CALIXARENES - PARADOXES AND PARADIGMS IN MOLECULAR BASKETS

Calixarenes are [ln]metacyclophanes that can be prepared in one-flask syntheses from ceminp-alkylphenols and formaldehyde. Their history is a facet of phenol-formaldehyde chemishy which had its genesis in the nineteenth century laboratories of Adolph von Baeyer. Their structures were partially elucidated by Zinke and by Niederl in the 1940's, but incomplete data concerning their synthesis and purification created troubling paradoxes. As a result of the research by groups in Mainz, Parma, and St Louis in the late 1970s and early 1980s the paradoxes have been resolved and a paradigm has been created that has elicited widespread and increasing attention to these compounds. The paradigm, which states that cyclic oligomers of various sizes are formed from the condensation of certain p-substituted phenols and formaldehyde, provides the guideline for easily obtaining p-rert-butylcalix[4], calix[6], and calix[l]arenes in a single step. The AlC13 catalyzed removal of the rerr-butyl groups then makes possible the introduction of a variety of substituents, and several methods have been developed for this purpose. The conformational mobility of the calixarenes, which played a key role in the aforementioned paradox, has been well-studied both in the solid state and in solution. It can often be curtailed by appropriate substitution on the phenolic oxygen atoms which freezes the calixarene into a particular conformation. Calixarenes are especially amactive as cornplexing agents and enzyme mimics, because their basket-like shape allows them to act as hosts to guest molecules of appropriate size and character. As the result of the ease with which calixarenes can now be synthesized, functionalized, and conformationally fixed much effort is currently being devoted to such studies. © 1990, IUPAC.