Glycosidic bond construction

In the carbohydrate chemistry arena, the Tsuji-Trost reaction has been applied to construct N-glycosidic bonds [53]. In the presence of Pd2(dba>3, the reaction of 2,3-unsaturated hexopyranoside 68 and imidazole afforded N-glycopyranoside 69 regiospecifically at the anomeric center with retention of configuration. In terms of the stereochemistry, the oxidative addition of allylic substrate 68 to Pd(0) formed the jc-allyl complex with inversion of configuration, then nucleophilic attack by imidazole proceeded with another inversion of the configuration. Therefore, the overall stereochemical outcome is retention of configuration. 

Besides the applications in the above-mentioned methodologies, trichloroaceti-midate chemistry has also been used in the past years to construct various oligosaccharides containing 1,2-cis linkages [456-465]. In some examples, notably by Kong and coworkers [460], 1,2-cis stereoselectivity was influenced by the glycosidic bonds originally present in either the donor or the acceptor. 

It has been shown that the stereochemistry of the glycosidic bond to which the carbohydrate component is attached at the neamine core is essential for antibacterial activity. The neomycin class aminoglycoside consists of a neamine core and a P-linked carbohydrate component attached at the 0-5 position, while the kanamycin class aminoglycoside consists of a neamine core with a-linked carbohydrate component attached at the 0-6 position. Since neamine is the pivotal component of both neomycin and kanamycin, a readily accessible library of nnnsnal sugars will provide opportunity for the facile construction of both classes of aminoglycosides via glycosylation approach. 

Danishefsky s total synthesis of vineomycinone B2 methyl ester [31] (Scheme 12.40) involves the construction of the sugar moiety possessing a C-glycosidic bond by a hetero Diels-Alder... 

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