Acid–base modulation of a versatile heteroditopic calix[6]arene based receptor†
Abstract
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