Calixarene

With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry.

Conformations can be locked in place with proper substituents replacing the hydroxyl groups which increase the rotational barrier.

One result was the discovery made in 1942 by Alois Zinke, that p-alkyl phenols and formaldehyde in a strongly basic solution yield mixtures containing cyclic tetramers.

Concomitantly, Joseph Niederl and H. J. Vogel obtained similar cyclic tetramers from the acid-catalyzed reaction of resorcinol and aldehydes such as benzaldehyde.

A number of years later, John Cornforth showed that the product from p-tert-butylphenol and formaldehyde is a mixture of the cyclic tetramer and another ambiguous cyclomer.

In the early 1970s C. David Gutsche recognized the calix shape of the cyclic tetramer and thought that it might furnish the structure for building an enzyme xenologue.

His attention to these compounds came from acquaintance with the Petrolite company's commercial demulsifiers, made by ethoxylation of the still ambiguous products from p-alkylphenols and formaldehyde.

He also determined the structures for the cyclic tetramer, hexamer, and octamer, along with procedures for obtaining these materials in good to excellent yields.

He then established procedures for attaching functional groups to both the upper and lower rims and mapped the conformational states of these flexible molecules.

Chemists of University of Parma, Giovanni Andreetti, Rocco Ungaro and Andrea Pochini were the first to resolve X-ray crystallographic images of calixarenes.

The Niederl cyclic tetramers from resorcinol and aldehydes were studied in detail by Donald J. Cram, who called the derived compounds "cavitands" and "carcerands".

[17][18] Calixarenes are of interest as enzyme mimetics, components of ion sensitive electrodes or sensors, selective membranes, non-linear optics[19] and in HPLC stationary phases.

Calixarenes accelerate reactions taking place inside the concavity by a combination of local concentration effect and polar stabilization of the transition state.