New spirocyclic Δ2-isoxazoline derivatives related to selective agonists of α7 neuronal nicotinic acetylcholine receptors

A set of structural analogues of spirocyclic quinuclidinyl- Δ²-isoxazolines, characterized as potent and selective α7 nicotinic agonists, was prepared and assayed for binding affinity at α7 and α4β2 neuronal nicotinic acetylcholine receptors (nAChRs). The investigated derivatives (3a-3c, 4a-4c, 5a-5c, 6a-6c, and 7a-7c), synthesized via the 1,3-dipolar cycloaddition of nitrile oxides to suitable dipolarophiles, showed an overall reduced affinity at the α7 subtype when compared with their model compounds. Solely Δ²-isoxazolines 3a, 3b, and 6c maintained a binding affinity in the nanomolar range at the α7 nAChRs (K_i = 230, 420 and 700 nM, respectively). The quaternary ammonium salt 6c retained also a noteworthy α7 vs. α4β2 subtype selectivity, whereas 3a and 3b showed a sharp reduction in selectivity compared with 1a and 1b, their quinuclidinyl higher homologues.