“The red cage”: implementation of pH-responsiveness within a macrobicyclic pyridinium-based molecular host

We present herein the implementation of pH-responsiveness into a new polycationic macrobicyclic structure, namely what we have termed the “red cage”. The hydrolytically-stable cryptand-like compound has been prepared in a relatively high yield in aqueous media by a kinetically-controlled hydrazone-exchange reaction, promoted by the unusual high stability of the new hydrazone C 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000N bonds formed. In organic media the macrobicycle was found not able to complex model aromatic substrates. In buffered aqueous solutions, as a comparison, the “red cage” was found able to recognize them, but the binding was observed to be more efficient in acidic form of the cyclophane compared with its conjugate base.