<i>N</i><sup>6</sup>-Cycloalkyl- and <i>N</i><sup>6</sup>-Bicycloalkyl-<i>C</i>5′(<i>C</i>2′)-modified Adenosine Derivatives as High-Affinity and Selective Agonists at the Human A<sub>1</sub> Adenosine Receptor with Antinociceptive Effects in Mice
N6-(endo-norborn-2-yl)adenosine derivatives, 5′-chloro-5′-deoxy-CPA (1) and 5′-chloro-5′-deoxy-(±)-ENBA (3) displayed the highest affinity in the subnanomolar range and relevant selectivity for hA1 vs the other human receptor subtypes. The higher affinity and selectivity of 5′-chloro-5′-deoxyribonucleoside derivatives 1 and 3 for hA1 AR vs hA3 AR compared to that of the parent 5′-hydroxy compounds CPA and (±)-ENBA