Pentenyl donor

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-... 

A representative example of this strategy is depicted146 in Scheme 21. The glycosyl donor 107 was glycosylidated with glycosyl acceptor 108 in the presence of triethyl-silyl (TES) triflate to produce disaccharide 109 in 89% yield. The acceptor 108 can be also converted into a glycosyl donor by simple acylation, to afford pentenyl donor 110... 

By Nucleophilic Displacement. - The Amadori compound 7 was synthesized by reaction of 2,3 4,5-di-0-isopropylidene-D-fructopyranose 1-triflate with the requisite primary amine, followed by acid-catalysed deprotection. Syntheses of 2-azido-2-deoxy-a-D-glucopyranosides of myo-inositol, involving either inversion at C-2 of the corresponding a-mannoside 2-triflate or direct glycosylation with a pentenyl donor, are further detailed in Chapter 10. ... 

We have reported several examples of regioselective glycosidation of diols 29). In order to facilitate comparisons, we maintained a level playing field by use of -pentenyl donors. Furthermore, manno donors were studied first since. 

Notes and discussion. Although it had been well documented that protecting groups could influence the reactivity of glycosyl donors [1, 2], Fraser-Reid et al. demonstrated for the first time that an activated pentenyl donor la could be selectively activated in the presence of a second, deactivated, pentenyl acceptor 2a. The disaccharide 3a thus obtained could be further activated to allow formation of a glycopeptide [32]. 

Trost coined the term chemoselective to describe the process where one of two similar functional groups is made to react, while the other is not affected or affected to a lesser extent [1]. This discrimination usually results from nuanced changes to a reagent s behavior, e.g., by addition of a salt. A typical example is the modification of sodium borohydride reduction by addition of cerium chloride in the Luche process, whereby the double bond of an ot-enone is not saturated during the reaction [85]. Similarly, a new approach to chemoselective reaction of n-pentenyl donors was soon to emerge. 

The observations depicted in Scheme 13a taught us the best n-pentenyl donors for assembling the pseMdotrisaccharide 109. And since NPOEs and disarmed NPGs display similar regio-preferences, we could now apply the NPOE/Yb(OTf)3 synergy (see Scheme 12) to enhance and improve upon the options in Scheme 13. 

It was reported that selective activation of the SBox glycosyl donors over SEt and O-pentenyl glycoside acceptors can also be achieved in the presence of AgOTf... 

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