CoachA crown ether is a macrocyclic polyether, such as 18-crown-6, . A cryptand is a bicyclic polyether with two nitrogen bridgeheads, which encloses a cation in three dimensions in a cryptate. A cyclodextrin is a cyclic oligomer of glucose units, in the shape of a truncated cone with a hydrophobic cavity and hydroxy groups on its rims. A calixarene is a cup-shaped macrocycle of phenol units joined by methylene bridges.
![Left: 18-crown-6, a ring of twelve carbon atoms (corners) and six oxygen atoms, with a potassium ion at its centre bound by the six oxygens. Right: the [2.2.2]cryptand, three chains of two ether oxygens between two nitrogen bridgeheads, schematic: the cation is held inside a three-dimensional cage, by six oxygens and the two nitrogens.](/images/onecourse/chapters/chemistry-4/b3-supramolecular/fig-8ce9fbbe0866.svg)