Exploration of Diazaspiro Cores as Piperazine Bioisosteres in the Development of sigma 2 Receptor Ligands

A series of sigma 2R compounds containing benzimidazolone and diazacycloalkane cores was synthesized and evaluated in radioligand binding assays. Replacing the piperazine moiety in a lead compound with diazaspiroalkanes and the fused octahydropyrrolo[3,4-b] pyrrole ring system resulted in a loss in affinity for the sigma 2R. On the other hand, the bridged 2,5-diazabicyclo[2.2.1]heptane, 1,4-diazepine, and a 3-aminoazetidine analog possessed nanomolar affinities for the sigma 2R. Computational chemistry studies were also conducted with the recently published crystal structure of the sigma 2R/TMEM97 and revealed that hydrogen bond interactions with ASP29 and pi-stacking interactions with TYR150 were largely responsible for the high binding affinity of small molecules to this protein.