N-Arylsulfonylindole derivatives as serotonin 5-HT(6) receptor ligands

Development and crystallography-aided SAR studies of multifunctional BuChE inhibitors and 5-HT6R antagonists with β-amyloid anti-aggregation properties

The lack of an effective treatment makes Alzheimer's disease a serious healthcare problem and a challenge for medicinal chemists. Herein we report interdisciplinary research on novel multifunctional ligands targeting proteins and processes involved in the development of the disease: BuChE, 5-HT₆ receptors and β-amyloid aggregation. Structure-activity relationship analyses supported by crystallography and docking studies led to the identification of a fused-type multifunctional ligand 50, with remarkable and balanced potencies against BuChE (IC₅₀ = 90 nM) and 5-HT₆R (K_i = 4.8 nM), and inhibitory activity against Aβ aggregation (53% at 10 μM). In in vitro ADME-Tox and in vivo pharmacokinetic studies compound 50 showed good stability in the mouse liver microsomes, favourable safety profile and brain permeability with the brain to plasma ratio of 6.79 after p.o. administration in mice, thus being a promising candidate for in vivo pharmacology studies and a solid foundation for further research on effective anti-AD therapies.

Novel N-Arylsulfonylindoles Targeted as Ligands of the 5-HT6 Receptor. Insights on the Influence of C-5 Substitution on Ligand Affinity

A new series of twenty-two C-5 substituted N-arylsulfonylindoles was prepared with the aim of exploring the influence of C-5 substitution on 5-HT₆ receptor affinity. Eleven compounds showed moderate to high affinity at the receptor (K_i = 58–403 nM), with compound 4d being identified as the most potent ligand. However, regarding C-5 substitution, both methoxy and fluorine were detrimental for receptor affinity compared to our previously published unsubstituted compounds. In order to shed light on these observations, we performed docking and molecular dynamics simulations with the most potent compounds of each series (4d and 4l) and PUC-10, a highly active ligand previously reported by our group. The comparison brings about deeper insight about the influence of the C-5 substitution on the binding mode of the ligands, suggesting that these replacements are detrimental to the affinity due to precluding a ligand from reaching deeper inside the binding site. Additionally, CoMFA/CoMSIA studies were performed to systematize the information of the main structural and physicochemical characteristics of the ligands, which are responsible for their biological activity. The CoMFA and CoMSIA models presented high values of q² (0.653; 0.692) and r² (0.879; 0.970), respectively. Although the biological activity of the ligands can be explained in terms of the steric and electronic properties, it depends mainly on the electronic nature.

Synthesis and Reactivity of Uhle’s Ketone and Its Derivatives

Uhle’s ketone and its derivatives are highly versatile intermediates for the synthesis of a variety of 3,4-fused tricyclic indole frameworks, i.e. indole alkaloids of the ergot family, that are found in various bioactive natural products and pharmaceuticals. Therefore, the development of a convenient preparative method for this structural motif as well as its opportune/useful derivatization have been the subject of longstanding interest in the fields of synthetic organic chemistry and medicinal chemistry. Herein, we summarize recent and less recent methods for the preparation of Uhle’s ketone and its derivatives as well as its main reactivity towards the synthesis of bioactive substances. Regarding the preparation, it can be roughly classified into two categories: (a) using 4-unfunctionalized and 4-functionalized indole derivatives as starting materials to construct a fused six-member ring, and (b) constructing the indole ring through intramolecular cycloaddition. Principally, the reactivity of the cyclic Uhle’s ketone shown here is derived from the classical electrophilicity of the carbonyl carbon or the acidity of the α-hydrogen and, though less intensively investigated, chemical reactions that induce ring expansion to form novel ring skeletons.

