Synthesis and Biological Evaluation of Novel Allosteric Enhancers of the A1 Adenosine Receptor Based on 2-Amino-3-(4’-Chlorobenzoyl)-4-Substituted-5-Arylethynyl Thiophene

A Sonogashira coupling strategy was employed to synthesize a new series of allosteric modulators for the A1 adenosine receptor based on the 2-amino-3-(p-chlorobenzoyl)-4-substituted 5-thiophene skeleton, with a two-carbon (rigid or flexible) linker between the 5-position of the thiophene ring and a (hetero)aryl or alkyl moiety. Among the compounds characterized by the presence of a common phenylacetylene moiety at the 5-position of the thiophene ring, the neopentyl substitution at the 4-position supported a strong activity. In the series of 4-neopentyl derivatives, the presence of an acetylene spacer at the 5-position of the thiophene is optimal for activity, while reduction of the acetylene to an ethyl moiety decreased activity, both in functional and binding assays. Derivatives 4e, 4g-h, 4j, 4l and 4m were the most promising compounds in binding (saturation and competition) and functional cAMP studies, being able to potentiate agonist [3H]CCPA binding to the A1 receptor, with 4e as the best compound of the series. The latter compound also retarded the dissociation of another radiolabeled agonist, [3H]NECA, from the receptor.