Cyclophanes as Synthetic Analogs of Enzymes and Receptors

In memoriam Iwao TabushiSince the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme-substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor-acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force-field calculations together with computer graphics are powerful tools in the design of water-soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates inla rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.

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Pentamakrocyclische Tris‐Kronen. – Selektive Bindung von Kationen, Anionen und Neutralverbindungen

Wallon

Werner

Müller

et al. 1990

Chem. Ber.

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The tris-crown host compounds 1 -4 were synthesized for the first time. The X-ray crystal structures of 2 and of the 1:3 complex of 1 and KSCN were determined. Whereas in the free crown 2 a less well preorganized molecular cavity is encountered, in the complex of 1 with KSCN one potassium ion is bound to each crown ether unit. One of the three SCN-anions is situated in the interior of the cavity and in addition is disordered in such a way as to be bound to the K+ using partially the S atom of the SCN-ion. Organic guest molecules like 1,2-, 1,3-, 1,5-, 2,6-, and 2,7-naphthalenediol as well as P-naphtho1 in acidic water solution are bound selectively in the interior of the cavities of the host molecules 2 and 3, but not by 1 and 4, the cavities of which seem to be too small and too large, respectively. In contrast, the binding of cr-naphthol and of smaller phenolic guest molecules like phenol, resorcinol, pyrocatechol, phloroglucinol, pyrogallol, and 1,2,4-trihydroxybenzene with the hosts 1 -4 is much less pronounced ('H-NMR highfield shifts).Pentamakrocyclische Kronenstrukturen der Typen 1 -5 sind bisher nicht bekannt 'I. Drei Kronenether-Einheiten sind hier durch starre aromatische Einheiten in einem definierten Abstand zueinander orientiert. Wir haben nun solche Strukturen dargestellt und zum einen den intramolekularen Abstand der Kronenether-Einheiten durch Einbau verschiedener aromatischer Abstandshalter I -111, die for-I1 111 ma1 eine dreizahlige Drehachse besitzen, schrittweise gezielt verandert. AuRerdem wurden die Komplexierungseigenschaften der Kronenether-Bausteine variiert, indem die Gro13e der verwendeten Diaza-Kronen mit Hilfe der Anzahl der Ethylenglycolether-Kettenglieder verandert wurde. Untersucht wurde ferner, inwieweit die vororientierten Kronenether-Einheiten dieser Verbindungen ein kooperatives Verhalten bei der Komplexierung verschiedener geladener und neutraler organischer Gastverbindungen zeigen.

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Cyclophanes as Synthetic Analogs of Enzymes and Receptors

Cited by 4 publications

References 142 publications

Cyclophane zur Komplexierung von Neutralmolekülen

Cyclophane zur Komplexierung von Neutralmolekülen

Complexation of Neutral Molecules by Cyclophane Hosts

Pentamakrocyclische Tris‐Kronen. – Selektive Bindung von Kationen, Anionen und Neutralverbindungen

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