Oligosaccharides armed-disarmed protocol

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. 

A major improvement of the NPGs method involved the discovery of the armed-dis-armed protocol for oligosaccharide assembly [149]. This finding came from the observation that acyl-protected NPGs (disarmed) reacted more slowly than the corresponding alkyl-protected (armed) derivatives. This difference, which can be rationalized on the bases of the inductive effect of the C-2 group, has been successfully exploited for the synthesis of trisaccharides (Scheme 29). 

---