Cone conformation

Thus, it has been shown that calix[4]aryl esters exhibit remarkably high selectivity toward Na [11-14]. This is attributable to the inner size of the ionophoric cavity composed of four 0CH2C=0 groups, which is comparable to the ion size of Na, and to the cone conformation that is firmly constructed on the rigid ca-lix[4]arene platform (Scheme 2). 

Shinkai [15] concluded that p-zert-butyl calix[n]ar-ene tetra esters form stable monolayers at the air-water interface and the metal responds, therein, quite differently from that in solution. They reported that examination of the metal template effect on the conformer distribution established that when the metal cation present in the base used serves as a template, the cone conformer results are predominant [16]. Hence, Na in... 

NaH should serve as an efficient template ion to yield cone conformers. Conformer distribution for the reaction of p-tert-h Wy calix[4]arene and ethyl bromoacetate is given in Table I. 

Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. 

Increasing the reducing agent/W ratio and using Na-naphthalenide led to the isolation of the two-electron-reduced, diamagnetic compound 19. The C2v symmetry of the h NMR spectrum and the X-ray analysis are in agreement with the cen-trosymmetric structure sketched in Scheme 2 for 19 [W = W, 2.614(1) A]. The six-coordination of the metal and the inclusion of the alkali metal cation removes the planarity ofthe 04 core and the cone conformation of the calix[4]arene. The sodium cation within the calix[4]arene cavity is r 3-bonded to two opposite arene rings. 

A different synthetic access to a 1 -metallacyclopropene, which can be a versatile organometallic synthon, is displayed in Scheme 33. The mono-alkyne derivatives of W(IV)-calix[4]arene are easily accessible through the thermal displacement of cyclohexene from 32 using the appropriate acetylenes. The reaction led to complexes 34 and 172-174. The proposed 3-metallacyclopropene has been confirmed from the spectroscopic and the X-ray data. The H NMR data reveal a cone conformation of the calixarene with a four-fold symmetry, for which the... 

In this context, divalent presentation of this promising candidate has been investigated by the authors using the fixed cone conformation of calix[4]arene to allow introduction of the molecules onto the upper rim, and their projection into the same portion of space, thus mimicking, to some extent, a small portion of the natural cell-surface ganglioside. The synthesis of the divalent neoglycoconjugate was based on initial functionalization of the corresponding diacid with an aminated spacer and further introduction of squaric acid moieties. 

Calixarenes, when in their cone-conformation (54), represent versatile host systems for metalated container molecules and many examples have been reported in the literature (55-61). Reinaud and coworkers have carried out extensive work concerned with calix[6]arenes that are functionalized at the small rim by nitrogen arms (62), aiming to reproduce the hydro-phobic binding site of mononuclear zinc and copper metalloen-zymes. A recent example is the calix[6]arene ligand L1 (Fig. 3), in which a tris(2-methylpyridyl)amine unit covalently caps the calixarene small rim (63). The ligand forms copper complexes of... 

Interestingly, the macrocycles can adopt two different conformations A and B (Fig. 34), which are reminiscent of the partial cone and cone conformations of the calixarenes. A detailed... 

Fig. 12 Brick-wall 2D network of complex (C-methylcalix[4]resorcinarene).2(4,4 -bipyri-dine).0.5(decamethylruthenocene).2(ethanol).The 24host molecule is in aflattened cone conformation. Disordered guest molecules are not shown and occupy the apparent cavities [58]... 

An unusual scoop conformation of 24 has been characterised in the hydrate (C-methylcalix[4]resordnarene)(H20) [65]. The scoop conformation is a hybrid of the bowl and flattened cone conformations and there are two intramolecular O-H... 0 hydrogen bonds from adjacent resorcinol units. Adjacent 24 molecules form O-H... 0 hydrogen bonds and there are additional links via water molecules resulting a 2D sheet structure. The sheets pack in a bi-layer arrangement. 

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. 

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. 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

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