Conformation bowl-shape

Strapped tetramines show conformationally rigid coordination environments that can lead to novel stability properties. The coordination properties of bowl-shaped cavities or adamanzanes such as the ligand l,5,9,13-tetraazabicyclo-[7.7.3]nonadecane (95) were investigated.745 The... 

Further examples of cavitand-type structures include hw-cyclo-triveratrylene derivatives such as (257) (Gabard Collet, 1981 Canceill, Lacombe Collet, 1986) and the bowl-shaped hosts represented by (258) - the base of the bowl is formed by the four methyl groups. Once again, the shape of these molecules is maintained by conformational constraints. Cavitand (258) is able to accommodate simple solvent molecules such as dichloromethane and chloroform. Moreover, its cavity is large enough to form inclusion complexes with up to four molecules of water (Moran, Karbach Cram, 1982). 

Calixarene and their resorcinarene relatives are similar to CDs with respect to their ability to form a concave cavity in which guests can bind. Calixarenes are conformationally more flexible than the resorcinarenes, and depending on the substituents attached to their wider upper rim or to the narrower lower rim they may exist in a highly-symmetric bowl-shaped, so-called cone conformation or in several other conformations that do not exhibit an as perfect cavity as does the cone conformation (Scheme 9). Cationic guests, such as alkali metal or ammonium ions. 

From the assemblies of fused five- and six-membered rings identifiable on the surface of 1, corannulene (8) is the smallest fragment that retains a bowl-shaped conformation. However, there are other known hydrocarbons that adopt bowl (or... 

As would be expected, semibuckminsterfiillerene 27 is predicted to adopt a bowl-shaped conformation by ab initio 3-21G calculations. However, while 27 is a fascinating compound, it does not represent a symmetrical half of the buckmin-sterfullerene surface. That is, when the carbon framework of 27 is removed from... 

All of the theoretical methods that allow for geometry optimization predict the bowl-shaped conformation of 8 to predominate over the planar alternative. However, the calculated energy differences between the two conformations differ drastically and span the range of 0.8 to 84 kcal/mol. Moreover, the calculated energy difference between the plants and bowl-shaped conformers is quite important since it represents the barrier for bowl-to-bowl inversion if (a) planar corannulene represents the transition state (TS) for the process, and (b) the entropic term in AG is negligible. Condition (a) seems to be fulfilled, since vibrational frequency calculations give one and only one imaginary frequency for the planar structure... 

Similarly, 28 is expected to exist in a rigid, bowl-shaped conformation with symmetry. The planar structure was found to be 77.3 kcal/mol higher in energy... 

MNDO calculations suggest a stacked-bowl arrangement of the two tetraan-ionic corannulene moieties in a convex-to-concave geometry with four lithiums inside the sandwich as the global energy minimum (Figure 11 Thus, in agreement with theoretical studies on the isolated tetraanions (see above), the corannulene tetraanion also prefers a bowl-shaped conformation when the countercations are included in the calculations. 

Figure 12. (a) Macrocyclic resorcinol tetramer 8 has a rigid bowl shaped conformation with the long alkyl tails in an all-axial and all-c/s configuration, (b) Model of K flow in a hypothetical bimolecular ion channel composed of tail-tail dimer of 8. The ion passes through the inner pore of macrocyclic resorcinol unit which acts as a selectivity filter and then to the area formed by the loose assembly of long alkyls. ... 

Cyclic peptides containing multiple Abz residues adopt bowl-shape conformations the rigid amino acid provides a cavity in the cyclic peptide that may be exploited for molecular recognition experiments. Thus, c[-Xaa-w-Abz-]3 binds phosphomonoestersj25 while serine protease activity is observed in c[-Ser-m-Abz-His-m-Abz-Asp-m-Abz-Ala-m-Abz-] following binding of the substrate in the cavity. 26 ... 

Figure 7.27 (a) Bowl-shaped conformation of CTV (7.37) and (b) saddle conformation of the related... 

Fig. 23 The outcome of thermodynamically controlled imine condensations was suggested to be a function of the conformational preference of the diimine linker that predisposes the bowl-shaped reactants with a minimal amount of strain.134 Reprinted with permission from Ref.134 (For color version of this figure, the reader is referred to the web version of this book.)...

A single crystal X-ray diffraction analysis has been carried out on both the chloro- and the phenyl-substituted subphthalocyanines 2.284 and 2.287 (Figures 2.3.2 and 2.3.3). These analyses served to show that subphthalocyanines lie in a bowl-shaped conformation. This is, of course, very different from the near-planar conformation of the parent phthalocyanines. Presumably, this bowl-shaped structure accounts, in part, for the decreased molar absorptivities of the subphthalocyanines relative to their phthalocyanine parents . Nevertheless, despite the non-planar nature of these macrocycles, the subphthalocyanines are capable of supporting an induced diamagnetic ring current (as judged by NMR spectroscopy). Thus, they may appropriately be considered as being aromatic. 

We have recently shown that it is possible to extend the cavities of bowl-shaped molecules known as resorcin[4]arenes (e.g. C-methylcalix[4]resorcinarene 2), su-pramolecularly, using hydrogen bond acceptors as extender units. As a starting point, we chose readily available 2 as a platform for the assembly process. Indeed, solid-state studies had revealed the ability of 2 to adopt a bowl-like conformation... 

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,... 

Still and coworkers also described some remarkably selective peptide receptors. Tricyclic systems 40-42 represent an interesting sequence. These molecules adopt bowl-shaped conformations in which the central aromatic ring serves as the "base." and the outer rings of amide groups define the rim. The first example 40 was found to bind a range of N-Boc a-amino acid amides with excellent enantioselectivities, peaking at 130 7 for valine derivative 43 in Receptor 41 was then used to... 

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