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

Due to their beneficial side effect profile, NMDA receptor antagonists interacting selectively with the allosteric ifenprodil binding site of the GluN2B subunit are of major interest for the treatment of neurological and neurodegenerative disorders. A series of benzo[7]annulen-7-amines 6 was designed by conformational restriction of ifenprodil (1). At first the benzo[7]annulen-7-one 11 was prepared in a three-step synthesis comprising of a double Knoevenagel condensation of phthalaldehyde (7) with dimethyl 3-oxoglutarate (8), hydrogenation of 9 and saponification/decarboxylation of 10. Reductive amination of the ketone 11 with primary amines and NaBH(OAc)₃ led to the secondary amines 6a–d, cis-6h and trans-6i. The tertiary amines 6e–g were obtained by S_N2-substitution of the nosylate 13. Although H-bond forming substituents in 2- and 5-position are missing, the amines 6 exhibit high affinity towards GluN2B containing NMDA receptors. A distance of four to five bond lengths between the basic amino moiety and the phenyl ring in the side chain appears to be optimal for high GluN2B affinity. The phenylcyclohexylamine cis-6h and the 4-benzylpiperidine 6g show the highest GluN2B affinities (K_i = 2.3 nM and 2.9 nM, respectively). With respect to selectivity against the PCP binding site, σ₁ and σ₂ receptors the phenylpiperazine 6f is the most promising GluN2B antagonist.     

2-Methyltetrahydro-3-benzazepin-1-ols – The missing link in SAR of GluN2B selective NMDA receptor antagonists

The NMDA receptor containing GluN2B subunits represents a promising target for the development of drugs for the treatment of various neurological disorders including neurodegenerative diseases. In order to study the role of CH₃ and OH moieties trisubstituted tetrahydro-3-benzazepines 4 were designed as missing link between tetra- and disubstituted 3-benzazepines 2 and 5. The synthesis of 4 comprises eight reaction steps starting from alanine. The intramolecular Friedel-Crafts acylation to obtain the ketone 12 and the base-catalyzed elimination of trifluoromethanesulfinate (CF₃SO₂^?) followed by NaBH₄ reduction represent the key steps. The GluN2B affinity of the cis-configured 3-benzazepin-1-ol cis-4a with a 4-phenylbutyl side chain (K_i?=?252?nM) is considerably lower than the GluN2B affinity of (R,R)-2 (K_i?=?17?nM) indicating the importance of the phenolic OH moiety for the interaction with the receptor protein. Introduction of an additional CH₃ moiety in 2-position led to a slight decrease of GluN2B affinity as can be seen by comparing the affinity data of cis-4a and 5. The homologous phenylpentyl derivative cis-4b shows the highest GluN2B affinity (K_i?=?56?nM) of this series of compounds. According to docking studies cis-4a adopts the same binding mode as the cocrystallized ligand ifenprodil-keto 1A and 5 at the interface of the GluN2B and GluN1a subunits. The same crucial H-bonds are formed between the C(O)NH₂ moiety of Gln110 within the GluN2B subunit and the protonated amino moiety and the OH moiety of (R,R)-cis-4a.     

Thiophene bioisosteres of GluN2B selective NMDA receptor antagonists: Synthesis and pharmacological evaluation of [7]annuleno[b]thiophen-6-amines

Thiophene bioisosteres of potent GluN2B receptor negative allosteric modulators were prepared and evaluated pharmacologically. The five-step synthesis of 4,5,7,8-tetrahydro[7]annuleno[b]thiophen-6-one (10) was considerably improved by carboxylation of thiophene-3-carboxylic acid (8) in the first reaction step. Reductive amination and alkylation led to three homologous series of secondary and tertiary phenylalkylamines 5, 11 and 12. Metalation, reaction with 1-formylpiperidine and subsequent reduction provided hydroxymethyl derivatives 15 and 16, which had been designed as bioisosteres of phenols. 2-Bromo derivatives 18 were obtained by bromination of ketone 10 with NBS and subsequent reductive amination. High GluN2B affinity was achieved with [7]annuleno[b]thiophenes bearing a 3-phenylpropylamino or 4-phenylbutylamino moiety (e.g. 5c: K_i = 5.9 nM; 11d: K_i = 9.0 nM). Tertiary ethylamines 12 showed lower GluN2B affinity than tertiary methylamines 11 or secondary amines 5 (e.g. 5c: K_i = 5.9 nM; 11c: K_i = 6.0; 12c: K_i = 51 nM). A Br-atom or a hydroxymethyl moiety in 2-position were less tolerated by the GluN2B receptor. Very similar relationships between the structure and GluN2B affinity and structure and σ affinity, in particular σ₂ affinity, were detected. A slight preference for the ifenprodil binding site of GluN2B receptors over σ₁ and σ₂ receptors was found for methylamines 11c (≈2-fold) and 11d (≈1.5–2-fold) as well as for bromo derivative 18c (≈3-fold).     

