A series of 24 1-aryl-6-benzyl-7-hydroxy-2,3-dihydroimidazo[1,2-a]pyrimidine-5(1H)-ones was designed as antinociceptive compounds acting through opioid receptors with additional serotoninergic activity. The compounds, similarly as previously published series, lack the protonable nitrogen atom which is a part of classical opioid receptor pharmacophore and is necessary to interact with the conserved Asp(3.32) in the opioid receptor binding pocket. The compounds were obtained in one-step cyclocondensation of 1-aryl-4,5-dihydro-1H-imidazol-2-amines diethyl 2-benzylmalonate or diethyl 2-(2-chlorobenzyl)malonate under basic conditions. Almost all the tested compounds exerted strong antinociceptive activity, but surprisingly, it was not reversed by naloxone; thus, it is not mediated through opioid receptors. It makes it possible to conclude that addition of one more aromatic moiety to the non-classical opioid receptor pharmacophore results in the compounds which are not opioid receptor ligands. The lack of activity of one of the tested compounds may be attributed to low blood-brain barrier permeation or unfavorable distribution of electrostatic potential and HOMO and LUMO orbitals.
Keywords: Antinociceptive compounds; Central nervous system activity; Imidazo[1,2-a]pyrimidines.