Molecular Tweezers with Additional Recognition Sites.

A new synthetic access to molecular tweezers with one or two aliphatic phosphate ester groups in the central benzene spacer-unit is presented. Alkynyl ester groups offer the prospect to attach additional functional units by click chemistry and greatly broaden the scope of these tools for chemical biology. We present two alternative strategies: the trichloroacetonitrile method involves activation of only one OH group of each phosphoric acid substituent by way of trichloroacetimidate intermediates and subsequent introduction of an aliphatic ester alcohol moiety. The method is versatile, robust and combines simple workup with high yields. Mono- and disubstituted novel host structures are thus accessible in a convenient way. Alternatively, the phosphoramidite strategy activates the hydroquinone precursor by way of phosphoramidite intermediates and couples the desired ester alcohols followed by mild oxidation to the desired phosphate esters. Each step of the synthesis is carried out at very mild conditions and allows to combine sensitive host candidates and recognition elements. After neutralization of the phosphoric acids to water-soluble tri- and tetra-anions the cavities of the new tweezer derivatives are open to bind lysine and arginine as well as peptidic guests. The concept of introducing clickable alkynyl phosphates to free OH groups may be transferred to other major macrocyclic host classes to introduce additional recognition elements, biomolecules or fluorescence labels.