Threading of an Inherently Directional Calixarene Wheel with Oriented Ammonium Axles

The threading of monostoppered alkylbenzylammonium axles 7(+) and 8(+) with the calix[6]-wheel 3 can occur by both routes of entering the macrocyde 3 in the cone conformation: passage through the upper rim and the through the lower rim. Thus, under thermodynamic conditions, with both the axles 7(+) and 8(+), the two possible orientations of calix[2]pseudorotaxane, namely, endo-benzyl and endo-alkyl, are formed by a stereoselectivity controlled by the endo-alkyl rule. Interestingly, by H-1 NMR monitoring of the threading process between 8(+) and 3, we revealed two calix[2]-pseudorotaxane isomers in which the calix-wheel adopts 1,2,3-alternate and cone conformations, which represent the kinetic and thermodynamic species, respectively. Finally, the synthesis of ammonium-based oriented calix[2]rotaxane is here described.