Calixarene

A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry. Calixarene nomenclature is straightforward and involves counting the number of repeating units in the ring and including it in the name. A calix[4]arene has 4 units in the ring and a calix[6]arene has 6. A substituent in the meso position Rb is added to the name with a prefix C- as in C-methylcalix[6]arene The word calixarene is derived from the Greek calix or chalice because this type of molecule resembles a vase (or cup) and from the word arene that refers to the aromatic building block. Calixarenes are generally produced by condensation of two components: an electron-rich aromatic compound, classically a 4-substituted phenol, and an aldehyde, classically formaldehyde. The scope for the aromatic component is broad diverse. The key attribute is susceptibility toward hydroxyalkylation.The related resorcinarenes and pyrogallolarenes are produced from resorcinol and pyrogallol, respectively. The aldehyde most often used is formaldehyde, while larger aldehydes, like acetaldehyde, are usually required in condensation reactions with resorcinol and pyrogallol to facilitate formation of the C4v symmetric vase conformation. Additionally, substituted aldehydes and some heterocycles (e.g. furan) may be used to impart additional functional groups onto the pendent groups of resorcinarenes and pyrogallolarenes. Calixarenes can be challenging to synthesize, producing instead complex mixtures of linear and cyclic oligomers. With finely tuned starting materials and reaction conditions, synthesis can also be surprisingly efficient. Calixarenes are sparingly soluble as parent compounds and have high melting points. Calixarenes are characterised by a three-dimensional basket, cup or bucket shape. In calix[4]arenes the internal volume is around 10 cubic angstroms.