1 Introduction

2 Synthesis

2.1 Disconnection A: Cyclization Reaction of the Opportune 3,4-Disubstituted Indole

2.2 Disconnection B: Intramolecular Friedel–Crafts Cyclization

2.3 Disconnection B: Intramolecular Cyclization via Metal–Halogen Exchange

2.4 Disconnection C: Intramolecular Diels–Alder Furan Cycloaddition

2.5 Disconnection D: Intramolecular Dearomatizing [3 + 2] Annulation

3 Reactivity

3.1 Use of Uhle’s Ketone for Lysergic Acid

3.2 Use of Uhle’s Ketone for Rearranged Clavines

3.3 Use of Uhle’s Ketone for Medicinal Chemistry

4 Conclusion and Outlook

Concise catalytic asymmetric synthesis of (R)-4-amino Uhle's ketone

A practical and asymmetric synthesis of (R)-4-amino-5-oxo-1,3,4,5-tetrahydrobenz[cd]indole, an enantiopure framework shared by most ergot alkaloids, was accomplished. Our method involves a Rh(i)-catalyzed 6-exo-trig intramolecular cyclization of an appropriate 4-pinacolboronic ester d-tryptophan aldehyde followed by the oxidation of the resulting secondary benzylic alcohol with a Cu(i)-ABNO catalyst and final deprotection under acidic conditions. This new procedure offers significant advantages over previous synthetic approaches, including brevity, mild reaction conditions, preservation of chiral integrity, and high overall yield and avoids the use of stoichiometric amounts of strongly basic and pyrophoric organometallic reagents.

The effect of the intramolecular C–H⋯O interactions on the conformational preferences of bis-arylsulfones – 5-HT6 receptor antagonists and beyond

The development of compounds with enhanced activity and selectivity by a conserved spatial orientation of the pharmacophore elements has a long history in medicinal chemistry. Rigidified compounds are an example of this concept. However, the intramolecular interactions were seldom used as a basis for conformational restraints. Here, we show the weak intramolecular interactions that contribute to the relatively well-conserved geometry of N1-arylsulfonyl indole derivatives. The structure analysis along with quantum mechanics calculations revealed a crucial impact of the sulfonyl group on the compound geometry. The weak intramolecular C–H⋯O interaction stabilizes the mutual "facing" orientation of two aromatic fragments. These findings extend the pharmacological interpretation of the sulfonyl group role from the double hydrogen bond acceptor to the conformational scaffold based on intramolecular forces. This feature has, to date, been omitted in in silico drug discovery. Our results should increase the awareness of researchers to consider the conformational preference when designing new compounds or improving computational methods.

The impact of weak intramolecular C–H⋯O interactions on the conformational stability of bis-arylsulfones is discussed, suggesting different role of sulfonyl group in the ligand – 5HT₆ receptor interaction.

Discovery and Development of 1-[(2-Bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole Dimesylate Monohydrate (SUVN-502): A Novel, Potent, Selective and Orally Active Serotonin 6 (5-HT6) Receptor Antagonist for Potential Treatment of Alzheimer's Disease

Optimization of a novel series of 3-(piperazinylmethyl) indole derivatives as 5-hydroxytryptamine-6 receptor (5-HT₆R) antagonists resulted in identification of 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate monohydrate (5al, SUVN-502) as a clinical candidate for potential treatment of cognitive disorders. It has high affinity at human 5-HT₆R (K_i = 2.04 nM) and selectivity over 100 target sites which include receptors, enzymes, peptides, growth factors, ion channels, steroids, immunological factors, second messengers, and prostaglandins. It has high selectivity over 5-HT_2A receptor. It is orally bioavailable and brain penetrant with robust preclinical efficacy. The combination of 5al, donepezil, and memantine (triple combination) produces synergistic effects in extracellular levels of acetylcholine in the ventral hippocampus. Preclinical efficacy in triple combination and high selectivity over 5-HT_2A receptors are the differentiating features which culminated in selection of 5al for further development. The Phase-1 evaluation of safety and pharmacokinetics has been completed, allowing for the initiation of a Phase-2 proof of concept study.