Modification of the 4-phenylbutyl side chain of potent 3-benzazepine-based GluN2B receptor antagonists

Excitotoxicity driven by overactivation of NMDA receptors represents a major mechanism of acute and chronic neurological and neurodegenerative disorders. Negative allosteric modulators interacting with the ifenprodil binding site of the NMDA receptor are able to interrupt this ongoing neurodamaging process. Starting from the potent 3-benzazepine-1,7-diol 4a novel NMDA receptor antagonists were designed by modification of the N-(4-phenylbutyl) side chain. With respect to developing novel fluorinated PET tracers, regioisomeric fluoroethoxy derivatives 11, 12, 14, and 15 were synthesized. Analogs 19 and 20 with various heteroaryl moieties at the end of the N-side chain were prepared by Sonogashira reaction and nucleophilic substitution. The fluoroethyl triazole 37 was obtained by 1,3-dipolar cycloaddition. In several new ligands, the flexibility of the (hetero)arylbutyl side chain was restricted by incorporation of a triple bond. The affinity towards the ifenprodil binding site was tested in an established competition assay using [³H]ifenprodil as radioligand. Introduction of a fluoroethoxy moiety at the terminal phenyl ring, replacement of the terminal phenyl ring by a heteroaryl ring and incorporation of a triple bond into the butyl spacer led to considerable reduction of GluN2B affinity. The phenol 15 (K_i = 193 nM) bearing a p-fluoroethoxy moiety at the terminal phenyl ring represents the most promising GluN2B ligand of this series of compounds. With exception of 15 showing moderate σ₂ affinity (K_i = 79 nM), the interaction of synthesized 3-benzazepines towards the PCP binding site of the NMDA receptor, σ₁ and σ₂ receptors was rather low (K_i > 100 nM).     

Development of fluorinated CB2 receptor agonists for PET studies

A convergent strategy was followed to modify systematically carbazole based CB₂ receptor ligands. The length of the N-(fluoroalkyl) group (n in 7), the length of the alkanamide (m in 7) and the substitution pattern of the phenyl moiety (X and Y in 7) were varied systematically. The highest CB₂ affinity was found for the 2-fluoroethyl substituted carbazole derivative 20a (K_i = 5.8 nM) containing the propionamide and the 2-bromo-4-fluorophenyl moiety. According to docking studies 20a fits nicely into the binding pocket of the CB₂ receptor, but elongation of the fluoroethyl side chain leads to a different binding mode of the ligands. The high CB₂ affinity together with the high selectivity over the CB₂ subtype qualifies the fluoroethyl derivative 20a to be developed as a PET tracer.     

Pyridine analogues of spirocyclic σ1 receptor ligands

Spirocyclic benzopyrans 2 interact with high affinity and selectivity with σ₁ receptors. Bioisosteric replacement of the benzene ring of the benzopyran substructure with the electron rich thiophene ring (3) led to increased σ₁ affinity. Herein the synthesis and pharmacological evaluation of electron deficient pyridine bioisosteres 4 are reported. Homologation of the aldehyde 6 to afford the pyridylacetaldehyde derivative 8 was performed by a Wittig reaction. Bromine lithium exchange of the bromopyridine 8, addition to 1-benzylpiperidin-4-one and cyclization led to the spirocyclic pyrranopyridine 10. Hydrogenolytic removal of the N-benzyl moiety of 10 provided the secondary amine 11, which allowed the introduction of various N-substituents (12a–d). Cyclization of the hydroxy acetal 9 with HCl led to various modifications of the substituent in 3′-position. Generally the σ₁ affinity of the pyridine derivatives is reduced compared with those of the benzene and thiophene derivatives 2 and 3. However, the relationships between the structure and the σ₁ affinity follow the same rules as for the benzene and thiophene derivatives. The most promising σ₁ ligand within this class of compounds is the pyranopyridine 15 with a double bond in the pyran ring revealing a K_i-value of 4.6 nM and a very high selectivity (>217-fold) over the σ₂ subtype.     

