Calix pyridine

The Ciamician-Dennstedt reaction is currently the only way to date to make calix[4]pyridine 20.(Scheme 8.3.6) Four sequential treatments of calix[4]pyrrole 19 with sodium trichloroacetate results in all four possible geometric isomers of 20 (all four... 

The specific structure of [(H20)5Ni(py)]2+ was observed in the complexes with the second-sphere coordination of calix[4]arene sulfonate.715 There are two different [(H20)5Ni(py)]2+ cations in the complex assembly. In one the hydrophobic pyridine ring is buried in the hydrophobic cavity of the calixarene with the depth of penetration into the calixarene cavity being 4.3 A (Figure 9). The second independent [(H20)5Ni(py)]2+ cation is intercalated into the calixarene bilayer. 

Figure 7.39 X-ray crystal structure of (a) Na[Eu(H20)g]2[Eu(pyridine-./V-oxide)(H20)5(calix[5] arene-p-sulfonato)2] pyridine-A7-oxide 17.5H205cl (b) [Rh -OH E C)) ] [X+2H20 X-8H+]-33H20 where X = (p-sulfonatocalix[4]arene)2(Na([18]crown-6)]7 (reprinted from the Section Key Reference with permission from Elsevier).

Exploiting the same concept, Reinhoudt and Shinkai were able to obtain another example of enantioenriched chiral double-rosette made of achiral components. In this case achiral calix[4]arene bearing dimelamine moieties with pyridine functionalities were assembled with achiral cyanurate leading to racemic chiral double rosettes. The latter are perfect counterparts for chiral D-dibenzoyl tartaric acid via two-point hydrogen-bonding interactions thus... 

Hosseini and co-workers [49] described the synthesis of an infinite single strand helical supramolecular network 27 4 by mixing equimolar amounts of the calix[4]-arene derivative 27, bearing four pyridine groups as H-bond acceptors, with... 

Calix[4]arene 142 in the 1,3-alternate conformation has two pairs of pyridine residues (hydrogen-bond acceptor) pointing in opposite directions. Together with... 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

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. 

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 . 

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