Synthesis, GluN2B affinity and selectivity of benzo[7]annulen-7-amines

Divergent Synthesis of 17-nor-Cephalotane Diterpenoids through Developed Ynol-diene Cyclization

On the basis of a novel ynol-diene cyclization developed as a rapid access to tropone unit, the first divergent strategy to 17-nor-cephalotane diterpenoids has been successfully established. Combining with a bioinspired stereoselective dual hydrogenation, the divergent total synthesis of (+)-3-deoxyfortalpinoid F, (+)-harringtonolide, (-)-fortalpinoids M/N/P, and analog (-)-20-deoxocephinoid P have been achieved in 14-17 linear longest steps starting from commercially available materials.

Synthesis and Characterization of 1-Hydroxy-4,5-arene-Fused Tropylium Derivatives

The properties of 1-hydroxy-4,5-arene-fused tropyliums were assessed based on experimental and theoretical investigations. An X-ray crystallographic analysis revealed a decrease of bond alternation in the seven-membered ring of 1-hydroxy-4,5-benzotropylium derivatives compared with that of the parent 4,5-benzotropones, which is indicative of an increase in aromaticity upon protonation. NICS and AICD calculations also supported the increased aromaticity of 1-hydroxy-4,5-arene-fused tropylium. The pK_a values for a series of 1-hydroxy-4,5-arene-fused tropylium derivatives were also determined.

Polarized Helical Coumarins: [1,5] Sigmatropic Rearrangement and Excited-State Intramolecular Proton Transfer

The tandem process of phenol addition to a cyclic α,β-unsaturated ester followed by intramolecular transesterification and [1,5] sigmatropic rearrangement affords a series of helical coumarins based upon a previously unknown 3-amino-7-hydroxybenzo[3,4]cyclohepta[1,2-c]chromen-6-one core. These novel polarized coumarins, possessing a β-ketoester moiety, have been employed to synthesize more rigid and helical coumarin–pyrazolones, which display green fluorescence. The enhanced emission of coumarin–pyrazolones in polar solvents depends on the nature of the S₁ state. The coumarin–pyrazolones are predicted to have two vertical states close in energy: a weakly absorbing S₁ (¹LE) followed by a bright S₂ state (¹CT). In polar solvents, the ¹CT can be stabilized below the ¹LE and may become the fluorescent state. Solvatochromism of the fluorescence spectra confirms this theoretical prediction. The presence of an N—H···O=C intramolecular hydrogen bond in these coumarin–pyrazolone hybrids facilitates excited-state intramolecular proton transfer (ESIPT). This process leads to a barrierless conical intersection with the ground electronic state and opens a radiationless deactivation channel effectively competing with fluorescence. Solvent stabilization of the CT state increases the barrier for ESIPT and decreases the efficiency of the nonradiative channel. This results in the observed correlation between solvatochromism and an increase of fluorescence intensity in polar solvents.

Synthesis and pharmacological evaluation of fluorinated benzo[7]annulen-7-amines as GluN2B-selective NMDA receptor antagonists

Because of their neuroprotective potential, GluN2B-selective ligands are of great interest for the treatment of various neurological and neurodegenerative disorders. Fluorinated benzo[7]annulen-7-amines, capable for PET, were synthesized by combining fluorinated phenylalkylamines with differently substituted ketones. Relationships between substitution pattern and GluN2B affinity as well as selectivity towards σ₁ and σ₂ receptors were investigated. Two promising ligands (18a and 20c) were selected for further pharmacological evaluation. Besides a slight serotonin transporter (SERT), norepinephrine transporter (NET), and hERG affinity, they did not show interaction with other targets. Furthermore, the pK_a value of a set fluorinated ligands, bearing the fluorine atom in different positions, was determined.

