O-Glycosidation Reactions

Thioglycosides are important glycoside donor for O-glycosidation reactions. Das et al demonstrated that the InCla-TiCU catalytic pair converts sugar acetates to thioglycosides efficiently, with moderate to excellent yields and mostly high selectivity for the P-anomer [150] (Figure 8.63). 

Capozzi G, Mannocci F, Menichetti S, Nativi C, Paoletti S. Versatile intermediate for complete a/p stereocontrol in O-glycosidation reactions. Chem. Commun. 1997 2291-2292. 

Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an —OH group) and an-... 

The use of trichloroimidates for the preparation of ethers is an effective method for O-alkylation of alcohols [27]. This method has found widespread use in the protection of alcohols as benzyl ethers since the corresponding trichlorobenzylimi-date is inexpensive and commercially available. The mechanism involves activation of the imidate with a catalytic amount of a strong acid (typically TfOH) which leads to ionization of the electrophile and the formation of carbocation which is rapidly trapped by an alcohol. For the preparation of sec-sec ethers, this protocol has been limited to glycosidation reactions, due to the SN1 nature of the reaction which often leads to diastereomeric mixtures of products [26],... 

Similarly, ketose sugars participate in polysaccharide formation by reaction of their ano-meric carbon with a hydroxyl of another monosaccharide to create a ketal linkage. The acetal and ketal bonds within polysaccharides are termed o-glycosidic linkages. 

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