Improvement of σ1 receptor affinity by late-stage C–H-bond arylation of spirocyclic lactones

The direct C–H-bond arylation of the complex spirocyclic lactones 13, 14, and 18 allows the introduction of diverse aryl moieties in the last step of the synthesis. A selective α-arylation of the thiophene moiety was performed with the catalytic system PdCl₂/2,2′-bipyridyl/Ag₂CO₃, whereas the β-position of the thiophene ring was addressed by using the alternative catalytic system PdCl₂/P[OCH(CF₃)₂]₃/Ag₂CO₃. Due to electronic and steric reasons the arylation of the five-membered lactone 18 occurred in both α-positions providing 4′-mono-, 6′-mono- and 4′,6′-diarylated thiophenes 22–26a–c. Compounds with an additional aryl moiety at the ‘upper left (top)’ position (1′-position of 13, 3′-position of 14, 4′-position of 18) showed increased σ₁ affinity compared to the non-arylated parent compounds. A phenyl moiety at the ‘left’ position (2′-position in 20a) also increased the σ₁ affinity but to a lower extent. A considerable reduction of σ₁ affinity was observed after introducing an aryl moiety in 6′-position of 18, which might result from shielding the tertiary amine, which is crucial for interaction with the σ₁ receptor. The discussion of the experimental results is supported by high-level quantum chemical DFT-calculations of the NBO-charges of 13 and 18 and the relative energies of the related arylated products.     

From NMDA receptor antagonists to discovery of selective σ2 receptor ligands

Following previous studies focused on the search for new molecules targeting GluN2B-containing NMDA, a small series of 1-(1H-indol-3-yl)-2-(4-phenylpiperidin-1-yl)ethanone derivatives has been synthesized by using Microwave Assisted Organic Synthesis (MAOS). Given that GluN2B ligands frequently exert off-target effects we also tested their affinity towards sigma receptors. Binding assay revealed that only the 1-(5-hydroxy-1H-indol-3-yl)-2-(4-phenylpiperidin-1-yl)ethanone (7a) retained GluN2B affinity. Interestingly, the 5-methoxyindoles 5a and 6a were efficient and selective ligands toward σ₂ receptor (K_i values of 10 nM and 20 nM, respectively). Thus, in this case the discovery of new σ₂ receptor selective ligands was an unexpected result emerging from the screening of cross-activity against other CNS receptors.     

Synthesis of new antimicrobial pyrrolo[2,1-a]isoquinolin-3-ones

The attractive structure of the pyrroloisoquinoline moiety, together with its potential antimicrobial activity, encouraged us to prepare six 8-substituted and seven 8,9-disubstituted-1,2,3,5,6,10b-hexahydropyrrolo[2,1-a]isoquinolin-3-ones in a few steps with good yields. We applied a convenient methodology via double intramolecular cyclization conducted by a Bischler-Napieralski cyclodehydration-imine reduction sequence, which is widely employed in the synthesis of isoquinoline alkaloids. Therefore, we synthesized three series of these pyrrolo[2,1-a]isoquinolin-3-ones characterized by the substituent at the 8-position or 8,9-positions of the aromatic ring: (a) different side chains are attached to an 8-OH group (series 1); (b) a chlorine atom is attached to the 8-position (series 2); and (c) 8- and 9-carbons are bearing an identical group (series 3). The compounds bearing a benzylic moiety at the 8-position, for example, 8-benzyloxy-pyrrolo[2,1-a]isoquinolin-3-one (1a) and 8-(4-fluorobenzyloxy)-pyrrolo[2,1-a]isoquinolin-3-one (1e), as well as, a 8-chloro-9-methoxy moiety including the 8-chloro-9-methoxy-pyrrolo[2,1-a]isoquinolin-3-one (2a), provided the most fungicide and bactericide agents, respectively.     