Negative allosteric modulators of the GluN2B NMDA receptor with phenylethylamine structure embedded in ring-expanded and ring-contracted scaffolds

A set of GluN2B NMDA receptor antagonists with conformationally restricted phenylethylamine substructure was prepared and pharmacologically evaluated. The phenylethylamine substructure was embedded in ring expanded 3-benzazocines 4 as well as ring-contracted tetralinamines 6 and indanamines 7. The ligands 4, 6 and 7 were synthesized by reductive alkylation of secondary amine 11, reductive amination of ketones 12 and 16 and nucleophilic substitution of nosylates 14 and 17. The moderate GluN2B affinity of 3-benzazocine 4d (K_i = 32 nM) translated into moderate cytoprotective activity (IC₅₀ = 890 nM) and moderate ion channel inhibition (60% at 10 μM) in two-electrode voltage clamp experiments with GluN1a/GluN2B expressing oocytes. Although some of the tetralinamines 6 and indanamines 7 showed very high GluN2B affinity (e.g. K_i (7f) = 3.2 nM), they could not inhibit glutamate/glycine inducted cytotoxicity. The low cytoprotective activity of 3-benzazocines 4, tetralinamines 6 and indanamines 7 was attributed to the missing OH moiety at the benzene ring and/or in benzylic position. Docking studies showed that the novel GluN2B ligands adopt similar binding poses as Ro 25-6981 with the central H-bond interaction between the protonated amino moiety of the ligands and the carbamoyl moiety of Gln110. However, due to the lack of a second H-bond forming group, the ligands can adopt two binding poses within the ifenprodil binding pocket.

Impact of hydroxy moieties at the benzo[7]annulene ring system of GluN2B ligands: Design, synthesis and biological evaluation

In this study, the impact of one or two hydroxy moieties at the benzo[7]annulene scaffold on the GluN2B affinity and cytoprotective activity was analyzed. The key intermediate for the synthesis of OH-substituted benzo[7]annulenamines 11-13 and 17 was the epoxyketone 8. Reductive epoxide opening of 8 resulted with high regioselectivity in the 5-hydroxyketone 9 (Pd(OAc)₂, HCO₂H, phosphane ligand) or the 6-hydroxyketone 10 (H₂, Pd/C), whereas hydrolysis in aqueous dioxane led to the dihydroxyketone 14. Reductive amination of these ketones with primary amines and NaBH(OAc)₃ afforded the benzo[7]annulenamines 11-13 and 17. In receptor binding studies 5-OH derivatives 11 and 12 showed higher GluN2B affinity than 6-OH derivatives 13, which in turn were more active than 5,6-di-OH derivative 17a. The same order was found for the cytoprotective activity of the ligands. The tertiary amine 12a with one OH moiety in 5-position represents the most promising GluN2B negative allosteric modulator with a binding affinity of K_i = 49 nM and a cytoprotective activity of IC₅₀ = 580 nM. In the binding pocket 12a shows a crucial H-bond between the benzylic OH moiety and the backbone carbonyl O-atom of Ser132 (GluN1b). It was concluded that a 5-OH moiety is essential for the inhibition of the NMDA receptor associated ion channel, whereas a OH moiety in 6-position is detrimental for binding and inhibition. An OH or CH₂OH moiety at 2-position results in binding at the ifenprodil binding site, but very weak ion channel inhibition.

Synthesis and receptor binding of thiophene bioisosteres of potent GluN2B ligands with a benzo[7]annulene-scaffold

The involvement of NMDA receptors containing the GluN2B subunit in neurodegenerative disorders including Alzheimer's and Parkinson's disease renders this NMDA receptor subtype an interesting pharmacological target. The aim of this study was the bioisosteric replacement of benzene, methoxybenzene and aniline moieties of known potent GluN2B selective NMDA receptor antagonists by a thiophene ring. In a nine-step synthesis starting from commercially available propionic acid 9 the thiophene derivative 7a was obtained as a bioisostere of the potent GluN2B ligands cis-3 and trans-3. [7]Annuleno[b]thiophene 8a without a benzylic OH moiety was prepared in a six-step synthesis starting from carboxylic acid 18. 8a represents a bioisostere of potent GluN2B ligands 4 and 5. [7]Annulenothiophene 8a without a benzylic OH moiety reveals approx. 8-fold higher GluN2B affinity (K _i = 26 nM) than the analogous thiophene derivative 7a with a benzylic OH moiety (K _i = 204 nM). Both thiophene bioisosteres show a slight preference for GluN2B receptors over both σ receptors. The data indicate that the bioisosteric replacement of benzene or substituted benzene rings by a thiophene ring is well tolerated by the NMDA receptor. Furthermore, the benzylic OH moiety seems not to be essential for high GluN2B affinity.

