Effects of Conformational Restriction of 2-Amino-3-benzoylthiophenes on A1 Adenosine Receptor Modulation

2-Amino-3-benzoylthiophenes (2A3BTs) have been widely reported to act as allosteric enhancers (AEs) at the A1 adenosine receptor (A1AR). Herein we describe the synthesis of a series of 1-aminoindeno[1,2-c]thiophen-8-ones and a series of (2-aminoindeno[2,1-b]thiophen-3-yl)(phenyl)methanones as conformationally rigid analogues of the 2A3BTs. These compounds were screened using a functional assay of A1AR-mediated phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in intact Chinese hamster ovary (CHO) cells to identify both potential agonistic effects as well as the ability to allosterically modulate the activity of the orthosteric agonist, N6-(R-phenylisopropyl)adenosine (R-PIA). All of the 1-aminoindeno[1,2-c]thiophen-8-ones (14a−c and 17a−f) proved either to be inactive or behaved as antagonists in the functional assay. However, the (2-aminoindeno[2,1-b]thiophen-3-yl)(phenyl)methanones with para-chloro substitution (compounds 25b, 25d, and 25f) did significantly augment the R-PIA response, indicating a positive allosteric effect.