Synthesis of Upper Rim Covalently Linked Double Calix[6]arenes

A series of new double calix[6]arenes obtained through the 1,3,5-upper rim linkage of two calix[6]arene units with imino or 1,4-phenylendiimino bridges has been synthesised. These compounds were conceived as hosts for selected organic cations but since they are molecular cages with different grades of accessibility of the internal cavity, they gave some insight on the way 1,3,5-trimethoxycalix[6]arenes interact with guests and, in particular on the pathway guests choose to access the cavity. 1H NMR studies showed that the distance between the two calixarene caps and the dimension of the equatorial portals are the key control elements on the efficiency and selectivity of binding. It was thus hypothesised that the entrance of the guests into the cage takes place exclusively from the equatorial portals and, in general, from the upper rim of the calix[6]arene.