Donors, armed/disarmed

Entry Donors " armed/disarmed nis (equiv) Product ratio 216 217 Yieid (%)... 

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). 

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... 

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). 

Ley and coworkers also showed that the degree of arming/disarming is a continuum and proposed that identification of donors with several different levels of reactivity would make it possible to prepare rather complex oligosaccharides by a one-pot procedure without the need for intermediate purifications. Subsequently, Chi-Huey Wong and coworkers reported a comprehensive study determining relative reactivities of... 

Ley and coworkers reported the one-pot synthesis of tetrasaccharide 176 by combining selective activation with reactivity-based armed-disarmed glycosylation [113]. As shown in Scheme 6.35, the first chemoselective coupling was achieved via selective activation of the fluoride donor 173 over the selenoglycoside acceptor 174. The resulting disaccharide intermediate was more armed than the bifunctional... 

In searching for the origin of the regioseiectivity observed in the formation of trisaccharides 206 and 212 (O Scheme 28 and Scheme 29) several factors were considered. The reactions in O Scheme 28 and Scheme 29c were carried out with excess NIS promoter, conditions under which the intermolecular halonium ion transfer (responsible for the armed-disarmed effect) is not operative. A study of the three types of -pentenyl donors indicated that their relative reactivities were in the order NPOE > armed > disarmed (e. g. 202a > 64 > 200) [111]. Therefore, the most and the least reactive donors have chosen their preferred —OH in the final trisac-... 

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