Conformation bridged calixarenes

Figure 7.17 Concave 1,10-phenanthrolines can also be obtained in one-step or two-step macrocyclizations starting from appropriately 2,9-disubstituted 1,10-phemmthrolines. Here the TL conformer (X, Y = polyethylene glycol orY = polymethylene) of a two step macrocyclization starting from a 1,10-phenanthroline containing monomacrocycle is shown. For a one-step macrocyclization see Figure 7.10, and for a bridged calixarene see Figure 7.15...

K. -L Koike. M. Preparation and conformational studies of ethylene-bridge calixarene-analogous macrocyclic meta-cyclophanes. Chem. Ber. 1993. 126. 2501. 

Another way of fixing the cone conformation has been achieved recently by the connection of two opposite para-positions by an aliphatic chain of appropriate length producing a bridged calixarene (135) (135) (Figure 2.58). 

Abstract. The synthesis of 1,2- and l,3-calix[4]-Z w-crowns, double calix[4]arenes and double calixcrowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-altemate conformation of the 1,3-calix[4]- w-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na /Cs selectivity was exploited in separation processes using them as carriers in transport through supported liquid membranes (SLMs). The best Na "/Cs selectivity (1/45 000) was observed for the naphthyl derivative 7. Calix(aza)crowns and 1,3-calix[4]-/ w-(aza)-crowns were also produced through the preliminary formation of the Schiff base-calixarenes, which were further hydrogenated. The syntheses consisted of the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridged calixarene or by the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented. 

As in the case of carbon-bridged calixarenes, conformational behaviors of azacalixarenes in solution have been investigated by means of temperature dependent H NMR spectroscopy. It was reported that azacalixarenes 4a-f, 7b-d, 7f, 7g, 8a, 20b, and 20f were conformationally flexible in solution [17-19,24,25]. Gong et al. pointed out from their NMR studies of compounds 7b, 7f, 20b, and 20f that the lack of steric hindrance and intramolecular hydrogen bonds were responsible for their high conformational mobihty in solution, as compared to carbon-bridged calixarenes [24]. 

The third chapter, Azacahxare A New Class of Calixarene Family by H. Tsue, K. Ishibashi, and Rui Tamura presents recent developments in syntheses, conformations, and inclusion properties of nitrogen-bridged calixarene derivatives possessing a [l/i]metacyclophane unit. Since just the replacement... 

In contrast, porphyrinogens 16 (Fig. 5) possess only 16 Ti-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28-30]. 

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. 

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. 

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