Resorcarenes Calixarenes

Fig. 1 Molecular architecture two fascinating nanometer-scale supramolecular species and the corresponding macroscopic counterparts, (a) Resorcarene-calixarene carcerand [7] and the Battistero of Pisa (Italy), (b) Norbomylogous-type compound [8] and the Olina medieval bridge, Modena (Italy). The geometries of the molecules are constructed by molecular mechanics calculations...

The synergistic effect was only found in mixed stationary phases that have a special selectivity. Those stationary phases were CD, crown ether, liquid crystal-hne, resorcarene, calixarene, AgNOs, and others. Crown ether, CD, cahxarene, and resorcarene possess cyclic moieties with cavity-like structures that are able to form inclusion complexes with metal ions and organic molecules. Liquid crystalhne stationary phases have temperature-dependent ordered structures and the retention is governed by the solute s length-to-breadth ratio. AgNOs retards olefins by the formation of loose adducts. Together with the above special selectivity stationary phases, they have already been the focal point of sup-ramolecular chemistry. 

In view of the greater reactivity of resorcinol, its cyclic tetramers are synthesized by the acid catalized condensation with less reactive aldehydes (Fig. 7.2.4) [lc]. However, in contrast with calixarenes 221, only hexamer resorcarene has been reported [8]. In view of different relative configurations at the CHR bridges, 225 can exist in form of four diastereomers [lc]. 

Besides these classical aromatics and polyaromatic hydrocarbons, other very important classes or arene molecules are porphyrins [60, 61], phthalocyanins [61, 62], porphycens [63], calixarenes [64], resorcarenes [64], cydophanes [47], dendrimers [65], elementa-arenes [66], organometallic arene (hexahapto) [67], benzyne (dihapto), and aryl- and benzyl (monohapto) complexes [68], inorganic pyridine and polypyridine complexes [69], fullerenes [70, 71], and... 

In this chapter we present an overview of chiral structures realized on the basis of calixarenes and resorcarenes.8 Clearly this cannot be an exhaustive survey, due to the enormous development calixarene chemistry has experienced during the last decade (mainly the last 5 years). Rather, it is a survey of the potential possibilities that are offered by this class of macrocyclic molecules—illustrated by selected examples of what has been done and indicating what might be done in the future. 

It seems appropriate at the outset to review some basic features of calixarenes and resorcarenes. Four basic conformations have been discussed for calix[4]arenes... 

Chiral groups may be attached to both the narrow and the wide rims of the calixarene skeleton I (or even to the bridges14) and this may be done by chemical modification of the calixarene or by the use of appropriate chiral phenols for the calixarene synthesis. With resorcarenes II chiral groups may be introduced at the hydroxy groups or at the 2-position between the OH-functions. Their attachment at the bridges (CHR ) was also realized. 

The three structures shown at the right of the second line represent important families of receptor molecules that have been extensively studied in their own right. The aromatic spherand grew out of crown ether chemistry while calixarene chemistry developed on a separate evolutionary pathway. Still, their similarity is apparent. The structures of calixarenes and resorcarenes (also resorcinarenes) are included here because they have often been fused to macrocycles to form complex receptor systems. A few examples of fused receptor systems are discussed below. 

Calixarenes are macrocyclic molecules synthesized with high yield by condensation of appropriate arenes and aldehyde derivatives. Calix means bowF in Latin and Greek, and this phrase reflects the shape of the tetramer, which usually adopts a bowl or beaker-like conformation. Gutsche first introduced the name calixarene for this class of compounds [38]. Several authors have exhaustively reviewed the chemistry and synthetic procedures, which lead to different structural modifications of calixarenes [39-42]. In general, three types of calixarenes derivatives are known first, metacyclophanes (type 1) second, those obtained by condensation of formaldehyde with phenol (type 11), and third, those obtained by reaction with resorcinol (type III) (Scheme 6). The latter modifications are also called resorcarenes to distinguish calixarenes of type III from those of type II. 

Only few investigations have been published on the gas-phase ion chemistry of host-guest complexes of calixarenes. With the advent of ESI mass spectrometry, especially when combined with ion-trap and FT-ICR mass spectrometry, this field has started to be developed. Binding selectivities of alkali metal ions to cahxarene-based crown ethers and open-chain ethers have been studied , the inclusion of neutral guests into the protonated resorcarene-based cavitand hosts by gas-phase ion-molecule reactions with amines have been studied and the formation of capsules from various calixarene tetraether derivatives and alkylammonium ions as ionic guests (notably enabling their detection by mass spectrometry) have been described recently . ... 

The basic skeleton of these compounds, calixarenes in the original (narrow) sense, is that of a [l ]metacyclophane and it is appropriate to include into the family of calixarenes also those cyclic oligomers, in which the phenolic hydroxy groups are situated in exo-position at the wide rim . Here especially cyclic compounds II (nearly exclusively tetramers) built up by resorcinol units (eventually substituted in the 2-position) are important and will be called resorcarenes within this article. Alternatively calixresorcinols is used for n, in distinction to calixphenols for I. 

This last example demonstrates already one of the future lines. As the chemistry of calixarenes and resorcarenes is more and more understood, these fascinating molecules lend themselves as building blocks for the construction of increasingly larger and more and more sophisticated structures. It is hoped that this fascination could be transferred at least in part to the reader. 

No general consent exists on the trivial name of these compounds. This short form resorcarenes has the advantage of comprising the same number of syllables as calixarenes . It would be reasonable to use calixarenes as the general name for the whole class of such [l ]metacyclophanes, and to distinguish, where necessary, between calixphenols, calixresorcinols, calixnaphthols, calixpyrogallols etc. 

The term cyclophane literally means any organic molecule containing a bridged aromatic ring. Thus specific cyclophanes such as the calixarenes and resorcarenes that we encountered in Section... 

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