The binding site for competitive NMDA receptor antagonists is on the NR2 subunit, of which there are four types (NR2A-D). Typical antagonists such as (R)-AP5 have a subunit selectivity of NR2A > NR2B > NR2C > NR2D. The competitive NMDA receptor antagonist (2R,3S)-(1-biphenylyl-4-carbonyl)piperazine-2,3-dicarboxylic acid (PBPD, 16b) displays an unusual selectivity with improved relative affinity for NR2C and NR2D vs NR2A and NR2B. Analogues of 16b bearing aroyl or aryl substituents attached to the N(1) position of piperazine-2,3-dicarboxylic acid have been synthesized to probe the structural requirements for NR2C/NR2D selectivity. A phenanthrenyl-2-carbonyl analogue, 16e, had >60-fold higher affinity for NR2C and NR2D and showed 3-5-fold selectivity for NR2C/NR2D vs NR2A/NR2B. The phenanthrenyl-3-carbonyl analogue (16f) was less potent but more selective, having 5- and 7-fold selectivity for NR2D vs NR2A and NR2B, respectively. Thus, antagonists bearing bulky hydrophobic residues have a different NR2 subunit selectivity than that of typical antagonists.