Calixarene-like Cyclooligomers

Over a century ago, Baeyer isolated a cyclic tetramer (80) from the acid-catalyzed condensation of pyrrole and acetone, and these compounds have been recently studied as anion binders and given the name calix[4]pyrroles . Compounds that have acquired the name calix[3]indoles can be prepared by 

Sessler, J. L. Krai, V. Lynch, V. J. Am. Chem. Soc. 19%, 118, 5140 see this journal article for earlier references to these compounds. 

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The distribution of products in the case of the cyclisation of 2,6-bis(bromomethyl)-4-methoxypyridine depends mainly on dilution conditions and the temperature. At temperatures of about -100 to -90 °C, formation of open-chain products is decreased. Use of high-dilution conditions leads to odd-numbered main products like the trimer 37 (12%) and pentamer 38 (2%) [58]. Increase of concentration produces a broader spectrum of cyclooligomers. Although even-numbered conventional calixarenes are readily obtained, improvements in the synthetic yields for odd-numbered calixarenes have been slowly achieved [59, 60]. 

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