Modified calixarenes and new calixarene-like macrocycles

A combined computational (molecular mechanics calculations) and experimental (synthesis, NMR studies, X-ray crystallography) approach was used for the determination of the structure, conformational equilibria and dynamic processes of several novel calixarene derivatives. Calix[4]arenes and calix[5]arenes substituted at one or two of the methylene bridges by alkyl or aryl groups were prepared by (2+2) or (3+2) fragment condensation. NMR data indicate that all these substituted calixarenes adopt a cone conformation. In the alkanediyl calix[4]- and calix[5]arenes, an alkyl substituent at the bridge prefers an equatorial position. In the calix[4]arenes, both the axial and equatorial conformers are populated when the substituent is phenyl, while only the equatorial conformer is present in the calix[5]arenes. The distinct conformational behavior of the phenyl substituent was rationalized in terms of destabilizing steric interactions present in the equatorial conformer of the calix[4]arene derivatives. A calixarene possessing pairs of vicinal exo and endo hydroxyl groups was prepared by fragment condensation. The compound adopts a 1,2-alternate conformation, and its rotational barrier is similar to that of endo-dihydroxycalix[4]arene, indicating that the additional intramolecular hydrogen bond between the exo-OH groups does not decrease the flexibility of the molecule. Solvent-induced conformational shifts were found for two exo-calixarenes. (orig.)Available from TIB Hannover: F00B234 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEGerman-Israeli Foundation for Scientific Research and Development (GIF), Oberschleissheim (Germany)DEGerman