Asymmetric calixarenes

In this section we will discuss not only asymmetric calixarenes (Cj-symmetry), but also those C2-symmetrical derivatives, which are prepared in a similar way. 

Synthesis of Dissymmetric and Asymmetric Calixarenes Chiral Calixarenes... 

With the increasing demand for chiral nonracemic compounds, stereoselective methods for the synthesis of 1,3-oxazine derivatives and applications of enantiopure 1,3-oxazines in asymmetric transformations have gained in importance in the past decade, as reflected by the increasing trend in the number of publications on this topic, and accordingly by the share of this topic in the present compilation. The limited size of this survey and the scope of this chapter do not allow a discussion here of the applications of 1,3-oxazines in polymer chemistry and the synthesis and properties of 1,3-benzoxazine-containing hetero-calixarenes. 

A variety of functional groups such as halogen, nitro and azophenyl substituents at the 4-position in the phenol can be introduced into the calixarene by this specific method and it gives also the opportunity for the synthesis of asymmetric compounds (ref.72,73). The structural substituent requirement is the presence ofthe four phenolic units (A,B,C, D) arranged ABCC, ABCD or simply a single... 

We synthesized chiral calixarene 9 by the reaction of (leH) and (S )-l-bromo-2-methylbutane [16]. In an aqueous system / -(5)-2-methylbutyloxybenzenesulfonate 10, a noncyclic analogue, did not give a perceptible CD spectrum at 220-300 nm. Probably, the asymmetric carbon in 10 is too far from the chromophoric benzene ring to affect it intramolecularly. In contrast, 9 gave a clear CD spectrum in this... 

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... 

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

Although syntheses of calixarenes which have no plane of symmetry have been reported, optical resolutions were unsuccessful because of the rapid inversion of aromatic rings at room temperature [8]. The first chiral resolution was shown to have an asymmetrically substituted calixarene fixed in a cone conformation by O-alkylation [9]. 

In 2011, Li used a chiral calixarene (CAT-31) to perform an asymmetric version of the Biginelli reaction (Table 6). CAT-31 produced the Biginelli adducts in moderate yields and enantioselectivities (54% 44% ee). As possible additives, a piperidine-TFA salt and p-toluic acid were used to provide the Biginelli adducts in 42% yield and 68% ee. The monomer did not efficiently promote enantioselectivity in these reactions. From all tested aldehydes, those with substituents in the metfl-position had a better ee (80-98%), while those with substituents in the para-position had a lower ee (20-69%). The proposed mechanism by the authors involved the enamine intermediate, which interacts with the calixarene cavity in some manner, as confirmed by NMR experiments [42]. 

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