Conformational analysis of calixarene derivatives substituted at the methylene bridges

The conformation of tetrahydroxycalix[4]arene derivatives substituted at two opposite methylene bridges or at a single bridge by an alkyl or aryl group is reviewed. The cone form with an equatorial substituent is the lowest energy conformation of calixarenes possessing one bridge substituted by an alkyl group, whereas both the axial and equatorial conformers are similarly populated if the substituent is an aryl group. In the cone conformation of calixarenes possessing two opposite bridges substituted in trans fashion by a pair of groups, one group must necessarily be located in an axial position. The steric interactions ensuing from the presence of an axial group destabilize the cone conformation, rendering the 1,2-alternate form the lowest energy conformation in some derivatives.