Computational investigation of the antagonism effect towards GluN2B-Containing NMDA receptor: Combined ligand-based and target-based approach

The interaction of GluN2B-Containing NMDA Receptor with 18 antagonists were investigated by a combined ligand-based and target-based approach. First, two distinct pharmacophore models were generated for antagonists which cluster in two groups with Catalyst (HipHop module). The pharmacophore of "ifenprodil group" antagonists includes three hydrophobic groups, one H-bond donor and one H-bond acceptor, the pharmacophore of "EVT101 group" antagonists involves one aromatic ring, two hydrophobic groups and one H-bond acceptor. Docking results and pharmacophore model confrontation allow the pharmacodynamic characteristics to be weighted and structural information integrated. Which results in the proposal of two interaction models inside the GluN2B binding cavity for two groups of antagonists. The interaction model of "ifenprodil group" antagonists consists of one hydrophobic group, one H-bond donor, one H-bond acceptor and an aromatic ring, while on the other hand, the interaction model of "EVT101 group" antagonists includes three hydrophobic groups and an aromatic ring.

Deconstruction - reconstruction approach to analyze the essential structural elements of tetrahydro-3-benzazepine-based antagonists of GluN2B subunit containing NMDA receptors

The role of the phenolic and benzylic OH moieties for the interaction of tetrahydro-3-benzazepine-1,7-diol 3d with GluN2B subunit containing NMDA receptors was analyzed by their stepwise removal. Elimination of trifluormethanesulfinate from 10 and 13 represent the key steps in the synthesis. Removal of phenolic OH moiety led to 5-fold reduced GluN2B affinity of 4d compared with 3d. Additional removal of the benzylic OH moiety (5d) resulted in further reduced GluN2B affinity but increased σ₁ and σ₂ affinities. Introduction of a NO₂ (6d) or NH₂ moiety (7d) decreased the GluN2B affinity. 3-Benzazepin-1-ol 4i with the N-phenylcyclohexyl side chain showed the highest GluN2B affinity of this series of compounds (K_i = 2.2 nM) and, moreover, high selectivity over the PCP binding site, σ₁ and σ₂ receptors. In docking studies 3-benzazepines (S)-4-7 adopt the same binding poses as ifenprodil and display the same crucial interactions. Unexpectedly, the high-affinity ligands (S)-4i, (S)-4j, and (S)-6i were not able to inhibit the glutamate/glycine evoked current in two-electrode voltage clamp measurements and the cytotoxic effects of glutamate/glycine on transfected cell lines.

Do GluN2B subunit containing NMDA receptors tolerate a fluorine atom in the phenylalkyl side chain?

The influence of an F-atom in the side chain of benzo[7]annulen-7-amines on the affinity towards GluN2B subunit containing NMDA receptors and the selectivity over related receptors was investigated. The synthesis of 5a and 5b was performed by reductive amination of the ketone 6 with primary alkanamines 14a and 14b bearing an F-atom in β-position. The GluN2B affinities of non-fluorinated and fluorinated ligands 4 and 5 are almost identical. The low impact of the F-atom on GluN2B affinity was unexpected, as it influences several chemical and physicochemical properties of the ligands. However, introduction of the F-atom led to reduced selectivity over σ receptors. Whereas 5a and 5b display still a 2-3-fold preference for GluN2B over σ1 receptors, they show almost the same affinity to GluN2B and σ2 receptors.