Calix arenes, building blocks

Cavitands and Calix[n]arenes Building Blocks for Supramolecular Capsules. 101... 

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

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

In the last two decades, chiral receptors containing amidic functions were designed almost exclusively for binding protected amino acids [49-57], oligopeptides [54,58], and lactic [59], tartaric [60,61] or camphoric acid derivatives [62]. Usually, chiral building blocks such as spirobifluorene [49, 60], binaphthalene [51,57],or amino acid chains containing macrocycles [52-56,58] were employed. An interesting receptor was synthesized via connection of the calix[4]arene moiety with an aza-crown derivative [61]. 

Calix[4]arene is a very popular building block for the design of molecules with well-defined mutual positions of substituents. The attachment of amidic residues to this molecule resulted in receptors 35 [65] and 36 [66]. Upon the H NMR titration of 35 by tetrabutylammonium salts of amino acids in acetone-d6y moderate chiral recognition was observed N-Ac-L-Ala K=4,900 M-1, AT-Ac-D-Ala K=5,700 M"1, AT-Ac-L-Phe iC=7,900 M-1, AT-Ac-D-Phe K= 10,500 M"1. The receptors 36a-c formed stable complexes with AT-tosyl-L-phenylalaninate in deuteriochloroform (36a K= 1,626 M"1, 36b 1C=4,836 M"1, 36c K=6y924 M-1). Unfortunately, the measurement using AT-tosyl-D-phenylalaninate has not been reported. 

Abstract Calix[n]arenes represent a well-known family of macrocyclic molecules with a broad range of potential applications in many branches of supramolecular chemistry. Because of their preorganisation, calix[n] arenes are frequently used as building blocks and molecular scaffolds in the construction of more elaborate systems, such as artificial enzyme biomimetics and receptors. This review is focused on the recent development of calixarene-based anion receptors. 

The principal idea of this present essay was to show how the unique preorganization of functional groups in self-assembled dimers of tetra-urea calix[4]arenes can be used to prepare novel multi-rotaxanes and -catenanes or topologically even more complex molecules and supramolecular structures. We will conclude by summarizing some related studies in which calixarenes were used in a different way as building blocks for the construction of such structures or assemblies. 

For an analogous example including a rigid, chiral calix[4]arene like building block see Okada, Y., Miao, Z., Akiba, M. and Nishimura, J. 

Figure 8 Schematic representation of koilands (hollow molecular building blocks bearing two preorganised cavities oriented at a 180° angle) centrosymmetric koiland (a), noncen-trosymmetric koilands based on electronic differentiation (b and c), geometrical differentiation (d) and both electronic and geometrical differentiation (e). A and B represent calix[4]arene units and M, Mi and M2 atoms used to fuse two calix units.

The calix[ ]arene ( indicates the number of aromatic units in the metacyclophane) building block provides a three-dimensional skeleton that is needed to fix different donor ligands in the optimal spatial positions for binding one ion selectively [94,95]. Calix[n]arenes are easily accessible from the base-eatalyzed condensation of p-tert-butylphenol and formaldehyde [96]. Gutsche proposed to define the two faces of a calix[4]arene molecule as the lower (phenolic groups) and the upper rim. The calix[4]arene with free hydroxyl groups is conformationally flexible and the... 

Various examples are known for all possible bis-crown ether derivatives from calix[4]arenes l,2 3,4-bis-crowns in the cone (39) and 1,2-alternate (40) and 1,3 2,4-bis-crowns in the 1,3-alternate conformation (41). Figure 5 gives a survey. Especially, the latter series is weU developed, comprising examples with identical and different ether loops, including structures describable as calixcryptands (41g). Compounds 41a (n = 6) were studied as ligands for the removal of caesium from nuclear wastes in analogy to their mono-crown counterparts in the 1,3-alternate conformation (37) . The 1,2 3,4-bis-crown-3 (39a, n = 3), on the other hand, is an important building block, since its calix[4]arene skeleton is fixed in a nearly perfect C4v-symmetrical cone conformation. ... 

A. Arduini, A. Pochini, A. Secchi, Rigid calix[4]arene as a building block for the synthesis of new quaternary ammonium cation receptors, Eur. J. Org. Chem., 2000, 2325-2334. 

Inclusion complexation has developed to becoming another widely exploited supramolecular interaction for the formation of supramolecular polymer networks, mostly in water [197, 198]. Several classes of macrocycles have been developed, including crown ethers [199, 200], porphyrins [201, 202], cyclophanes [203], catenanes [204], cavitands [205, 206], cryptophanes [207], calix[n]arenes [208], and carcerands [209]. Macrocyclic-based supramolecular gels can either be formed from low molecular weight precursors or from macromolecular building blocks. The following discussion focuses on the latter. 

Calixarenes, for example, 2.120, have already been discussed as hosts for cations and both cation and neutral molecules simultaneously in Sections 2.3.6 and 2.5.5, respectively. Resorcarenes e.g. 2.121) are very closely related to calixarenes and are sometimes referred to as calixresorcarenes. They are prepared from the acid-catalysed condensation of resorcinol (1,3-dihydroxybenzene) with aldehydes, a reaction first carried out by Baeyer in 1872. This macrocycle is arguably one of the most important molecules in the field, as resorcarenes serve as the basic building blocks for many cavitands and (hemi)carcerands. Both calixarenes and resorcarenes posses a shallow bowl-shaped conformation in their most stable forms. However, [4]resorcarenes (the [4] prefix refers to the ntimber of resorcinol units that make up the molecule, just as in calix[n]arenes) tend to produce a... 

Various (thia)calix[4]arenes and calix[4]resorcinols can be used as building blocks for the construction of nanocapsules. The process of binding/release is... 

In this article we will illustrate our new strategy with a few examples of receptor molecules composed of calix[4]arenes in combination with several other building blocks that exhibit unique complexation properties. Moreover we have found new routes for the selective introduction of functional groups both in calix[4]arenes and resorcinarenes. One of these methods comprises the key step in the synthesis of calix[4]arene-based carcerands in which guest molecules are permanently encapsulated in an asymmetric environment. Finally, the first example of a holand, a molecule with a large rigid cavity of nanosize dimensions, will be described. 

Combination of Calix[4]arenes with Other Building Blocks... 

For the design of new receptor molecules with unique complexation properties calix[4]arenes have been combined with several other building blocks making use of the methods for their selective functionalization that were developed. In this paragraph the combination of calix[4]arenes with glycol chains (8), a terphenyl (9), and a uranyl containing salophen moiety (10) will be discussed. 

Using calix[4]arene as building blocks, highly selective receptors have been developed for potassium, sodium, silver, lead, and a number of other cations. For the first test of their performance in CHEMFETs, these receptor molecules were incorporated in plasticized PVC membranes. The excellent response of the CHEMFETs containing the silver and lead receptor, respectively, is given in Figure 3. 

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