P-Nitrophenyl thioglycosides

Thus, it seems that the concept of anomeric electronic activation-deactivation at the anomeric center taking precedence over armed-disarmed in the remainder of the pyranose ring might have reasonable validity, but, in many instances, the difference in reactivity of the p-nitrophenyl thioglycoside versus the p-acetamidophenyl thioglycoside is not enough to make this work (Scheme 1l).94... 

An alternative could be steric activation-deactivation at the anomeric centers of the donor and acceptor. An example using this concept is shown in Scheme 12. In the electronic activation-deactivation concept, deactivated p-nitrophenyl thioglycosides can, after functioning as glycosyl acceptors, be transformed into... 

As shown, the p-nitrophenyl thioglycoside (109) was glycosylated with the thiophenyl galactoside (110) to give the 3-(1 4) hnked disaccharide (111). Fucosylation of the free 0-3 with... 

Thioamides 226 and 227, resulting from replacement of the amide carbonyl by thiocarbonyl in p-nitrophenyl 2-acelamido-2-deoxy-/3-D-gluco-and galacto-pyranosides, were shown to inhibit 2-acetamido-deoxy-/i-i> glucosidase from Turbatrix aceti.174 In contrast, the related p-nitrophenyl thioglycosides exhibited almost no inhibitory effect. In the light of the recent results of Knapp and co-workers,386 using the 4-methylumbelliferyl de-... 

Considering the observed differences in reactivities between ethyl and phenyl thioglycosyl donors, it was anticipated that selective activation could also be achieved when active p-A-acetamidophenyl thioglycosyl donors were used in the presence of latent p-nitrophenyl thioglycosides acting as acceptors. For this purpose, p-nitrophenyl 2,3,4-tri-0-benzoyl-l-thio- 3-D-galactopyranoside 17 was initially used as the acceptor (Scheme 7). 

The application of diazo coupling is somewhat limited by the availability of the p-aminophenyl glycosides, particularly of those of the oligosaccharides. Their precursors, the p-nitrophenyl glycosides, are usually obtained by the condensation of a per-O-acetylated glycosyl halide with p-nitrophenol in ethanol (Michael reaction)19,20 or by the reaction of a per-O-acetylated sugar with p-nitrophenol in the presence of a Lewis acid catalyst (Helferich reaction).21 p-Nitrobenzyl 1-thioglycosides have also been prepared by the condensation of the... 

In the next steps, the relative reactivities of armed and disarmed (active) p-Ai-acetamidophenyl 1-thioglycosides were established using perbenzoylated p-nitrophenyl 1-thioglycoside 17 as the acceptor (disarmed-latent) (Scheme 7). To this end, fully benzoylated and benzylated p-Ai-acetamidophenyl 1-thio-p-D-galactopyranosides 29 and 47 were glycosylated with 17 using TfOH in a 1.5 1 M ratio relative to the acceptor. Similar to the results obtained from 37 and 43,... 

Linear Hexasaccharides.— / Nitrophenyl a-maltohexaoside was made by enzymic transglycosylation (probably of enzyme bound maltopentaose) between maltohexaose and p-nitrophenyl a-Z)-glucopyranoside in water containing DMSO (40%) or butan-l-ol or propan-2-ol (20%). A methyl glycoside of the hexaose based on 4-deoxy-4-fonnylmethyl-o-2)-rhamnopyranose has been made by the thioglycoside procedure. ... 

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