Synthesis of expanded calix[ n]pyrroles and their furan or thiophene analogues

Abstract

meso-Dialkylporphyrinogen-like mixed cyclic oligomers containing furans, pyrroles and thiophenes have been synthesized by ‘3+1’, ‘3+2’ and ‘3+ketone’ approaches. Condensation of 2,5-bis[1-methyl-1(1 H-pyrrol-2-yl)-ethyl]furan with 2,5-bis[(α-hydroxy-α,α-dimethyl)furan or 2,5-bis[(α-hydroxy-α,α-dimethyl)thiophene resulted in the formation of the mixture of cyclic tetramer, cyclic octamer and cyclic hexamer possessing different numbers of furans (thiophenes) and pyrroles. The major product was found to be the cyclic tetramer in ‘3+1’ condensation while cyclic pentamer was the major product in ‘3+2’ condensation. Appreciable amounts of cyclic decamer was isolated in ‘3+2’ condensation. The formation of furan–pyrrole alternating cyclic hexamer in ‘3+1’ condensation indicates acid-catalyzed, reversible cleavage of the starting material during the reaction. In the same analogy, cyclic dodecamer was synthesized by ‘4+2’ approach. Synthesis of cyclic dodecamer and cyclic pentadecamer was also achieved by condensing 2,5-bis[1-methyl-1(1 H-pyrrol-2-yl)-ethyl]furan with ketones. The effect of catalysts, temperature, templates, solvent and concentration was examined and no appreciable changes in the product distribution were found.