Armed-disarmed

Transfer of the Armed-Disarmed Concept to Thioglycoside Glycosylation... 

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). 

Another efficient strategy, the armed-disarmed approach, developed by Fraser-Reid is based on the chemoselectivity principle [193,202]. According to this... 

Scheme 4.90 Arming-disarming properties of the SPyr glycosides [158].

Scheme 5.8 Armed/disarmed concept for chemoselective coupling of two w-pentenyl glycosides (NPGs). A 2-0-alkylated (armed) NPG 29 is much more readily activated than is a 2-0-acylated (disarmed) NPG 30.62...

The concept of armed/disarmed glycosyl donors was subsequently extended by other groups to thioglycosides21 and selenoglycosides.64 A similar strategy has been used by Friesen and Danishefsky to achieve chemoselectivity in electrophilic addition to glycal double bonds.65... 

Fraser-Reid and Armed/Disarmed Isopentenyl Glycosides... 

Scheme 11.3 Using a C2-protecting group for arming/disarming of an n-pentenyl glycoside for oxonium ion formation.

It was the use of NBS for NPG couplings that led to the discovery of the armed-disarmed protocol for oligosaccharide assembly [16]. Thus, it was observed that acyl-protected NPGs reacted more slowly than their alkyl-protected counterparts. These reactivity differences were analyzed, based on the inductive effect of the C-2 protecting group [18]. However, reactivity differences between substrates would not have been observed if NlS/Et3SiOTf had been the promoter because, under these conditions, both NPGs would have reacted by the time the TLC sample had been taken. 

In cases where the nature of the protecting groups does not allow the armed-disarmed strategy to be applied, two NPGs can still be coupled together by use of an intermediate dibromination step (see Scheme 7). Thus, depending on how the reaction is to... 

B. Fraser-Reid, Z. Wu, U. E. Udodong, and H. Ottosson, Armed/disarmed effects in glycosyl donors Rationalization and sidetracking, J. Org. Chem. 55 6068 (1990). 

Scheme 12.22. Synthesis of heptasaccharide 147 using the armed-disarmed glycosylation approach, byjona and co-workers [75],...

SCHEME 4.18 Selective glycosylation using the armed-disarmed glycosylation strategy. Cbz, benzyloxycarbonyl. 

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