Serotonin 5-HT6 Receptor Antagonists in Alzheimer's Disease: Therapeutic Rationale and Current Development Status

Alzheimer's disease (AD) is the most common cause of dementia in elderly people. Because of the lack of effective treatments for this illness, research focused on identifying compounds that restore cognition and functional impairments in patients with AD is a very active field. Since its discovery in 1993, the serotonin 5-HT₆ receptor has received increasing attention, and a growing number of studies supported 5-HT₆ receptor antagonism as a target for improving cognitive dysfunction in AD. This article reviews the rationale behind investigations into the targeting of 5-HT₆ receptors as a symptomatic treatment for cognitive and/or behavioral symptoms of AD. In addition to describing the available clinical evidence, this article also describes the purported biochemical and neurochemical mechanisms of action by which 5-HT₆ receptor antagonists could influence cognition, and the preclinical data supporting this therapeutic approach to AD. A large number of publications describing the development of ligands for this receptor have come to light and preclinical data indicate the procognitive efficacy of 5-HT₆ receptor antagonists. Subsequently, the number of patents protecting 5-HT₆ chemical entities has continuously grown. Some of these compounds have successfully undergone phase I clinical studies and have been further evaluated in clinical phase II trials with variable success. Phase II studies have also revealed the potential of combining 5-HT₆ receptor antagonism and cholinesterase inhibition. Two of these antagonists, idalopirdine and RVT-101, have been further developed into ongoing phase III clinical trials. Overall, 5-HT₆ receptor antagonists can reasonably be regarded as potential drug candidates for the treatment of AD.

Synthesis and in vitro anti-HIV-1 activity of a series of N-arylsulfonyl-3-propionylindoles

Fifteen N-arylsulfonyl-3-propionylindoles (3a-o) were prepared and preliminarily evaluated as in vitro inhibitors of human immunodeficiency virus type-1 (HIV-1). Three compounds 3c, 3g and 3i exhibited potent anti-HIV-1 activity with effective concentration (EC(50)) values of 0.8, 4.0 and 1.2 μg/mL, and therapeutic index (TI) values of 11.7, 16.6 and 84.1, respectively. N-(m-Nitro)phenylsulfonyl-3-propionyl-6-methylindole (3i) exhibited the most promising and best activity against HIV-1 replication. The cytotoxicity of these compounds was assessed as well.

Design for carbon–carbon bond forming reactions under ambient conditions

The carbon–carbon (C–C) bond forms the ‘backbone’ of nearly every organic molecule, and lies at the heart of the chemical sciences! Let us explore designing of carbon–carbon frameworks at ambient conditions.

Copper supported on MWCNT-guanidine acetic acid@Fe3O4: synthesis, characterization and application as a novel multi-task nanocatalyst for preparation of triazoles and bis(indolyl)methanes in water

The synthesis of a new supported copper (Cu) nanocatalyst, with highly dispersed particles, based on magnetic guanidine acetic acid (GAA) functionalized multi-wall carbon nanotubes (MWCNT), Cu/MWCNT-GAA@Fe₃O₄, is reported.

Therapeutic Potential of 5-HT6 Receptor Agonists

Given its predominant expression in the central nervous system (CNS), 5-hydroxytryptamine (5-HT: serotonin) subtype 6 receptor (5-HT6R) has been considered as a valuable target for the development of CNS drugs with limited side effects. After 2 decades of intense research, numerous selective ligands have been developed to target this receptor; this holds potential interest for the treatment of neuropathological disorders. In fact, some agents (mainly antagonists) are currently undergoing clinical trial. More recently, a series of potent and selective agonists have been developed, and preclinical studies have been conducted that suggest the therapeutic interest of 5-HT6R agonists. This review details the medicinal chemistry of these agonists, highlights their activities, and discusses their potential for treating cognitive issues associated with Alzheimer's disease (AD), depression, or obesity. Surprisingly, some studies have shown that both 5-HT6R agonists and antagonists exert similar procognitive activities. This article summarizes the hypotheses that could explain this paradox.

Cyclopropyl-tryptamine analogues: synthesis and biological evaluation as 5-HT(6) receptor ligands

Conformational restrictions: Based on the pharmacophore model for 5-HT(6) receptor ligands (shown), tryptamine analogues bearing a cyclopropyl ring on the α-position of the tryptamine side chain were synthesized and evaluated against 5-HT receptors. N,N-Dimethyl-1-arylsulfonyltryptamine derivatives exhibited promising selectivity for 5-HT(6) over 5-HT(1a) and 5-HT(4) receptors and interesting activity against 5-HT(6) (K(i) =∼0.15 μM; IC(50) =∼0.20 μM).

