Glycosyl phosphate formation

Figure 25.7 Glycoprotein formation occurs by initial phosphorylation of the starting carbohydrate to a glycosyl phosphate, followed by reaction with UTP to form a glycosyl uridine 5 -diphosphate. Nucleophilic substitution by an -OH (or -NH2) group on a protein then gives the glycoprotein.

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. 

Scheme 6.5 Glycosyl phosphates functioning as glycosyl donors in P-selective formation of...

Scheme 6.9 Formation of P-mannosides using glycosyl phosphates...

It seemed of interest to check whether the type of biosynthetic mechanism involved may be related to the structure of the polymeric chain formed. In the block mechanism of chain assembly, formation of polyprenyl glycosyl diphosphates through transfer of glycosyl phosphate is a necessary step of the process. Thus far, only UDP-activated monosaccharides have been found to participate in this reaction (see Section 11,3). Consequently, the presence of a monosaccharide residue of this group in the main chain of a polymer may be regarded as a necessary condition for assembly of the chain through the block mechanism. 

Although the anomeric configuration of the glycosyl phosphate in natural lipid A had not been elucidated at that time, it was assumed to be a and we employed a sufficient long reaction time for this phosphorylation to secure formation of a-phosphates in the following synthesis. Meanwhile the natural glycosyl configuration was established to be a (6) and our choice is fortunately correct. 

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