Negative allosteric modulators of N‐methyl‐ d‐aspartate receptors containing the GluN2B subunit represent promising drug candidates for the treatment of various neurological disorders including stroke, epilepsy, and Parkinson's disease. To increase the bioavailability and GluN2B affinity, the phenol of the potent benzazepine‐based inhibitor, WMS‐1410 (3), was replaced bioisosterically by a benzoxazolone moiety and the phenylbutyl side chain was conformationally restricted in a phenylcyclohexyl substituent. A four‐step, one‐pot procedure transformed the oxazolo‐benzazepine 7 into the phenylcyclohexyl derivative 11. The same protocol was applied to the methylated analog 12, which unexpectedly led to ring‐contracted oxazolo‐isoquinolines 18. This rearrangement was explained by the additional methyl moiety in the 8‐position inhibiting the formation of the planar intermediate iminium ion with phenylcyclohexanone. The allyl protective group of 11 and 18 was removed with RhCl3 and HCl to obtain the tricyclic compounds 5 and 19 without substituent at the oxazolone ring. The structures of the rearranged products 18 and 19 were elucidated by X‐ray crystal structure analysis. The oxazolo‐isoquinoline trans‐18 with allyl moiety (Ki = 89 nM) and the oxazolo‐benzazepine 5 without substituent at the oxazolone ring (Ki = 114 nM) showed GluN2B affinity in the same range as the lead compound 3. In two‐electrode voltage clamp measurements, 5 displayed only weak inhibitory activity.

To improve the metabolic stability and receptor selectivity of ifenprodil (1), the benzoxazolone moiety of besonprodil ( 2) and the 3-benzazepone moiety of WMS-1410 (3) were merged to obtain oxazolobenzazepines of type 4. The 5-(hydroxyethyl)benzoxazolone 7 representing the first key intermediate was prepared in four steps starting with the 4-(2-hydroxyethyl)phenol (8). Mitsunobu reaction of primary alcohol 7 with N-sulfonylated glycine esters established the necessary side chain. The intramolecular Friedel-Crafts acylation of acid 12a containing the N-tosyl protective group led upon decarbonylation exclusively to the tricyclic tetrahydroisoquinoline 14. Protection of the amino moiety by the stronger electron-withdrawing triflyl group resulted in the desired 3-benzazepine 15 without the formation of analogous isoquinoline. The triflyl protective group was cleaved off by K 2 CO 3 -induced elimination of trifluoromethanesulfinate. In a one-pot three-step procedure, various oxazolobenzazepinediones 15 were obtained, which were reduced to afford the desired secondary alcohols 18.

Tricyclic tetrahydrooxazolo[4,5-h]-[3]benzazepin-9-ols 22 were designed as phenol bioisosteres of tetrahydro-3-benzazepine-1,7-diols. Key features of the synthesis are the introduction of the trifluoromethylsulfonyl and allyl protective groups at the heterocyclic N-atoms. Two methods were developed to convert the triflyl-protected ketone 16 into tricyclic alcohols 21 bearing various N-substituents. According to the first method, trifluoromethanesulfinate was removed by K 2 CO 3 . Following the selective reduction of the imino moiety of 17 with NaBH(OAc) 3 afforded the aminoketone 18, which was reductively alkylated and reduced. According to the second method, both the imine and the ketone of the iminoketone 17 were reduced with NaBH 4 to yield the aminoalcohol 20, which was alkylated or reductively alkylated to form tertiary amines 21f-21r. In the last step, the allyl protective group of 21 was removed with RhCl 3 and HCl to obtain oxazolones 22. In receptor binding studies using [ 3 H]ifenprodil as radioligand ketone, 22m showed the highest GluN2B affinity (K i = 88 nM). However, a reduced affinity toward GluN2B subunit-containing N-methyl-D-aspartate (NMDA) receptors was observed for oxazolones 22 compared to bioisosteric 3-benzazepine-1,7-diols. High selectivity of 22m for the ifenprodil binding site of GluN2B-NMDA receptors over the 1-(1-phenylcyclohexyl)piperidine binding site and σ 2 receptors was observed, but only negligible selectivity over σ 1 receptors. In two-electrode voltage clamp experiments, the 4-phenylbutyl derivative 22d (K i = 422 nM) demonstrated 80% inhibition of ion flux at a concentration of 1 µM. The differences in GluN2B affinity and inhibitory activity are explained by docking studies. In conclusion, 22d is regarded as a novel scaffold of highly potent GluN1/GluN2B antagonists.

