Conformation partial cone

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]. 

Calixarene derivatives involving crown ethers like 233 exhibit complexation ofalkali or ammonium cations typical of crown ethers that is especially effective in the partial cone conformation [26]. By a proper functionalization selective synthetic receptors for specific hosts have been developed. For instance, two 2,4-diaminotriazine groups at diametrical positions of the upper rim of a... 

When these crowned, ionizable calixarenes contain no t-Bu groups in the 4-position (MCI1, MCI2, MCI3, MCI4, and MCI5), the increased molecular flexibility causes the extraction to shift to higher pH (cone conformer), the selectivity for Ba2+ to disappear (partial cone conformer) or to be less pronounced (1,3-alternate conformer).141... 

Shinkai and coworkers have capitalized on this conformation flexibility in their designs of several pyrene-derivatized calixarene chemosensors. The rotated phenyl ring of the partial cone conformer of 37 allows two pyrene units to more easily interact by decreasing steric hindrance at the lower rim [376], Addition of Li+, Na+, and K+ ions enforces cone formation and the disruption of the initially formed excimer. Accordingly, the metal ions are detected by a decrease in pyrene excimer emission and concomitant increase in the pyrene fluorescence. 

Chiral di- and tripropyl ethers, 59b and 60b for example, have been synthesized as shown in Figure 14.121 Complete O-alkylation of the monobenzyl ether was possible with n-propyl iodide in THF/DMF with NaH as base. The cone and the partial-cone conformers were formed in a 1 1 ratio and could be separated chroma-tographically. Alkylation with n-propyl bromide in the presence of Cs2C03 in acetone gave the di-O-propylated compound 59a, in which both propoxy groups in anti orientation, in good yield. The cleavage of the benzyl ether with trimethylsilyl bromide led to the final products 59b and 60b. 

Calix[4]arene tetraethers can be converted into mono Cr(CO)3 complexes 72, which makes one of the phenolic units different from the others (AAAB).135 Thus, a single chiral compound is obtained from the 1,2-altemate conformer, while for the partial cone conformer only one of the three possible products is asymmetric (compare Figure 13). Mono derivatives fixed in the cone and 1,3-altemate conformation contain a symmetry plane. 

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

This is necessary, not to end up with an untractable mixture of isomeric or similar compounds, since there are four monoethers or four (etraethers in the partial cone conformation possible, to mention just two examples. 

Direct tri-0-alkylation of calix[4]arenes has been reported to lead to the syn/syn isomer using bases such as BaO, BaO/Ba(OH)2 or CaH2 in DMF " . The tribenzoate of the p-unsubstituted calix[4]arene, one of the first examples of selectively derivatized calix[4]arenes, was obtained as the anti-syn isomer (benzoyl chloride/pyridine) , while the tribenzoate of 2a obtained in toluene with iV-methylimidazole as base (70% yield) was described as the syn-syn isomer, assuming a partial cone conformation with an inverted phenol ring . Both tris(3,5-dinitrobenzoates) of 2a were obtained by acylation with 3,5-dinitrobenzoyl chloride/l-methylimidazole. While 95% of the syn-syn isomer was formed in acetonitrile, 70% of the anti-syn isomer was obtained in chloroform . 

The dependence on the reaction conditions was shown also for the iyw-l,3-diallyl ether of which in CH3CN gives 5yn-5yn-diether/ester with PhCOCl/NaH, while reaction with PhCOCl in the presence of pyridine led to the awfi-yyw-diether/ester °. The formation of an anti-syn 1,3-diether/ester derivative was also achieved by barium(II) ion assisted monodeacylation of a l,3-crown-5 diacetate in the partial cone conformation . ... 

Syntheses and ion selectivities of the tri-amide derivatives of hexahomotrioxa-calix[3]arene. Remarkably large metal template effect on the ratio of cone vs. partial-cone conformers. H. Matsumoto, S. Nishio, M. Takeshita and S. Shinkai, Tetrahedron, 1995, 51, 4647. 

Relatively few X-ray structures of calix[4]arenes in the partial cone conformation have been reported (see ref. 1, pp. 106-107), most of them involving tetra-0-... 

---