Breathing crown ethers

Schroder and Witt ° have reported the synthesis of crown ethers having fluctuating ring sizes which they have termed breathing crown ethers . The structures are based on the bullvalene subunit and, as the tetracyclic subunit undergoes Cope rearrangement, the size of the macroring likewise varies. The synthetic steps follow the conventional routes used for the preparation of crown ethers and are illustrated in Eq. (3.44). 

Crown ethers with fluctuating ring size (a supply of activation energy < 20 kcal moE suffices to vary the ring size reversibly), which have been called breathing crown ethers , have been synthesized which incorporate bullvalene into the ring. For bullvalene with two equal substituents, twelve positional isomers, i.e. four each of the 1,2-, 1,3-, and 1,4-isomers, can be formulated, so that for (55 n = 1) and (55 n = 2) the ring size varies between eleven and thirteen members, and between twenty and twenty-two members, respectively. 

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