Glycosylation conventional

Conventionally, the use of NaH or Cs2C03 as a base for the reaction of glycosyl hemiacetals with C13CCN often yields the thermodynamically favored a-glycosyl trichloroacetimidates, whereas the use of K2C03 often yields kinetically controlled P-glycosyl trichloroacetimidates (Scheme 3.26). The use of l,8-diazabicyclo[5.4.0] undec-7-ene (DBU) often provides a/p mixtures, mostly favoring a-products. 

The conventional thinking in organic synthesis indicates that primary hydroxyls are more reactive than secondary hydroxyls [96], Under this assumption, the glycosylation of triol 157 to furnish diol 158 by exclusive glycosylation at the... 

Although addition of activated phosphoramidite to hemiacetals of manno-pyranoses under thermodynamic control has been reported to deliver exclusively a-phosphates in some cases,43 anomeric mixtures with preponderance of a-anomer have been reported in other examples.10,44 Since formation of phosphorotetrazolidite is a rate-limiting step of the process, initial activation of phosphoramidite followed by addition of nucleophilic hemiacetal should accelerate condensation and favour the formation of the thermodynamic a-product. Indeed, reaction of hemiacetal 101 with dibenzyl phosphorotetrazolidite assured exclusive a-selectivity of the resulting glycosyl phosphate 102.43 The accumulation in the reaction mixture of mildly acidic 1H-tetrazole, which is liberated upon reaction of tetrazolidite with hydroxylic component, could also favour predominant formation of the a-phosphate (Scheme 18, A). Conventional hydrogenolysis afforded the a-mannosyl phosphate 103. 

The high specificity and stereoselectivity of enzymes, as well as the mild conditions under which they react, make enzyme-catalyzed reactions versatile tools in the synthesis of glycoconjugates. In some instances, an enzymic one-step transformation affords higher yields then the conventional and more-complex chemical synthesis. The application of enzymes in glycopeptide synthesis is under active development for selective deprotection and glycosylation purposes. 

Alkyl and glycosyl isocyanates and isothiocyanates are produced in good yield under phase-transfer catalytic conditions using either conventional soluble catalysts or polymer-supported catalysts [32, 33]. Acyl isothiocyanates are obtained under similar conditions [34]. A-Aryl phosphoramidates are converted via their reaction with carbon disulphide under basic conditions into the corresponding aryl isothiocyanates, when the reaction is catalysed by tetra-n-butylammonium bromide [35]. 

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