Synthesis of benzopyrano[4,3-b](N-arylsulfonyl)indoles and benzopyrano[3,4-b](N-arylsulfonyl)indoles via intramolecular palladium-catalyzed aryl-aryl coupling reaction

A series of benzopyrano[3,4-b](N-arylsulfonyl) indole derivatives and benzopyrano[4,3-b](N-arylsulfonyl) indole derivatives were synthesized from 2- or 3-methylindole via intermolecular S( N )2 reaction and subsequent intramolecular palladium-catalyzed aryl-aryl coupling reaction for the first time. It was suggested that, besides using the Fischer cyclization, benzopyrano[4,3-b]indoles and benzopyrano[3,4-b]indoles could also be prepared via intermolecular S( N )2 reaction and sequential intramolecular palladium-catalyzed coupling reaction.

Synthesis and biological evaluation of benzoisothiazole derivatives possessing N,N-dimethylformimidamide group as 5-HT₆ receptor antagonists

A series of novel N,N-dimethyl-N'-(5-(Ar-sulfonamido) benzo[d]isothiazol-3-yl)formimidamides was designed and synthesized as 5-HT(6) ligands. Here N,N-dimethyl formimidamides was used as a replacement for an aminoethyl moiety. In vitro functional assays demonstrated compounds 9b and 9i significantly inhibited the 5-HT-induced Ca(2+) increases (9b; IC(50)=0.36 μM and 9i; IC(50)=0.44 μM), indicating that 9b and 9i were potent 5-HT(6) receptor antagonists. Compounds 9i also showed good selectivity on the 5-HT(6) over 5-HT(4) and 5-HT(7) receptors.

Tryptamine derivatives as novel non-nucleosidic inhibitors against hepatitis B virus

A series of tryptamine derivatives were synthesized and evaluated for their anti-hepatitis B virus (HBV) activity and cytotoxicity in the HepG2.2.15 cell line. The preliminary SAR was discussed. Compounds 2e and 4a showed potent antiviral activity (IC(50)=0.4 and <1 μM, respectively) and low cytotoxicity (CC(50)=40.6 and >25 μM, respectively).

Separation and identification of heterocyclic nitrogen compounds in biooil derived by fast pyrolysis of chicken manure

N-heterocyclics were separated from a biooil, generated by the pyrolysis of chicken manures by column chromatography over neutral alumina and silica, and identified by Pyrolysis Field Ionization Mass Spectrometry (Py-FIMS) and Electrospray Ionization Mass Spectrometry (ESI-MS). Identities of chemical structures, whose presence was indicated by ESI-MS, were confirmed by comparing the Collision-Induced Dissociations (CID's) mass spectra of unknown and standards. The following seven base structures were identified: pyrazine, benzoquinoline, carbazole, phenylpyridine, indole, pyrazole and pyridine. Available hydrogens bonded to ring carbons and nitrogens on the seven N-heterocyclics were increasingly substituted by alkyl groups, mainly methylene groups (m/z 14) to yield mono-, di-, tri- methyl N-heterocyclics. In some instances, longer alkyl chains, such as ethyl, propyl, up to heptyl groups were the substituents.

Novel and Potent 5-Piperazinyl Methyl-N 1-aryl Sulfonyl Indole Derivatives as 5-HT6 Receptor Ligands

The exclusive distribution of 5-HT6 receptors in the brain regions associated with learning and memory makes it an ideal target for cognitive disorders. A novel series of 5-piperazinyl methyl-N 1-aryl sulfonyl indoles were designed and synthesized as 5-HT6R ligands. Most of the synthesized compounds are potent when tested by in vitro radioligand binding assay. The lead compound from the series does not have the CYP liabilities and is active in an animal model of cognition.

Substituent diversity-directed synthesis of indole derivatives

This paper reports a versatile, good-yielding, solution-phase method that is a substituent diversity-directed synthesis of 1H-indoles (6-13, 17-20) and 1-hydroxyindoles (14, 15) starting from commercially available 1,5-difluoro-2,4-dinitrobenzene. The synthetic products possessed the maximum six diversity points.

