Vase conformation

The vase conformation can be stabilized by a seam of eight intramolecular amide hydrogen bonds forming a macrocyclic ring (compound 33) that surrounds the open... 

That receptors 22a and 22b contain an inwardly directed hydrogen bond acceptor is not directly evident. These compounds structurally relate to cavitand 3 in that they contain amide groups at three aromatic resides, which stabilize the cavitand s vase conformation by intramolecular hydrogen bond formation. The remaining aromatic wall contains an appended Kemp s triacid residue such that its free carboxyl group is oriented toward the inside of the cavity. 

Cavitands 89 flutter between a C2v-symmetric kite conformation (favoured at low temperature) and a C4v-symmetric vase conformation (favoured at higher temperatures) in which the walls are upright oriented see Reference 397a... 

Fig. 6.3 Top Schematic presentation of the redox-tuned molecular switching. Bottom Redoxswitching cavitand systems 4a and 4b with a vase conformation in the reduced state, stabilized by four intramolecular hydrogen bonds...

The authors also studied the encapsulation of guests in the halogen-bonded container 59 (Fig. 32.22). 1,4-dioxane 60 and 1,4-dithiane 61 were assayed as guests using a non-competitive solvent (mesytilene) and 2 % of 3,5-dimethylbenzyl alcohol, to stabilize the cavitands vase conformation by... 

Recent examples of this kind of methodology can be found, for example, in the work of Rebek et al. [10] The catalyst used is a cavitand armed with a Zn salen-type complex (Figure 1.2). The cavitand adopts a vase-like conformation that is stabilized by a seam of hydrogen bonds provided by the six secondary amides. The structure of the catalyst permits a slow dynamic exchange between free and bound guest (reactant) on the H NMR time-scale that is controlled by the folding and unfolding of the cavitand. 

Collet, J.-P., Park, D., Lesty, C., Soria, J., Soria, C., Montalescot, G., and Weisel, J. W. (2000). Influence of fibrin network conformation and fibrin fiber diameter on fibrinolysis speed Dynamic and structural approaches by confocal microscopy. Arterioscler. Thromb. Vase. Biol. 20, 1354-1361. 

Philosophers have seemingly spoken of this Matter only to conceal it at least when there is a question of designating it particularly. But when they speak of it in general they enlarge very much upon its qualities and properties they give to it all the names of the individuals of the Universe, because they say it is the principle and basis of them. Investigate, say the Cosmopolite, Tract I), and see if what you propose to do, is conformable to what Nature can do. See what materials she employs and what vase she uses. If you wish to do only what she does, follow her step by step. If you wish to do better, see what can best serve for this purpose but remain always in natures of the same kind. If,... 

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). 

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 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,... 

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). 

Considering a molecular model of the cyclic tetramers, Gutsche introduced the term calix[4]arenes [1,2] ( calix — vase, bowl, cup), a term that emphasizes the cup-like structure of these macrocycles, which have all four aromatic rings oriented in one direction (a so-called cone conformation). The word arene indicates the presence of aromatic rings in the macrocyclic ensembles. Later the term was extended to include large macrocyclic compounds ([l ]metacyclophanes) and macrocycles, in which the methylene bridge is replaced by a heteroatom. The size of the cycle is indicated by a number placed in square brackets between the roots caUx and arene nature and position... 

Calixarenes, shown schematically in Fig. 24, are prepared by the condensation of p-tert-butylphenol with paraformaldehyde. In the cone conformation, calix[4]arene has the shape of a chalice or vase. Figure 25 exhibits this geometry, and the inclusion of a toluene molecule is apparent. Closer observation of Fig. 25 shows that an Na+ ion is also held in the base of the chalice. Thus, this cal-ixarene can include both neutral molecules and ions at the same time. The similarity to the cyclodextrins is clear, but the calixarenes are not as rigid. 

Calix[ ]arenes and calix[ ]pyrroles can dramatically change their conformation. Vase-shaped calix[ ]arenes have two rotation modes of the phenolic units the ojygen-through-the-annulus rotation and the para-substituent-through-the-annulus rotation (Figure 4.2). ... 

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