Calix arene dimelamines

FIGURE 4.2. Formation of the double rosette assemblies l3 (DEB)6, from its building blocks calix[4]arene dimelamine 1 and DEB. 

These double rosettes can be easily extended to tetrarosettes by connecting two calix[4]arene dimelamines by a flexible linker unit X (Figure 4.3).11,12 A tetrarosette assembly consists of fifteen components, three tetramelamines 2 and twelve barbiturate/cyanurate derivatives held together by 72 hydrogen bonds and it has a size of 3.0 x 3.1 x 2.7 nm. The thermodynamic equilibrium for the assembly 23 (DEB)i2 is reached within seconds after mixing the two different components. Similarly, hexarosettes 33 (DEB)ig are prepared... 

The multimeric assembly reported in Fig. 6 forms spontaneously when three calix[4]arene dimelamine units and six barbiturates or cyanurate are mixed in a 1 2 ratio in apolar solvents. These assemblies, made of nine achiral com-... 

Chiral calix[4]arene dimelamines are weakly CD active, while on the contrary chiral double rosettes show strong absorptions with Aemax ... 

Similar chiral supramolecular assemblies are also observed from achiral calix[4]arene dimelamine derivatives with chiral cyanurates thus leading to D3 symmetry species [21]. Analogously to chiral dimelamines, predominantly one of the two possible diastereomeric double-rosette assemblies was observed in most cases and it is noteworthy that the same (R) chiral moiety when mounted in the dimelamine is located on the left-hand side of the calix[4]arene and induces (M) chirality, while when implemented in the cya-nurate is placed on the right-hand side and therefore promotes P chirality (Fig. 8). 

Three equivalents of a calix[4]arene derivative, diametrically substituted at its upper rim with two melamine units, have also been demonstrated to yield well-defined box-like assemblies on interaction with six equivalents of 5,5-diethylbar-bituric acid. The product, which is based on an extended rosette motif, consists of nine different components held together by 36 hydrogen bonds. It is stable in apolar solvents and remains so even at very low concentrations. Further, the structure remains intact in the solid state, as confirmed by an X-ray diffraction study. In an extension of this study, three enantio-pure calix[4]arene-dimelamine derivatives were employed for a parallel study. In this instance, the chiral information in the individual calixarene derivatives proved successful in controlling the conformation of the final assembly - a homochiral supramolecular entity was the result. 

Fig. 8 Schematic representation of highly diastereoselective non-covalent synthesis achiral dimelamine calix[4]arene with enantiopure chiral cyanurate components lead to chiral double rosettes with d.e. as high as 96% (R)-cyanurate induces complete (P) chirality, while (S) favors only (M) chirality...

Exploiting the same concept, Reinhoudt and Shinkai were able to obtain another example of enantioenriched chiral double-rosette made of achiral components. In this case achiral calix[4]arene bearing dimelamine moieties with pyridine functionalities were assembled with achiral cyanurate leading to racemic chiral double rosettes. The latter are perfect counterparts for chiral D-dibenzoyl tartaric acid via two-point hydrogen-bonding interactions thus... 

Fig. 7 Chiral double-decker rosettes formed by dimelamine calix-[4]-arenes and barbiturates and cyanurates. (From Ref. [18].) One of the helical forms, the P. is shown.

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