Sugar acceptors

In the initial studies, as briefly described earlier, the methyl a-2-thioglycoside of Neu5Ac 5 [22,24], or the -1 1 anomeric mixture 11 [26,27], which can be almost quantitatively prepared from 9 in one step, was coupled with lightly protected sugar acceptors such as 14,16, and 18 in the presence of DMTST in acetonitrile to give exclusively the sialyl a(2—>3)- or sialyl ot(2—>6)-D-galactose or lactose derivatives (20, 22, and 24) in 50-70% yields even in large-scale reactions [26],... 

The lactonized phenyl 2-thioglycosides of dimeric (26) [41] and trimeric (27) [42] sialic acids, which were prepared from the corresponding oligosialic acids according to the procedure employed for the monosialyl derivatives, were each coupled with the suitably protected sugar acceptors (14, 16, 17, and 19) by use of NIS-TfOH as a promoter in acetonitrile, as just described for the monosialyl glycoside synthesis (Fig. 3). 

Table 2 NIS-TfOH-Promoted Oligo-sialylation of Sugar Acceptors by Phenyl 2-thioglycosides of Dimeric and Trimeric NeuSAc in Acetonitrile...

The polyprenols obtained from tobacco and Arum maculatum (solanesol and spadicol), which are all E, are the exceptions. Another interesting fact is that, in most seed-plants, the polyprenols are allylic, as is bacterial undecaprenol. Saturated or slightly modified polyprenols, such as animal dolichol, seem to be present only in fungi. Any of those plant polyprenols, in a phosphorylated form, can be potential sugar acceptors for transfer reactions. Experiments were performed with phosphorylated allylic polyprenols and membrane preparations from Phaseolus aureus. D-Mannose from GDP-D-mannose was incorporated into the exogenous, polyprenyl phosphates,29,49 52 but it was not possible to establish unequivocally the role of these polyprenols as lipid intermediates. 

The first evidence showing that plant polyprenols involved in glycosyl-transfer reactions are not allylic, but a-saturated, like animal dolichol (2), came from Pont Lezica and coworkers.23 The authors postulated that the presence of sugar acceptors having the properties of a-saturated polyprenyl phosphates may be a general feature of eukaryotic cells, in contrast to the a-unsaturated polyprenyl phosphates characteristic of prokaryotic cells. 

V. N. Shibaev, L. L. Danilov, T. N. Druzhinina, L. M. Gogilashvili, S. D. Maltsev, and N. K. Kochetkov, Enzymatic synthesis of Salmonella O-specific polysaccharide analogs from modified polyprenyl pyrophosphate sugar acceptors, FEBS Lett., 139 (1982) 177-180. 

Nicolaou [47] has applied this methodology in synthetic studies towards the antibiotic everinomicin 13,384-1. Glycosylation of immobilized fluoride donors such as 258 (Scheme 65) with sugar acceptors allowed the synthesis of disaccharides such as 259, attached to solid support through a selenium fink-... 

Finally, it should be mentioned that there is also a field of research which studies how functional groups introduced at different positions of a sugar acceptor can affect selectivity. This should provide valuable information concerning the nature of the active site and binding characteristics of the enzyme. 

Kiso, M, Anderson, L, Synthesis of disaccharides by the ferric chloride-catalyzed coupling of 2-acylamido-2-deoxy-P-D-glucopyranose 1-acetates to protected sugar acceptors, Carbohydr. Res., 72, C15-C17, 1979. 

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