Synthesis and crystal structure of p-acylated calix[4]arene ethers

Abstract

C-acylation of calix[n]arenes is an important reaction which has been primarily utilized for their further functionalization to provide conformers with varying shapes, cavity dimensions and molecular receptor characteristics that can bind ionic and neutral species in a selective and specific manner. The length of the alkyl chain at the upper or the lower rim of calixarenes can be adjusted as required to give derivatives which can span the channels and membranes and majorly influence transport phenomenon. As a part of our program to obtain calixarene based derivatives that can span and scan artificial membranes, C-acylation of calix[4]arene has been examined to yield peracylated and partially acylated calixarene ethers. 5,11,17,23-Tetraacetyl-25,26,27,28-tetramethoxycalix[4]arene has been obtained in 80 % yield by treatment of tetramethoxycalix[4]arene with acetyl chloride in the presence of aluminum chloride using dichloromethane as the solvent. The structure was established by the conversion to corresponding phenyl hydrazones and oximes. The tetraacetyltetramethoxycalix[4]arene 2a crystallized in a monoclinic lattice, space group P21/C with a = 10.320(2) A, b = 18.928(4) A, c = 18.421(4) A, β = 95.44(3)o, Z = 4. The corresponding methyl substituted O7 directs inwards towards the cavities of calix[4]arene to give an inward flattened partial cone conformation. Molecular packing shows the presence of intermolecular C–H···O, H-bonding interactions between methyl and methylene hydrogens and oxygens of the acetyl groups.