Calixarene variations

The preceding molecular baskets belong broadly to two classic classes of macrocycles namely the calixarenes and cycloveratrylenes. There are several other macrocycles with the potential to bind guests in an aromatic-rich cavity that are worthy of discussion. Of the vast amount of work that emanated from the Cram group, the rigid spherands and carcerands stand out. In addition there are the cyclophanes and Baeyer s early contribution to macrocyclic chemistry, resorcinar-enes and calixpyrroles. 

Other linear aldehydes may be used to prepare pyrogallol[4]arenes though some, such as pentanal, have unpleasant odours. 

Note Work wherever possible in a fume hood. 

Dissolve resorcinol (5.5 g, 50 mmol) in water (20 mL) in a 100 mL round-bottomed flask and add hydrochloric acid (5 mL, 37 per cent aqueous solution) dropwise with vigorous stirring. To the stirred solution, add redistilled acetaldehyde (2.6 mL, 2.2 g, 50 mmol) dropwise. Heat to 75 °C for 1 h then cool in ice and filter of the beige precipitate. Wash with cold water (ca. 5 mL) and recrystallize from ethanol (ca. 35 mL) to give C-methyl[4]resorcinarene (35) as a white powder. 

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Variation of the spacer length between CMPO groups attached to the 1,3-and 2,4-positions of the calixarene platform did not lead to appreciable improvements, in terms of extraction or selectivity. 

While the selective interactions of functionalised calixarenes with cations have been studied broadly for almost three decades, the application of cal-ixarene-based receptors for anion recognition is a relatively new research topic [2]. This review is focused on recent developments in the design and synthesis of calixarene-based anion receptors. Although the name calixarene was originally designated only for phenol-formaldehyde derivatives 1, recently many structural variations and mutations have been formed. Some of them, such as calixpyrroles [3], are widely used for anion recognition nevertheless, this review is restricted only to classical calixarenes 1 and newly discovered thiacalix-arenes 2 [4]. 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

Figure 2. SEM image of ultrahigh packed dot arrays resist pattern (a) and resist dot size variations (b) using calixarene (28-44 mC/cm 30 kV, a pitch of 25 nm x 25 nm).

The facile condensation reaction between formaldehyde and phenols or their derivatives provides a major route into rigid macrocycles used in supramolecular chemistry. Calixarenes, the best-known class of phenol-derived macrocycles, are prepared this way, as are spherands and their relatives. Cyclotriveratrylene, however, is an excellent exemplar of the molecular basket type of ligand and has been known for the best part of a century. The basic cyclotriveratrylene synthesis is shown in Figure 3.1. The original procedure by Mrs. Gertrude Maud Robinson [1] has since been refined by others and many variations are known [2,3]. 

The analyte can hardly be identified if the sensor substrate induces only weak and unspecific interactions. A new approach to overcome these problems is the development, using supramolecular chemistry, of sensor materials which allow the production of selective chemosensors. A large number of supramolecular receptors can be prepared by means of combinatorial chemistry [12], but the selective receptors described so far, such as cyclodextrins and calixarenes, offer only a relatively small number of distinct variations. 

A few other variations are included here to demonstrate the diversity of this class with regard to molecular shape. Vogtle et al. s multiple ansa compound (115, R = CH2CH2OCH2CH3), an unexpected product obtained from an attempt to synthesize biscalixarenes with aromatic linkers, has two bridges threaded through the macrocycle formed by joining the two calixarenes subunits [152]. Siegel et al. s... 

Due to their ability to form complexes selectively with metal cations and thereby solubilize such cations in liquid media of low dielectric constant, macrocyclic multidentate ligands have been widely investigated as carriers in metal ion transport processes for the past two decades (2-P). Such macrocycles include crown ethers, lariat ethers (crown ethers with a side arm which contains potential binding sites) (JO), calixarenes, and some cryptands. Studies of structural variations within a given... 

Hetero-calixarenes represent another interesting group of calixarene-related compounds that can be used for the interaction with nucleic acids and related compounds. Due to their constitutional variation their structural features significantly differ from the corresponding original calixarene scaffolds. Still there is a number of examples worth considering in the broad context of calixarenes and nucleic acids. 

Fig. 4. Variation in selectivity of PVC membrane ISEs as calixarene annulus size increases. The tetramer 1 is clearly sodium selective, while the three oxocalixarenes 18,19 and 20 have intermediate and rather poor overall selectivity. On the other hand, the hexamer 3 is obviously caesium selective.

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