Improved Procedure for the Synthesis of Calix[4]arenes with Four Amino Groups on the Wide Rim

Calix[4]arenes fixed in the cone conformation and bearing four amino functionalities on the wide rim, are well known and often used starting materials in calixarene chemistry. During our studies in this field we have noticed, that the safe procedures known (which use palladium on carbon as catalyst and hydrazine hydrate as reducing agent in methanol at reflux) not always fit well for the synthesis of all possible even rather simple derivatives of this type and undesired mixtures are usual outcome in these cases. High resolution MS and 1H NMR indicate, that the main impurity (nearly 30%) is a partially reduced calix[4]arene containing three amino- and one nitro groups. We have modified the synthetic procedure by changing the solvent to iso-propanol, which allowed to shorten the reaction times and to obtain the desired products in high purity and yields.