Among all crystalline complexes of calix-type calix[n]arenes those with organic molecules which are biologically relevant are especially of interest due to their potential medical and pharmaceutical applications. The co-crystallization of drugs with water-soluble calixarenes offers the opportunity to modify chemical and physical properties of APIs (active pharmaceutical ingredients) and to control drug conformation. Such co-crystallization can improve solubility, bioavailability and stability of pharmaceutically active molecules and/or eliminate polymorphism. In this article the solid-state interactions between calixarene hosts and biologically relevant guest molecules are summarized for the currently available structures solved by single-crystal X-ray crystallography.

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[structure: see text]. A diastereomeric mixture of chiral 25-(1S)-camphorsulfonyloxy-26-isopropoxycalix[4]arene 2a (de 15%) and 25-isopropoxy-26-((1S)-10-camphorsulfonyl)calix[4]arene 2b has been obtained by asymmetrical lower rim (1S)-camphorsulfonylation of the monoisopropoxycalix[4]arene. Pure diastereomer 2a has been obtained by simple crystallization, and its absolute configuration has been determinated by X-ray analysis. Enantiomerically pure inherently chiral 5,11-dibromo-26-isopropoxycalix[4]arene 4 has been synthesized by the upper rim dibromination of the diastereomer 2a followed by hydrolytical removal of the auxiliary camphorsulfonyl group.