Acid-base modulation of a versatile heteroditopic calix[6]arene based receptor

A new calix[6]crypturea (3) has been efficiently synthesized through a domino Staudinger/aza-Wittig reaction followed by a [1 + 1] macrocyclization step. In comparison to the previously reported tren-based calix[6]crypturea, this heteroditopic receptor 3 displays a more flexible and larger tris-ureido cap. Due to this structural alteration, 3 exhibits unique host-guest properties: (i) the protonation of its basic cap leads to a rigidification of the whole structure and, thus, allosteric control of the binding properties and selective guest switching processes are possible, (ii) its versatility is unprecedented in the literature since it can bind either neutral molecules, anions, primary/secondary ammonium ions, quaternary ammonium ions or contact ion pairs according to different modes of recognition and with a remarkable selectivity within each family of guest, (iii) cascade complexes even stable in a protic environment can be obtained. These remarkable features are nicely illustrated by the fact that, according to the nature of its counterion, an ammonium ion R ¹R ²NH ₂ ⁺ can be accommodated into the cavity either as an independent guest, as a contact ion-pair or as a cascade complex. All these results are reminiscent of biological receptors and validate the strategy that consists of designing receptors presenting a high flexibility that can be controlled by an external stimulus.