Meta-Substituted calix arenes

Based on the above, this chapter is focused on the synthesis and some selected properties of meta-substituted calix[4]arenes known in the state of the art. The review is by no means comprehensive, but rather should be meant as an overview of the most common ways leading to the wta-substituted isomers, showing the synthetic strategies and approaches that can address this least accessible position of a basic calix[4]arene skeleton. 

Scheme 3.5 Tungsten complexes of meta-substituted calix[4]arenes...

Very recently an unprecedented direct electrophilic substitution of calix[4] arenes leading to meta-substitution was described (see Sect. 3.7). Thus, the use of mercury (II) trifluoroacetate as an electrophile provides exclusively meta-... 

An interesting application was found for 25,27-dimethoxy derivative of calix[4] arene 5a, which exists exclusively as a cone conformation. This conformation is stable on the NMR time scale up to the highest accessible temperatures (125 °C in CDCI2-CDCI2), in other words, the cone-cone interconversion cannot he studied by NMR technique. On the other hand, as the meta substituted calixarene in the cone... 

Asymmetric calix[4]arenes also result from the incorporation of a single meta-substituted phenolic unit (9), an idea first realized by Vicens et al. [11] and subsequently picked up by Shinkai et al. [12]. Again, these compounds have been synthesized by 3+1 fragment condensation. 

The relation to the larger group of substituted cyclophanes (see Figure 15 in Chapter 2) is noted namely, calix[2]arene is an abbreviated name for 2,2 -dihydroxy-[l,l]-meta, metacyclophane. 

Calix[n]arenes are cyclic condensation products of para-substituted phenol derivatives and formaldehyde [29], They are highly interesting for the development of sensitive coatings due to their conformational flexibility and the ease by which they may be modified chemically. Chemical modification can be done either in the meta position, or by reactions at the hydroxy group. In this way, bulky substituents [30], chelating substituents [31], aromatic residues [32], crown ethers [33,34], peptides [35,36], etc. can be introduced. A first approach to combinatorial synthesis of calix[4]arene receptors has been published by Reinhoudt and co-workers [37,38], who prepared calixarenes with different substituents. In solution, these calixarenes lead to formation of hetero-oligomers with barbiturates, and these hetero-oligomers were detected by MALDI-TOF mass spectrometry and H-NMR spectroscopy. 

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