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 undergo a conformational change when 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. These glycosylations suggested a decrease in reactivity when binding Zn2+. The conformational effect of binding Zn2+ was therefore studied using a third glycosyl donor, unable to undergo conformational changes when binding Zn2+. From competition experiments, it was observed that the binding-induced conformational change increased the reactivity slightly compared to the glycosyl donor unable to undergo a conformational change.