Dissymmetric calixarenes

Key words Inherent chirality, asymmetric and dissymmetric calixarenes, resorcarenes. 

Synthesis of Dissymmetric and Asymmetric Calixarenes Chiral Calixarenes... 

The majority of chiral calixarenes are made either by affixing a chiral moiety to the upper or lower rim of an achiral calixarene or by establishing a dissymmetric... 

Often symmetry operations cannot be used in a simple way to classify chiral forms because, e.g., the molecule consists of a number of conformations. Therefore, independent of the symmetry considerations, a chemical approach to describe chiral molecules has been introduced by the use of structural elements such as chiral centers, chiral axis, and chiral planes. Examples for a chiral center are the asymmetric carbon atom, i.e., a carbon atom with four different substituents or the asymmetric nitrogen atom where a free electron pair can be one of the four different substituents. A chiral axis exists with a biphenyl (Figure 3.2) and chiral planes are found with cyclo-phane structures [17]. Chiral elements were introduced originally to classify the absolute configuration of molecules within the R, S nomenclature [16]. In cases where the molecules are chiral as a whole, so-called inherent dissymmetric molecules, special names have often been introduced atropiso-mers, i.e., molecules with hindered rotation about a helical molecules [18], calixarenes, cyclophanes [17], dendrimers [19], and others [20]. 

The unique feature of calixarenes is their concave structure that clearly distinguishes the upper and lower rims. When the inversion of the concave structure is hindered, an asymmetric/dissymmetric substitution pattern leads to the formation of chiral structures (Fig. 2.14). This type of chirality is known as inherent chirality. Calix[4]arenes were the first scaffolds for which the term inherent chirality was... 

In contrast, the H-NMR spectrum of 4 shows resonances for the tert-butyl protons at <5 1.28, for the methylene protons at 6 3.59, 5.13 (Jab = 15.1 Hz), and for the aromatic protons at S 7.02, 7.21, indicating a symmetrical structure. These signals correspond to symmetric structures, RRRR/SSSS or RSRS, described above. Recently, chiral calixarenes have been reported by many groups [22]. Bbhmer and coworkers [23] demonstrated the chirality of dissymmetric calix[4]arenes with... 

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