Inspired by besonprodil, the phenol of potent negative allosteric modulators of GluN2B‐N‐methyl‐d‐aspartate (NMDA) receptors was replaced by a benzoxazolone system. To increase the similarity to the lead compounds, an additional methyl moiety was installed in the 8‐position of tricyclic oxazolobenzazepines, resulting in compounds 6. The additional methyl moiety originates from alanine, which was introduced by a Mitsunobu reaction of benzoxazolylethanol 7 with N‐triflyl‐protected alanine methyl ester. A crucial feature of the synthesis was the protection of the oxazolone ring by an allyl moiety, which was cleaved off at the end of the synthesis by RhCl3‐catalyzed isomerization. Due to the additional methyl moiety, the intramolecular Friedel–Crafts acylation of acid 10 to afford ketone 11 required careful optimization to minimize the formation of the side product tetrahydroisoquinoline 16. Alkylation or reductive alkylation of secondary amine 13 led to diastereomeric oxazolobenzazepines cis‐14 and trans‐14, which were separated by flash chromatography. Phenylbutyl derivatives cis‐6a and trans‐6a revealed twofold higher GluN2B affinity than analog 5a without 8‐CH3 group. The methylated oxazolobenzazepines 6 and 14 did not interact with the phencyclidine binding site of NMDA receptors and σ2 receptors. However, the σ1 receptor preferred cis‐configured oxazolobenzazepines. The highest σ1 receptor affinities were obtained for cis‐14a (Ki = 26 nM) and cis‐6b (Ki = 30 nM).

The NMDA receptor plays a key role in the pathogenesis of neurodegenerative disorders including Alzheimer's and Huntington's disease, as well as depression and drug or alcohol dependence. Due to its participation in these pathologies, the development of selective modulators for this ion channel is a promising strategy for rational drug therapy. The prototypical negative allosteric modulator ifenprodil inhibits selectively GluN2B subunit containing NMDA receptors. It was conformationally restricted as 2-methyl-3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepine-1,7-diol, which showed high GluN2B affinity and inhibitory activity. For a better understanding of the relevance of the functional groups and structural elements, the substituents of this 3-benzazepine were removed successively (deconstruction). Then, additional structural elements were introduced (reconstruction) with the aim to analyze, which additional modifications were tolerated by the GluN2B receptor. The GluN2B affinity was recorded in radioligand receptor binding studies with the radioligand [3H]ifenprodil. The activity of the ligands was determined in two-electrode voltage clamp experiments using Xenopus laevis oocytes transfected with cRNA encoding the GluN1-1a and GluN2B subunits of the NMDA receptor. Docking studies showed the crucial interactions with the NMDA receptor protein. The deconstruction approach showed that removal of the methyl moiety and the phenolic OH moiety in 7-positon resulted in almost the same GluN2B affinity as the parent 3-benzazepine. A considerably reduced GluN2B affinity was found for the 3-benzazepine without further substituents. However, removal of one or both OH moieties led to considerably reduced NMDA receptor inhibition. Introduction of a NO2 moiety or bioisosteric replacement of the phenol by a benzoxazolone resulted in comparable GluN2B affinity, but almost complete loss of inhibitory activity. An O-atom, a carbonyl moiety or a F-atom in the tetramethylene spacer led to 6-7-fold reduced ion channel inhibition.