Calixarenes partial cone conformation

Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. 

Calixarene derivatives involving crown ethers like 233 exhibit complexation ofalkali or ammonium cations typical of crown ethers that is especially effective in the partial cone conformation [26]. By a proper functionalization selective synthetic receptors for specific hosts have been developed. For instance, two 2,4-diaminotriazine groups at diametrical positions of the upper rim of a... 

When these crowned, ionizable calixarenes contain no t-Bu groups in the 4-position (MCI1, MCI2, MCI3, MCI4, and MCI5), the increased molecular flexibility causes the extraction to shift to higher pH (cone conformer), the selectivity for Ba2+ to disappear (partial cone conformer) or to be less pronounced (1,3-alternate conformer).141... 

Shinkai and coworkers have capitalized on this conformation flexibility in their designs of several pyrene-derivatized calixarene chemosensors. The rotated phenyl ring of the partial cone conformer of 37 allows two pyrene units to more easily interact by decreasing steric hindrance at the lower rim [376], Addition of Li+, Na+, and K+ ions enforces cone formation and the disruption of the initially formed excimer. Accordingly, the metal ions are detected by a decrease in pyrene excimer emission and concomitant increase in the pyrene fluorescence. 

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

Conversion to a triether with CS2CO3 as the base gives the partial cone 109 (pathway F) as the major conformer from the 5yn-A,C-diether 106 or the cone conformer 110 (pathway H) from the syn-A,B-diether. Conversion of the triether 109 to the tetraether using CS2CO3 as the base then yields the partial cone conformer 114 (pathway M) as the major conformer when the p-substituent in the calixarene is f-Bu. When the p-substituent is hydrogen, however, the... 

Modified calixarenes that are conformationally immobile, fixed in either the cone or partial cone conformation, have been successfully synthesized by... 

In partial-cone conformation the calixarene cavity self-inclusion of a substituent belonging to the alternate aromatic moiety may occur. Such a situation is shown in... 

Interestingly, the macrocycles can adopt two different conformations A and B (Fig. 34), which are reminiscent of the partial cone and cone conformations of the calixarenes. A detailed... 

Calix[4]arenes are characterized by a three-dimensional basket shape, the internal volume being around 10 nm3. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is possible. In calix[4]arene, four conformations are possible cone, partial cone, 1,2-altemate, and 1,3-altemate. These four conformations are in dynamic equilibrium. Conformations can be locked, for instance, by placing a bulkier substituent than the ethyl group on the lower (or narrow) rim, this chain prevents the benzene units from rotating inside the calixarene cavity. 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

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