Traffic control: By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure).The possibility of obtaining full control on the direction of axle threading in calix[6]arene wheel 1 either from its upper or lower rim was evaluated in solution. To this aim, we prepared nonsymmetric axles characterised by a 4,4'-bipyridinium recognition unit with two alkyl side chains, one of which terminates with a stopper, and the other with either ammonium (2), hydroxy (3) or methyl (4 and 5) head groups. When the axles were mixed with 1 in apolar solvents at room temperature, the formation of oriented pseudorotaxanes derived from the threading of the axles from the upper rim was observed. The stability constants of such complexes are in the order of 10(7) m(-1) and are almost independent of the type of axle. A detailed thermodynamic and kinetic study revealed that stability constants and activation parameters for complex formation between 1 and axles 2 and 3 are of the same order of magnitude, suggesting a common threading process. However, upon heating a solution of 1 and 2 in benzene at 340 K, the formation of another supramolecular complex was observed, the structure of which is consistent with an oriented pseudorotaxane derived from the threading of axle 2 from the lower rim of the calixarene wheel. By carrying out the threading-stoppering reaction sequence between 1 and 2 in the presence of an excess of diphenylacetyl chloride, the orientational rotaxane isomers R1 and R2, derived from lower- and upper-rim threading, respectively, were collected in about a ratio of 7:3 as the unique chromatographic fraction. Our results suggest that at room temperature the threading process is under kinetic control for all axles. On increasing the temperature only the threading behaviour of axle 2 is substantially modified, most likely because the process becomes thermodynamically controlled owing to the peculiar recognition properties of the ammonium head of this axle.

A synthetic study to disclose the more appropriate manner by which two calix[6]arene units could be connected for the construction of an extended tubular structure was undertaken. As a result, a head-to-tail double calix[6]arene having the structure of an oriented nanotube that is about 2.6 nm long and 1.6 nm wide was prepared and characterized. This molecule is able to act as a wheel-type host and forms a supramolecular complex with an axle-type molecule, derived from 4,4'-bipyridinium (viologen), through very efficient self-assembly in solution. The properties of such a pseudorotaxane-type complex, which is stabilized by a combination of noncovalent interactions, were investigated in solution by UV/Vis absorption spectroscopy and voltammetric methods. These observations provide a clue about the location of the bipyridinium unit along the nanotube. In the solid state, the complex undergoes a further stage of self-assembly, thereby initiating extended oriented tubular structures. Crystallographic studies revealed that the positioning of the viologen dication in this asymmetric wheel is addressed by a complicated pattern of cooperative noncovalent intermolecular interactions that involve only one half of the host, whereas the remaining (more polar) half of the host is exploited to create long-range structural order that leads to a "secondary" structure of extended supramolecular channels that, in turn, self-assemble in the lattice, thus giving rise to a "tertiary" structure of parallel sandwiches of nanotubes.

The first example of a calix[4]arene amino acid is reported. It allowed the synthesis of the novel N,C-linked peptidocalix[4]arenes which self-assemble in low-polarity media to give chiral dimeric capsules held together by an antiparallel beta-sheetlike hydrogen-bonding motif.

In this paper we report the synthesis of the first examples of upper (wide) rim calix [4]arene amino acids 5 and 27, together with the conformational, self-assembly and molecular inclusion properties of the N,C-linked peptidocalix[4]arenes obtained from them. Whereas the dipropyl derivative 5 readily undergoes peptide synthesis allowing a small library of calix[4]arene pseudopeptides 12-21 to be obtained, the tetrapropoxy compound 27 preferentially gives upper-rimbridged derivatives (e.g., 28) which are formed through an intramolecular condensation reaction. The tetrapropoxycalix[4]arene pseudopeptide 33 shows conformational and self-assembly properties quite different to those of the dipropoxy derivatives 12-21. The observed differences are explained on the basis of the different conformational flexibilities of the two calix[4]arene scaffolds. Calixarene 5 is more rigid than 27 thanks to the presence of two OH groups at the lower (narrow) rim that are involved in strong intramolecular hydrogen bonds. Only peptidocalix[4]arenes 12-21 but not