Conformation pinched

In the presence of excess AgNOs, 5 forms an octanuclear complex [(en)Pt(UH)Ag]4(N03)8 4H2O (11), which from its partially resolved X-ray structure has been assigned a pinched cone conformation where four Ag centers bind to the uracil 0(2) donor atoms of the four uracil monoanions. On addition of NaCl, all four Ag dissociate to give 6. 

FIGURE 6-22 Induced fit in hexokinase. (a) Hexokinase has a U-shaped structure (PDB ID 2YHX). (b) The ends pinch toward each other in a conformational change induced by binding of o-glucose (red) (derived from PDB ID 1HKG and PDB ID 1GLK). 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

A single-crystal X-ray structure for a tetraloop compound shows the molecules (not unexpectedly) in a pinched cone conformation (see Figure 5.10a), although the time-averaged conformation deduced from the1H NMR-spectra is C4v-symmetrical (Figure 5.10b). 

The nitrate salt of the analogous [ (en)PtnAg(UrH, A (7),A (5),(9(2), 0(4)) 4]8+ cation exhibits a different structure, which can be formally derived from that of the pinched-cone conformer of Fig. 15,a, by coordination of silver ions to the uracilate oxygens. In all the octanuclear complexes, two metal units, Pt(en) and M, represent each corner of the boxes. These results demonstrate that, irrespective of the 120° angle between the Pt-N(l)... 

The regioselective incorporation of the phenolic units (apparent from the simple NMR spectrum) was confirmed by X-ray analysis for several derivatives and for the parent 97a itself. This shows the molecules in a strongly pinched cone conformation in which two opposite phenolic units are nearly parallel (effective C2 symmetry), a situation that would not be favorable for guest inclusion. This pinched cone conformation might be the energy minimum in solution but it could not be detected by NMR spectroscopy down to the lowest available temperature (150 K in CDFC12).187... 

The connection of adjacent phenolic units by directional bridges may be mentioned as an additional possibility. The C2-symmetrical lactone 102a is obtained in 36% yield from the tetrachloromethylated calix[4]arene tetrapropyl ether by reaction with salicylic acid. The formation of the Cs-symmetrical isomer is not observed in this case, probably since the most nucleophilic phenolate attacks at opposite rings, due to a pinched cone conformation of the tetrapropyl ether.196 In contrast, the similar cyclization with 3-hydroxymethyl-2-naphthol gave mainly the Cs isomer (Cs/C2 = 95/5). However, the C2 isomer 102b could be isolated and has been resolved by enantioselective HPLC (Chiralpak AD, a = 3.17).145... 

MM-calculations lead to an energy minimum with C2v -symmetry ( pinched cone conformation) forcalix[4]arenes (a) Harada, T. Ohseto, F. Shinkai, S. Tetrahedron 1994,50,13377-13394. (b) Harada, T. Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1995,2231-2242. 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

Calix[8]arenes are also found to act as a ditopic receptor in most cases. The calixarenes assume various pinched conformations in these complexes, resembhng two calix[4]arene cone units hnked together. Two 1 1 complexes, of Ca° and Eu, have been characterized where a roughly planar circular conformation is observed, with only two of the phenolic O atoms interacting with the metal atom. It is interesting to note that the free jo-t-butylcalix[8]arene has been found to crystallize as a pyridine solvate with a planar, circular conformation in one case, and a pinched conformation in a subsequent report. 

Fig. 12a, b a The structure of C4-acetone. b Possible orientation of the acetone molecule with respect to C6 in pinched cone conformation. T is ca. 120° and is equally divided into four angles of 30°. The angles between the twofold axes of calixarene and the CD3 vector averaged about the twofold axis of acetone are a, pi and p2 for the A, and B2 orientations. (Adopted from [48] by permission of the PCCP Owner Societies)... 

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). 

---