In order to prepare a [3]rotaxane w

In order to prepare a [3]rotaxane with a programmed tail-to-tail stereosequence, we designed thread 3b, the OH-bearing analogue of 2c, characterized by external alkylammonium units (Scheme 2). Clearly, in this case the orientation of the calixarene cavity toward the exterior should be favored leading to a tail-to-tail stereochemistry. In fact, the 1H NMR spectrum of a 1:2 mixture of TFPB salt of 3b and hexamethoxy-p-H-calix[6]arene 1a in CDCl3 showed again the presence of shielded alkyl resonances at negative values typical of endo-alkyl complexation and confirming the exclusive formation of tail-to-tail pseudo-[3]rotaxane (T,T)-6 (Scheme 2).