A quantitative structure-activity relationship study on serotonin 5-HT6) receptor ligands: indolyl and piperidinyl sulphonamides

The serotonin 5-HT(6) binding affinity of indolyl- and piperidinyl-sulphonamide derivatives has been analysed with topological and molecular features with DRAGON software. Analysis of the structural features in conjunction with the biological endpoints in combinatorial protocol in multiple linear regression (CP-MLR) led to the identification of 25 descriptors for modelling the activity. The study clearly suggested the role of an average Randic-type eigenvector-based index from adjacency matrix, VRA2, number of secondary aliphatic amines, nNHR, the sum of the topological distance between N and O, T(N...O), ring tertiary carbon atoms, nCrHR, and CH2RX type fragment, C-006, in a molecular structure to optimize the 5-HT(6) binding affinities of titled compounds. The PLS analysis confirmed the dominance of information content of CP-MLR identified descriptors for modelling the activity when compared with those of leftover ones.

Benzimidazole derivatives as new serotonin 5-HT6 receptor antagonists. Molecular mechanisms of receptor inactivation

On the basis of our previously described pharmacophore model for serotonin 5-HT(6) receptor (5-HT(6)R) antagonists, we have designed, synthesized, and pharmacologically characterized a series of benzimidazole derivatives 1-20 that represent a new family of potent antagonists at the human 5-HT(6)R. Site-directed mutagenesis and a beta(2)-adrenoceptor-based homology model of the 5-HT(6)R were used to predict the mode of binding of antagonist SB-258585 and the new synthesized ligands. Substitution of W6.48, F6.52, or N6.55 by Ala fully impedes compound 4 to block 5-HT-induced activation. Thus, we propose that D3.32 in TM 3 anchors the protonated piperazine ring, the benzimidazole ring expands parallel to EL 2 to hydrogen bond N6.55 in TM 6, and the aromatic ring is placed between TMs 3 and 5 in CH(2)-containing compounds and between TMs 3 and 6 in CO-containing compounds. This combined experimental and computational study has permitted to propose the molecular mechanisms by which the new benzimidazole derivatives act as 5-HT(6)R antagonists.

Functional reconstitution of the human serotonin receptor 5-HT(6) using synthetic transmembrane peptides

Seven transmembrane (7TM) synthetic peptides mimicking the alpha-helical TM domains of the human serotonin receptor subtype-6 (5-HT(6)) were autonomously reconstituted in detergent micelle and liposome environments. The degree of assembly of the 7TM peptides was characterized by monitoring the fluorescence resonance energy transfer (FRET) between donor and acceptor probes labeled at the amino termini of the second and fourth TM-peptides, respectively. The FRET efficiency of these peptides significantly increased when the 7TM peptides were reconstituted in liposome compare to detergent micelles. Furthermore, the 7TM peptides reconstituted in liposomes selectively bound to free serotonin and serotonin-conjugated magnetic beads, yielding a dissociation constant of 0.84 microM. These results show that the seven individual TM domains of 5-HT(6) can spontaneously assemble into liposomes in a conformation that mimics a native structure, and further demonstrate that specific interactions between TM helices play a critical role in the folding and stabilizing of GPCRs. The autonomous assembly of 7TM-peptides can be applied to the screening of agonists for GPCRs that are difficult to manipulate.

One-pot N-arylation of indoles directly from N-arylsulfonylindoles via consecutive deprotection and S(N)Ar Reactions with activated Aryl halides

An efficient one-pot step by step t-BuOK-mediated procedure for the synthesis of N-arylindoles has been developed in moderate to good yields. The protocol involves the consecutive deprotection of N-arylsulfonylindoles as latent indoles and subsequent S(N)Ar reactions with activated aryl halides. This tandem reaction affords an efficient and convenient preparation of N-arylindoles that benefit from prior indoles protection by arylsulfonyl group, and can shorten a reaction sequence and improve synthetic efficiency.

Identification of a series of benzoxazoles as potent 5-HT6 ligands

As part of our continuing efforts to identify therapeutics for CNS diseases such as schizophrenia and Alzheimer's disease (AD), we have been focused on the 5-HT(6) receptor in order to identify potent and selective ligands as a potential treatment for cognitive dysfunction. Herein we report the identification of a novel series of benzoxazole derivatives as potent 5-HT(6) ligands. The synthesis and detailed SAR of this class of compounds are reported. The compounds have been shown to be full antagonists in a cyclic AMP functional assay.