Design,synthesis, and structure–activity relationship of novel and effective apixaban derivatives as FXa inhibitors containing 1,2,4-triazole/pyrrole derivatives as P2 binding element

Four series of novel and potent FXa inhibitors possessing the 1,2,4-triazole moiety and pyrrole moiety as P2 binding element and dihydroimidazole/tetrahydropyrimidine groups as P4 binding element were designed, synthesized, and evaluated for their anticoagulant activity in human and rabbit plasma in vitro. Most compounds showed moderate to excellent activity. Compounds 14a, 16, 18c, 26c, 35a, and 35b were further examined for their inhibition activity against human FXa in vitro and rat venous thrombosis in vivo. The most promising compound 14a, with an IC₅₀ (FXa) value of 0.15 μM and 99% inhibition rate, was identified for further evaluation as an FXa inhibitor.     

Synthesis and pharmacological characterization of novel xanthine carboxylate amides as A2A adenosine receptor ligands exhibiting bronchospasmolytic activity

The carboxylate amides of 8-phenyl-1,3-dimethylxanthine described herein represent a new series of selective ligands of the adenosine A_2A receptors exhibiting bronchospasmolytic activity. The effects of location of 8-phenyl substitutions on the adenosine receptor (AR) binding affinities of the newly synthesized xanthines have also been studied. The compounds displayed moderate to potent binding affinities toward various adenosine receptor subtypes when evaluated through radioligand binding studies. However, most of the compounds showed the maximum affinity for the A_2A subtype, some with high selectivity versus all other subtypes. Xanthine carboxylate amide 13b with a diethylaminoethylamino moiety at the para-position of the 8-phenylxanthine scaffold was identified as the most potent A_2A adenosine receptor ligand with K_i = 0.06 μM. Similarly potent and highly A_2A-selective are the isovanillin derivatives 16a and 16d. In addition, the newly synthesized xanthine derivatives showed good in vivo bronchospasmolytic activity when tested in guinea pigs.     

Indole derivatives as dual-effective agents for the treatment of neurodegenerative diseases: Synthesis,biological evaluation,and molecular modeling studies

Several indole derivatives, that were highly potent ligands of GluN2B-subunit-containing N-methyl-d-aspartate (NMDA) receptor, also demonstrated antioxidant properties in ABTS method. In particular, the 2-(4-benzylpiperidin-1-yl)-1-(5-hydroxy-1H-indol-3-yl)ethanone (1) proved to be a dual-effective neuroprotective agent. With the aim to increase the antioxidant properties we added a catechol moiety onto piperidine moiety. The designed hybrid derivative 3,4-dihydroxy-N-[1-[2-(5-hydroxy-1H-indol-3-yl)-2-oxoethyl]piperidin-4-yl]benzamide (10) was the most effective antioxidant agent (>94.1 ± 0.1% of inhibition at 17 μM) and showed GluN2B/NMDA receptor affinity at low micromolar concentration (IC₅₀ 0.66 μM). By means of computational studies we explored the effect of the presence of this antioxidant fragment during the recognition process to binding pocket.     

Design,synthesis and evaluation against Mycobacterium tuberculosis of azole piperazine derivatives as dicyclotyrosine (cYY) mimics

Three series of azole piperazine derivatives that mimic dicyclotyrosine (cYY), the natural substrate of the essential Mycobacterium tuberculosis cytochrome P450 CYP121A1, were prepared and evaluated for binding affinity and inhibitory activity (MIC) against M. tuberculosis. Series A replaces one phenol group of cYY with a C3-imidazole moiety, series B includes a keto group on the hydrocarbon chain preceding the series A imidazole, whilst series C explores replacing the keto group of the piperidone ring of cYY with a CH₂-imidazole or CH₂-triazole moiety to enhance binding interaction with the heme of CYP121A1. The series displayed moderate to weak type II binding affinity for CYP121A1, with the exception of series B 10a, which displayed mixed type I binding. Of the three series, series C imidazole derivatives showed the best, although modest, inhibitory activity against M. tuberculosis (17d MIC?=?12.5?μg/mL, 17a 50?μg/mL). Crystal structures were determined for CYP121A1 bound to series A compounds 6a and 6b that show the imidazole groups positioned directly above the haem iron with binding between the haem iron and imidazole nitrogen of both compounds at a distance of 2.2?Å. A model generated from a 1.5?Å crystal structure of CYP121A1 in complex with compound 10a showed different binding modes in agreement with the heterogeneous binding observed. Although the crystal structures of 6a and 6b would indicate binding with CYP121A1, the binding assays themselves did not allow confirmation of CYP121A1 as the target.     

