Replacement of the Benzylpiperidine Moiety with Fluorinated Phenylalkyl Side Chains for the Development of GluN2B Receptor Ligands

The 4-benzylpiperidine moiety is a central structural element of potent N-methyl-d-aspartate (NMDA) receptor antagonists containing the GluN2B subunit. To obtain novel GluN2B ligands suitable for positron emission tomography, the benzylpiperidine moiety was replaced with fluorinated ω-phenylalkylamino groups. For this purpose three primary propyl- and butylamines 7 a–c and one butyraldehyde 7 d bearing a fluorine atom and an ω-phenyl moiety were prepared in 3- to 7-step syntheses. Compounds 7 a–d were attached to various scaffolds of potent GluN2B antagonists (scaffold hopping) instead of the original 4-benzylpiperidine moiety. Although benzoxazol-2-ones and indoles with a benzylpiperidine moiety show high GluN2B affinity, the corresponding fluorophenylalkylamine derivatives did not result in high Glu2B affinity. Moderate GluN2B affinity was observed for a 3-(fluoroalkyl)-substituted tetrahydro-1H-3-benzazepine (K_i=239 nm). However, high GluN2B affinity was obtained for the tetrahydro-5H-benzo[7]annulen-7-amines 12 a–c (K_i=17–30 nm). Docking studies resulted in the same binding pose for 12 a as for the lead compound Ro 25-6981. It can be concluded that some GluN2B ligands (benzoxazolones, indoles) do not tolerate replacement of the 4-benzylpiperidine moiety with flexible fluorinated phenylalkyl side chains, but other scaffolds such as tetrahydro-3-benzazepines and -benzo[7]annulenes retain interaction with NMDA receptors containing the GluN2B subunit.