Synthesis and biological evaluation of novel γ-carboline analogues of Dimebon as potent 5-HT6 receptor antagonists

Merging Rh-Catalyzed C-H Functionalization and Cascade Cyclization to Enable Propargylic Alcohols as Three-Carbon Synthons

Reported herein is a reactivity of propargyl alcohols as "Three-Carbon Synthons" in a Rh(III)-catalyzed C-H functionalization of acetanilides, leading to the synthesis of core structures of isocryptolepine, γ-carbolines, dihydrochromeno[2,3-b]indoles, and diindolylmethanes (DIM) derivatives. The transformation involves a rhodium(III)-catalyzed C-H functionalization and heteroannulation to yield indoles followed by a cascade cyclization with both external and internal nucleophiles to afford diverse products. The role of the hydroxy group, the key function of the silver additive, the origin of the reverse regioselectivity and the rate-determining step, are rationalized in conformity with the combination of experimental, noncovalent interaction analysis and DFT studies. This protocol is endowed with several salient features, including one-pot multistep cascade approach, exclusive regioselectivity, good functional group tolerance and synthesis of variety of molecular frameworks.

A Halogen-Atom Transfer (XAT)-Based Approach to Indole Synthesis Using Aryl Diazonium Salts and Alkyl Iodides

Indoles are among the most important N-heterocycles in pharmaceuticals. Here, we present an alternative to the classic Fischer indole synthesis based on the radical coupling between aryl diazoniums and alkyl iodides. This iron-mediated strategy features a double role for the aryl diazoniums that sequentially activate the alkyl iodides through halogen-atom transfer and then serve as radical acceptors. The process operates under mild conditions and enables the preparation of densely functionalized indoles.

Visible-Light-Irradiated Cascade Reaction of Indole-Tethered Alkenes to Access Tetracyclic Tetrahydro-γ-carbolines

A series of indole-derived alkenes have been designed and applied in a photocatalytic cascade reaction with bromodifluoroacetate esters, affording an unknown type of tetracyclic tetrahydro-γ-carboline derivative in up to 90% yields. Mechanistic studies suggest that the reaction proceeds with tetrahydro-γ-carboline as a key intermediate. The reaction tolerates a diverse pool of substrates, which provides an efficient method for the construction of tetracyclic tetrahydro-γ-carboline compounds.

Orthogonal Syntheses of γ-Carbolinone and Spiro[pyrrolidinone-3,3']indole Derivatives in One Pot through Reaction Telescoping

Herein we report a series of telescoping methodologies for one pot synthesis of biologically relevant γ-carboline derivatives 6 and spiro[pyrrolidinone-3,3']indole 7. Initially the three consecutive steps of cyclopropanation, phthalimide deprotection, and Boc-deprotection have been congregated in a single reaction vessel to afford a ∼1:1 mixture of 6 and 7. Next, careful optimization of the reaction sequence and the conditions generated an orthogonal approach to access compounds 6 and 7 exclusively. Air oxidation of the γ-carbolinones 6 afforded aromatic γ-carbolines 8.

Identification, Structure-Activity Relationship, and Biological Characterization of 2,3,4,5-Tetrahydro-1H-pyrido[4,3-b]indoles as a Novel Class of CFTR Potentiators

Cystic fibrosis (CF) is a life-threatening autosomal recessive disease, caused by mutations in the CF transmembrane conductance regulator (CFTR) chloride channel. CFTR modulators have been reported to address the basic defects associated with CF-causing mutations, partially restoring the CFTR function in terms of protein processing and/or channel gating. Small-molecule compounds, called potentiators, are known to ameliorate the gating defect. In this study, we describe the identification of the 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole core as a novel chemotype of potentiators. In-depth structure-activity relationship studies led to the discovery of enantiomerically pure 39 endowed with a good efficacy in rescuing the gating defect of F508del- and G551D-CFTR and a promising in vitro druglike profile. The in vivo characterization of γ-carboline 39 showed considerable exposure levels and good oral bioavailability, with detectable distribution to the lungs after oral administration to rats. Overall, these findings may represent an encouraging starting point to further expand this chemical class, adding a new chemotype to the existing classes of CFTR potentiators.

Stereodivergent assembly of tetrahydro-γ-carbolines via synergistic catalytic asymmetric cascade reaction

Enantiomerically enriched indole-containing heterocycles play a vital role in bioscience, medicine, and chemistry. As one of the most attractive subtypes of indole alkaloids, highly substituted tetrahydro-γ-carbolines are the basic structural unit in many natural products and pharmaceuticals. However, the syntheses of tetrahydro-γ-carbolines with high functionalities from readily available reagents are significant challenging. In particular, the stereodivergent syntheses of tetrahydro-γ-carbolines containing multi-stereogenic centers remain quite difficult. Herein, we report an expedient and stereodivergent assembly of tetrahydro-γ-carbolines with remarkably high levels of stereoselective control in an efficient cascade process from aldimine esters and indolyl allylic carbonates via a synergistic Cu/Ir catalyst system. Control experiments-guided optimization of synergistic catalysts and mechanistic investigations reveal that a stereodivergent allylation reaction and a subsequent highly stereoselective iso-Pictet-Spengler cyclization are the key elements to success.

Tetrahydro-γ-carbolines are the basic structural unit in many natural products and pharmaceuticals. Here, the authors report a synergistic Cu/Ir system to assemble chiral tetrahydro-γ-carbolines via a stereodivergent allylation followed by a stereoselective iso-Pictet-Spengler cyclization.

