A systematic exploration of bioisosteric replacements for furan and thiophene cores in a series of potent A<sub>2B</sub>AR antagonists has been carried out using the nitrogen-walk approach. A collection of 42 novel alkyl 4-substituted-2-methyl-1,4-dihydrobenzo[4,5]imidazo[1,2-<i>a</i>]pyrimidine-3-carboxylates, which contain 18 different pentagonal heterocyclic frameworks at position 4, was synthesized and evaluated. This study enabled the identification of new ligands that combine remarkable affinity (<i>K<sub>i</sub></i> < 30 nM) and exquisite selectivity. The structure-activity relationship (SAR) trends identified were substantiated by a molecular modeling study, based on a receptor-driven docking model and including a systematic free energy perturbation (FEP) study. Preliminary evaluation of the CYP3A4 and CYP2D6 inhibitory activity in optimized ligands evidenced weak and negligible activity, respectively. The stereospecific interaction between <i>h</i>A<sub>2B</sub>AR and the eutomer of the most attractive novel antagonist (<i><b>S</b></i>)-<b>18g</b> (<i>K<sub>i</sub></i> = 3.66 nM) was validated.