Design,synthesis and binding affinity of acetylcholine carbamoyl analogues

A series of acetylcholine carbamoyl analogues, cyclised at the carbamate moiety or at the cationic head or at both, were tested for binding affinity at muscarinic and neuronal nicotinic receptors (nAChRs). While no muscarinic affinity was found, submicromolar K_i values, similar to that of carbachol, were measured at α₄β₂ nAChRs for the enantiomers of 5-dimethylaminomethyl- and 5-trimethylammoniomethyl-2-oxazolidinone, 2 and 2a, and for (S)-N-methylprolinol carbamate (S)-3. Methylation of oxazolidinone nitrogen of 2 and 2a and of N-methylprolinol nitrogen of (S)-3 and, even more, hybridization of cyclic carbamate substructure (oxazolidinone) with cyclic cationic head (N-methylpyrrolidine) markedly lower the nicotinic affinity. Docking results were consistent with SAR analysis highlighting the interaction capabilities of (R)-2a and (S)-3 and the negative effect of intracyclic nitrogen methylation and of double cyclisation.     

Synthesis and structure–activity relationships of amide derivatives of (4,4-difluoro-1,2,3,4-tetrahydro-5H-1-benzazepin-5-ylidene)acetic acid as selective arginine vasopressin V2 receptor agonists

A series of (4,4-difluoro-1,2,3,4-tetrahydro-5H-1-benzazepin-5-ylidene)acetamide derivatives was synthesized, and their structure–activity relationships were examined in order to identify potent and selective arginine vasopressin V₂ receptor agonists. Attempts to substitute other chemical groups in place of the 2-pyridilmethyl moiety of 1a led to the discovery that potent V₂ binding affinity could be obtained with a wide range of functional groups. This structural tolerance allowed for the manipulation of other attributes, such as selectivity against V_1a receptor affinity or avoidance of the undesirable inhibition of cytochrome P450 (CYP), without losing potent affinity for the V₂ receptor. Some representative compounds obtained in this study were also found to decrease urine volume in awake rats.     

Discovery of 1,2,3,4-tetrahydropyrimido[1,2-a]benzimidazoles as novel class of corticotropin releasing factor 1 receptor antagonists

A new class of corticotropin releasing factor 1 (CRF₁) receptor antagonists characterized by a tricyclic core ring was designed and synthesized. Novel tricyclic derivatives 2a–e were designed as CRF₁ receptor antagonists based on conformation analysis of our original 2-anilinobenzimidazole CRF₁ receptor antagonist. The synthesized tricyclic derivatives 2a–e showed CRF₁ receptor binding activity with IC₅₀ values of less than 400?nM, and the 1,2,3,4-tetrahydropyrimido-[1,2-a]benzimidazole derivative 2e was selected as a lead compound with potent in vitro CRF₁ receptor binding activity (IC₅₀?=?7.1?nM). To optimize the pharmacokinetic profiles of lead compound 2e, we explored suitable substituents on the 1-position and 6-position, leading to the identification of compound 42c-R, which exhibited potent CRF₁ receptor binding activity (IC₅₀?=?58?nM) with good oral bioavailability (F?=?68% in rats). Compound 42c-R exhibited dose-dependent inhibition of [¹²⁵I]-CRF binding in the frontal cortex (5 and 10?mg/kg, p.o.) as well as suppression of locomotor activation induced by intracerebroventricular administration of CRF in rats (10?mg/kg, p.o.). These results suggest that compound 42c-R successfully binds CRF₁ receptors in the brain and exhibits the potential to be further examined for clinical studies.     

