Synthesis and Biological Evaluation of 2‑Amino-3-(4-chlorobenzoyl)-4-[(4-arylpiperazin-1-yl)methyl]-5-substituted-thiophenes. Effect of the 5‑Modification on Allosteric Enhancer Activity at the A1 Adenosine Receptor

We have recently reported a detailed structure–activity relationship study around a wide series of 2-amino-3-(4-chlorobenzoyl)-4-[(4-arylpiperazin-1-yl)­methyl]­thiophene derivatives as potent allosteric enhancers of the A1 adenosine receptor. In the current study, we have continued to explore the potential of these molecules by synthesizing of a novel series of analogues that share a common 2-amino-3-(4-chlorobenzoyl)­thiophene nucleus. Modifications were focused on varying the nature and the position of electron-withdrawing or electron-releasing groups on the phenyl of an arylpiperazine moiety attached at the 4-position of the thiophene ring by a methylene chain, combined with the presence of small alkyl groups (methyl or ethyl), bromine, or aryl moieties at the thiophene C-5 position. In this series of compounds, substitution at the 5-position had a fundamental effect on activity, with the 5-aryl group contributing additively to the allosteric enhancer activity. The thiophene C-5 aryl derivatives 4ad, 4ak, and 4al were the most active compounds in binding and functional experiments.