 

The mechanism for glycoside hydrolysis is just the reverse of glycoside formation. It involves two parts formation of a planar carbocation, followed by nncleophilic attack of H2O to form anomeric hemiacetals, as shown in Mechanism 27.2. 

N. M. Spijker, C. A. A. van Boeckel, Double stereodifferentiation in carbohydrate coupling reactions The mismatched interaction of donor and acceptor as an unprecedented factor governing a/fi ratio of glycoside formation, Angew. Chem. Int. Ed., 30 (1991) 180-183. 

Based on die highly developed art of oligosaccharide synthesis, orgamc-chemical, enzyme-catalyzed, and combined organic-enzyme-catalyzed syntheses of the Sialyl-Lewis structure and related oligosaccharides have been devised. Selected examples of these syntheses are discussed in Section IV, some of them including recently developed procedures of glycoside formation. 

The study of glycoside formation and breaking in the gas phase is not as esoteric as it might appear. The very short lifetimes of glycosyl oxocarbenium ions in solution require that their study be via indirect competitive kinetic experiments. Knowledge of the barrier for capture of these species, whether as free oxocarbenium ions or as ion molecule pairs, is difficult to obtain by direct methods. On the other... 

Depending on the stereochemical course of glycoside formation, i.e., whether retention or inversion of the configuration at the anomeric center is observed, glycosidases operate via two separate and distinct mechanisms (Schemes 2.221, 2.222)... 

The mechanism of glycoside formation is the same as that described in eq. 9.13 of Section 9.7. The acid catalyst can protonate any of the six oxygen atoms, since each has unshared electron pairs and is basic. However, only protonation of the hydroxyl oxygen at C-1 leads, after water loss, to a resonance-stabilized carbocation. In the final step, methanol can attack from either face of the six-membered ring, to give either the /3-glycoside as shown or the a-glycoside. 

The Fischer procedure is particularly good for the preparation of the alkyl pyranosides although a few crystalline furanosides have been obtained in this manner (9), Better methods for the furanosides are described later. The disaccharides are partially hydrolyzed under the conditions of glycoside formation as are also the acetyl groups of acetylated sugars. 

The mechanism for this process is analogous to that of glycoside formation. Two carbohydrates, in particular, D-ribose and 2-deoxy-D-ribose, form especially important A -glycosides. 

A more stereospecific course of glycoside formation by reaction of the polymer-bound C-1 bromide with the second monosaccharide hydroxyl component involves the initial formation of a c -l 2-orthoester. This undergoes an acid-catalyzed stereospecific isomerization (Kochetkov et al., 1967) to the corresponding glycoside (disaccharide). 

Synthesis of Monosaccharide Glycosides.- A review has been published on the synthesis of glycopeptides which covers aspects of glycoside formation and the linkage between sugars and serine or serine-containing peptides. ... 

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