1.3- Altemate conformation

For the three conformers, the binding sequence is Na > K > Rb > Cs. This is supported by energy component analysis on the trajectories, as well as by Free energy perturbation (FEP) calculations. Intrinsically, Cs+ has the weakest interactions with both hosts. The largest contribution of the cation/host interaction energy comes from the ether ring rather than from the aromatic moieties. Each complex displays a clear conformational preference. Sodium is most stable in the cone conformation, whereas cesium is most stable in the 1,3-altemate conformation. 

Calix[4]arenes-bis-crown constrained to a 1,3-altemate conformation present two binding sites on each crown, which are linked by a %-basic benzene tunnel. In the 1 1 complexes, the cation switches from one binding site to the other through this... 

Ungaro, R., Casnati, A., Ugozzoli, F., Pochini, A., Dozol, J.-F., and Hill, C., Rouquette, H. 1,3-Dialkoxycalix[4]arenecrowns-6 in 1,3-Altemate Conformation, Cesium-Selective Ligands that Exploit Cation-Arene Interactions, Angew. Chem., Int. Ed Engl. 33(14) (1994), 1506-1509. 

Asfari, Z., Bressot, C., Vicens, J., Hill, C., Dozol, J.-F., Rouquette, H., Eymard, S., Lamare, V., Toumois, B. Doubly Crowned Calix[4]arenes in the 1,3-Altemate Conformation as Cesium-Selective Carriers in Supported Liquid Membranes. Anal. Chem. 67 (1995), 3133-3139. 

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. 

An elegant synthesis of inherently chiral calix[4]arenes 75 in the 1,3-altemate conformation (type AABB) was recently described by Gutsche et al.136 It consists of the ring opening aminolysis of calix anhydride 74, which is available from the corresponding 1,3-altemate tetraacid 73 by reaction with oxalyl chloride in yields up to 60%. 

It should be mentioned that a tetraether in the 1,3-altemate conformation (54) or 1,2-altemate conformation (Cj) or an anti-1,3-diether (C,) are not chiral. 

The synthesis of p-t-butyltetrathiocalix[4]arene has been reported Gibbs and Gutsche [27], Interestingly, this compound (Figure 16) was found to exist in the 1,3-altemate conformation. 

The reaction of p-bu -calix[4]arene with 4-chloro-2-phenylquinazoline gave a mixture of the syn- and, 3-diether derivative 64. In solution, the syn form adopted predominantly the cone conformation, whereas in its crystals an energetically unfavorable 1,3-altemate conformation was present (2003NJC236). 

The first tetra-TTF calix[4] pyrrole conjugate 170 has been synthesized by the condensation of acetone and the corresponding monopyrrolo-TTF unit. This calix[4]pyrrole in its 1,3-altemate conformation acted as a host for neutral electron-deficient guests such as 1,3,5-trinitrobenzene, tetrafluoro-p-benzoquinone, tetrachloro-p-benzoquinone and p-benzo-quinone. The addition of chloride ions served to effect release of these guests and thus neutral substrate recognition process could be blocked by the addition of chloride anion (2004JA16296). 

The crystal structure of the calix[4Jindole 26a was elucidated and it was found that the molecule adopts a 1,3-alternate conformation in the solid state forming a "square" channel (Figure 29) that is analogous to what is seen in the 1,3-altemate conformation of calixarenes. ... 

Stoikov et alP synthesized novel p-tert-hnty thia-calix[4]arenes functionalized with hydrazide groups at the lower rim of the cone and the partial cone in the 1,3-altemate conformations. The affinity of the self-assembly to p-(Al +, Pb +) and c -(Fe +, Co +, Ni +, Cu ", Ag" ", Cd " ") block elements was investigated by... 

To summarize, l,3-calix[4]-6/5-crowns containing six oxygen atoms appeared to be a promising family of carriers for the selective removal of cesium from high salinity media, such as medium-level radioactive liquid wastes, with SLMs. By choosing a highly hydrophobic organic diluent, 2-NPOE, and a very lipophilic cafix[4]- /5-crown in the 1,3-altemate conformation suitable for cesium complexation over sodium, very selective and stable SLMs can be obtained (over a period of 50 days). 

A different pattern of reaction products was obtained with the ethyl 3 and isopropyl 5 cinnamic esters. In both cases, comparable chemical yields were obtained, but in addition to the 1,2-altemate (4a, 6a, respectively) and the flattened-cone (4b, 6b, respectively), a third type of stereoisomers (namely, 4c and 6c) emerged from the reaction mixture. An accurate analysis of H and NMR spectra pointed out the existence of two symmetry planes. DIF NOE experiments revealed the proximity of the methine and the methylene groups with the aromatic protons. Therefore, on the basis of only NMR evidences, 4c and 6c were assigned the 1,3-altemate conformation with the pseudoequatorial side chains in an all -cis position. Later, thanks to the obtainment of suitable crystals for X-ray analysis, the stereoisomer 4c was definitively reassigned the chair conformation [6]. 

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