Discovery
of chemical tools for the ionotropic glutamate receptors
continues to be a challenging task. Herein we report a diversity-oriented
approach to new 2,3-trans-l-proline analogs
whereby we study how the spatial orientation of the distal carboxylate
group influences the binding affinity and receptor class and subtype
selectivity. In total, 10 new analogs were synthesized and 14 stereoisomers
characterized in binding assays at native rat ionotropic glutamate
receptors, and at cloned human homomeric kainic acid (KA) receptor
subtypes GluK1–3. The study identified isoxazole analogs 3d,e, which displayed selectivity in binding
at native N-methyl-d-aspartate (NMDA) receptors
over native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic
acid (AMPA) and KA receptors, in the high nanomolar to low micromolar
range. Furthermore, analogs 3i-A/B showed a preference
in binding affinity for GluK3 over GluK1,2. Finally, analog 3j displayed high nanomolar affinity for native NMDA receptors
as well as for homomeric GluK3 receptors.