Conformationally Switchable Glycosyl Donors.

Glycosyl donors functionalized with 2,2'-bipyridine moieties on the 3-OH and 6-OH or the 2-OH and 4-OH undergoes a conformational change forming 1:1 complexes with Zn2+ ions. The pyranoside ring of the zinc complexes adopted axial-rich skew boat conformations. The reactivities of the two glycosyl donors were investigated by performing a series of glycosylations in the presence or absence of Zn2+ ions. The effect of binding Zn2+, on the reactivity, was studied using a third glycosyl donor, unable to undergo conformational changes when binding Zn2+. From competition experiments it was observed that the binding of Zn2+ increased the reactivity, when an axial rich conformation could be adopted, i.e. a conformational arming and an electronically disarming in balance.

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