Synthesis of 3-aza[4.4.3]propellanes with high σ1 receptor affinity

In order to obtain rigid σ₁ receptor ligands with defined orientation of pharmacophoric elements, the azapropellane scaffold was chosen. Schmidt rearrangement of propellan-8-ones 6 and 10 provided 3-azapropellan-4-ones 7 and 11. Benzylation of the secondary lactams 7 and 11 followed by LiAlH₄ reduction furnished the azapropellanes 4a and 4c, respectively. A second hydrophobic element was introduced by transformation of the alcohols 4a into carbamates 4b. The σ₁ affinity of the azapropellanes 4 is strongly dependent on the stereochemistry and the substitution pattern in 12-position. anti-configured azapropellanes anti-4a and anti-4b show higher σ₁ affinity than their syn-configured counterparts syn-4a and syn-4b. Conversion of the alcohol anti-4a into the carbamate anti-4b led to increased σ₁ affinity, but complete removal of the 12-substituent resulted in the highest σ₁ affinity (K_i(4c)?=?17?nM). It can be concluded that the propellane scaffold alone is able to form strong lipophilic interactions and stabilize the ligand–σ₁ receptor complex as does usually the primary hydrophobic region.     

Combination of cyclohexane and piperazine based κ-opioid receptor agonists: Synthesis and pharmacological evaluation of trans,trans-configured perhydroquinoxalines

Desymmetrization of the pseudochiral (2r)-configured cyclohexane-1,2,3-triamines 8 with dimethyl oxalate led to racemic aminoquinoxaline-2,3-diones 9. Selective introduction of the κ pharmacophoric structural elements pyrrolidine and 3,4-dichlorophenylacetamide with a two-carbon distance afforded conformationally restricted κ agonists 13–15 based on the quinoxaline ring system. In competitive radioligand receptor binding studies the benzylamine 13b, the secondary amine 14b, and the carbamate 15 displayed high κ receptor affinity. The K_i value of the lead compound derived methoxycarbonyl derivative 15 is 9.7 nM. However, the κ affinity of 15 is exceeded by 13b and 14b with a basic functional group instead of the methoxycarbonyl group in 1-position of the quinoxaline system. The chlorine atoms of the dichlorophenylacetyl residue are essential, since the corresponding phenylacetyl analogs show considerably reduced κ affinity. The potent κ ligands 13b, 14b and 15 are selective over the related μ- and δ-opioid receptors, σ₁, σ₂ and NMDA receptors. In the [³⁵S]GTPγS-binding assay 13b behaved as partial agonist with lower activity than U-69,593.     

Synthesis and cytotoxic effects of 2-thio-3,4-dihydroquinazoline derivatives as novel T-type calcium channel blockers

In our previous work, a series of 2-amino-3,4-dihydroquinazoline derivatives using an electron acceptor group was reported to be potent T-type calcium channel blockers and exhibit strong cytotoxic effects against various cancerous cell lines. To investigate the role of the guanidine moiety in the 2-amino-3,4-dihydroquinazoline scaffold as a pharmacophore for dual biological activity, a new series of 2-thio-3,4-dihydroquniazoline derivatives using an electron donor group at the C2-position was synthesized and evaluated for T-type calcium channel blocking activity and cytotoxic effects against two human cancerous cell lines (lung cancer A549 and colon cancer HCT-116). Among them, compound 6g showed potent inhibition of Ca_v3.2 currents (83% inhibition) at 10 µM concentrations. The compound also exhibited IC₅₀ values of 5.0 and 6.4 µM against A549 and HCT-116 cell lines, respectively, which are comparable to the parental lead compound KYS05090. These results indicate that the isothiourea moiety similar to the guanidine moiety of 2-amino-3,4-dihydroquinazoline derivatives may be an essential pharmacophore for the desired biological activities. Therefore, our preliminary work can provide the opportunity to expand a chemical repertoire to improve affinity and selectivity for T-type calcium channels.     

Exploration of the 5-bromopyrimidin-4(3H)-ones as potent inhibitors of PDE5

The substituents both at the 6-position of the 5-bromopyrimidinone ring and at the 5′-position of the phenyl ring of 5-bromopyrimidin-4(3H)-ones were explored. 5-Bromo-6-isopropyl-2-(2-propoxy-phenyl)pyrimidin-4(3H)-one was identified as a new scaffold for potent PDE5 inhibitors. The crystal structures of PDE5/2e and PDE5/10a complexes provided a structural basis for the inhibition of 5-bromopyrimidinones to PDE5. In addition, it was also found that there is a great tolerance for the substitution at the 5′-position of the phenyl ring of 5-bormopyrimidinones and the resulted compound 13a has the highest inhibition activity to PDE5 (IC₅₀, 1.7 nM).