Calix crater

Perceiving a similarity between the shape of a Greek vase known as a Calix Crater and the shape of the cyclic tetramer, as illustrated in Fig. 11., we assigned the name calixarene 3,4). If the calixarenes assume the shape designated as the cone conformation (see Sect. 5.1) they are seen to have cavities whose dimensions increase as the number of arene moieties in the maerocyclic array increases, as illustrated in Fig. 12. Whether the shapes of these cavities are time-invariant depends on the flexibility of the calixarene (see Sect. 5.2) whether the open conformation exists depends on intramolecular hydrogen bond interactions. There is evidence (see Sect. 5.1), for example, that the calix[6]arenes and calix[8]arenes exist in solution in nonpolar solvents in transannularly pinched conformations, as illustrated in Fig. 13. x-Ray crystallographic determinations have established that in the solid state the calix[4]-arenes exist in the cone conformation 29,30), the calix[5]arenes in the cone conformation 31,32), and the derivatives of the calix[6]arenes and calix[8]arenes in the alternate rather than cone conformation 33,34). 

Fig. 11. /7-Phenylcalix[4]arene (left) and Calix Crater (right)...

The calixarenes are a popular and versatile class of macrocycle formed from the condensation of a p-substituted phenol (e.g. p-tert-butylphenol) with formaldehyde. Since they contain bridged aromatic rings, they are formally members of the cyclophane family (Section 6.5). In cyclophane nomenclature they are termed substituted [l.l.l.ljmetacyclophanes . The descriptive name calixarene was coined by C. David Gutsche (Washington University, USA) because of the resemblance of the bowl-shaped conformation of the smaller calixarenes to a Greek vase called a calix crater (Figure 3.78). The number of phenolic residues is denoted by a number in square brackets. Thus the most common cyclic tetramer with p-f-butyl substituents is termed p-t-butyl-calix[4]arene (3.118). It is easy to understand why this appealing nomenclature has found wide acceptance within the field when it is compared to the Chemical Abstracts systematic name for 3.118, [19.3.1.U U l ]octacosa-l(25),3,5,7(28),9,ll,... 

Figure 3.78 p-f-fcMiy/-calix[4]arene (3.118) resembles a calix crater vase in shape. 

Alice Gutsche with molecular model and calix crater... 

Calixarenes are [1 Jmetacyclophanes (1) that acquired their name because of the resemblance of the shape of one of the conformers of the smallest member of their family to a type of Greek vase called a calix crater (Figure 1.1). The name was initially chosen to apply specifically to the phenol-derived cyclic oligomers, but it has subsequently taken on a more generic aspect and is now applied to a wide variety of structurally related types of compounds. The calixarenes were first discussed in comprehensive fashion in 1989 in the first volume of Monographs in Supramolecular Chemistry, where the literature on the subject that had been published up to that time was covered in reasonably complete detail in 222 pages. Since 1989, however, there has been such a rapid expansion of the field that a somewhat less comprehensive coverage of topics is now necessary if this... 

The calixarenes can be prepared by the condensation of p-tert-butylphenol and formaldehyde under basic conditions. Gutsche has recently reported the facile synthesis of the tetramer (128) known as p-tert-butyl calix [4] arene (125). The name calixarene comes from the similarity between the shape of a Greek vase known as a calix crater and the shape of the cyclic tetramer (Figure 2.54). x-ray crystallographic determinations have established that in the solid state the calix [4]arene does exist in this cone conformation (126,128). 

Figure 2.54 C.P.K. model of p-phenylcalix [4] arene (left) and calix crater (right). Reprinted with permission from Acc. Chem. Res. 16 (1983) 161, 1983 American Chemical Society.

Fig. 39.1 (a) Calix crater (b) CPK model of calixarene (Reproduced with permission from Ref [1]. Copyright 2008, Royal Society of Chemistry)... 

The name cahx[n]arenes was coined by C. D. Gutsche originally to describe cyclic oligomers built up by (4-substituted) phenolic units linked in 2- and 6-position via methylene bridges (I). It is deduced from the calix or cup-like conformation assumed especially by the tetra- and pentamer, which resembles an ancient Greek vase, known as cahx crater , while arene refers to the aromatic units, the number of which is indicated by [n]. AU hydroxy groups in the general formula I are found in cndo-position at the narrow rim of the macrocycle. 

The cyclic tetramer of p-t rf-butylphenol-formaldehyde condensates which maintains a cone-type conformation called a calix[4]arene from its cuplike shape, similar to that of a Greek crater vase, tends to complex with small organic compounds or metal ions. Further, incorporation of carbonyl groups such as ester, amide and ketone linkages into the phenolic oxygen atoms of the calixarene makes it function as an ionophore [1]. 

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