Thioglycosides disarmed” activation

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,... 

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... 

The armed-disarmed concept is tranferrable to having thioglycosides both as donors and acceptors91,92 (Scheme 9). Anomeric activation can, in favorable instances, also be used for this purpose93 (Scheme 10). 

Disarmed as well as armed (see Section V and Scheme 9) phenyl selenoglycosides are reported to be activated by silver triflate in the presence of potassium or silver carbonate in the presence of armed ethyl thioglycoside acceptors, to produce ethyl thiodisaccharides in excellent yields. 

Takeuchi, K, Tamura, T, Jona, H, Mukaiyama, T, A novel activating agents of disarmed thioglycosides, combination of trityl tetrakis(pentafluorophenyl)borate, iodine and 2,3-dichloro-5,6-dicyano-/7-benzoquinone (DDQ), Chem. Lett., 692-693, 2000. 

Initially, the method was based on the differential reactivity conferred on each of the partners by the nature of the protecting groups and by the intrinsic nucleophilicity of the thioaryl leaving groups. However, as the method evolved, it became clear that the choice of the promoter was also an important factor to take into consideration. Indeed, it was demonstrated that a disarmed thioglycoside could be activated in the presence of powerful thiophilic promoters such as A-iodosuccinim-ide/trifluoromethanesulfonic acid (NIS/TfOH), whereas it remained inactivated in the presence of weak thiophilic reagent such as iodonium dicollidine perchlorate or methyl triflate [43,45]. 

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