Recent developments on synthesis and biological activities of γ-carboline

γ-Carboline alkaloids are a family of natural and synthetic agents that have diverse bioactivities including antiviral, antibacterial, antifungal, antiparasitic, antitumor, anti-inflammatory, neuropharmacological activities and so on. They constitute an important class of pharmacologically active scaffolds that exhibit biological activity via diverse mechanisms. This review provides an update on the recent developments (2010-2017) in the synthesis and biological activities of these compounds. In cases where sufficient information is available, the mechanism and the structure-activity relationship (SAR) of biological activity are presented, and based on our expertise in the field and careful analysis of the recent literature, for the potential of γ-carboline alkaloids as medicinal drugs is proposed.

Bioisosteric matrices for ligands of serotonin receptors

The concept of bioisosteric replacement matrices is applied to explore the chemical space of serotonin receptor ligands, aiming to determine the most efficient ways of manipulating the affinity for all 5-HT receptor subtypes. Analysis of a collection of over 1 million bioisosteres of compounds with measured activity towards serotonin receptors revealed that an average of 31 % of the ligands for each target are mutual bioisosteres. In addition, the collected dataset allowed the development of bioisosteric matrices—qualitative and quantitative descriptions of the biological effects of each predefined type of bioisosteric substitution, providing favored paths of modifying the compounds. The concept exemplified here for serotonin receptor ligands can likely be more broadly applied to other target classes, thus representing a useful guide for medicinal chemists designing novel ligands.

The synthesis and comparative receptor binding affinities of novel, isomeric pyridoindolobenzazepine scaffolds

Compounds 7, 8, and 9, derived from the novel scaffolds 3, 5, and 6, were synthesized and evaluated in vitro. The b,c→c,d shift of the E-phenyl ring resulted in a large decrease (ca. 20- to 1000-fold) in binding to the 5-HT2A, 5-HT2C and H2, receptors, and a modest decrease (ca. 10- to 20-fold) in binding to the 5-HT5A, D2, D5, and α1D, receptors. The b,c→d,e shift resulted in a large decrease in binding to the 5-HT1D, 5-HT2C, 5-HT6, and H1 receptors, a modest decrease in binding to 5-HT1A, 5-HT5A and D2, D5, α2B, and H2 receptors, and a large increase in affinity to the 5-HT3, 5-HT6, and σ1 receptors.

Gold-catalyzed domino cycloisomerization/Pictet-Spengler reaction of 2-(4-aminobut-1-yn-1-yl)anilines with aldehydes: synthesis of tetrahydropyrido[4,3-b]indole scaffolds

A domino cycloisomerization/Pictet-Spengler reaction of 2-(4-aminobut-1-yn-1-yl)aniline with aldehydes has been achieved using a AuIPrCl (5 mol %)/AgSbF(6) (10 mol %) catalytic system to produce the corresponding 1-aryl-N-tosyl-2,3,4,5-tetrahydropyrido[4,3-b] indole derivatives in good yields. This is the first report on the synthesis of tetrahydro pyrido[4,3-b]indole scaffolds through as tandem 5-endo-dig cyclization and Pictect-Spengler reaction.

Second-generation histone deacetylase 6 inhibitors enhance the immunosuppressive effects of Foxp3+ T-regulatory cells

Second-generation Tubastatin A analogues were synthesized and evaluated for their ability to inhibit selectively histone deacetylase 6 (HDAC6). Substitutions to the carboline cap group were well-tolerated with substitution at the 2-position of both β- and γ-carbolines being optimal for HDAC6 activity and selectivity. Some compounds in this series were determined to have subnanomolar activity at HDAC6 with more than 7000 fold selectivity for HDAC6 versus HDAC1. Selected compounds were then evaluated for their ability to augment the immunosuppressive effect of Foxp3+ regulatory T cells. All compounds tested were found to enhance the ability of regulatory T cells to inhibit the mitotic division of effector T cells both in vitro and in vivo, suggesting that further investigation into the use of these compounds for the treatment of autoimmune disorders is warranted.

Dimebolin in dementia

The treatment options for Alzheimer's type dementia are limited to the use of acetyl-cholinesterase inhibitors, along with memantine in some cases, and offer variable success in the treatment of cognitive impairment and to improve or stabilize activities of daily living, behavioral abnormalities, and impairment of global function. This review examines dimebolin, a nonselective antihistamine drug that has been used in Russia in the past for its antihistamine effect and has now generated considerable interest in the treatment of Alzheimer's type dementia, following results in the initial trials (Doody et al., 2008) and is currently under further evaluation. This article considers various theories proposed to explain its effect in the treatment of dementia. The results from further trials will help to clarify the future of dimebolin as a potential treatment for chronic neurodegenerative disorders that include dementia that affects 25 million people worldwide.

Ruthenium-catalyzed γ-carbolinium ion formation from aryl azides; synthesis of dimebolin

A range of γ-carbolines were produced stereoselectively from ruthenium(III)-catalyzed reactions of 3-pyridyl substituted aryl azides. Other catalysts and conditions were neither as selective nor as high-yielding. This method was used to synthesize dimebolin in a concise and efficient manner.

The gramine route to pyrido[4,3-b]indol-3-ones - identification of a new cytotoxic lead

A novel approach to 3-oxo-γ-carbolines was worked out starting from methyl indol-2-ylacetate via a gramine derivative. After quaternization, ammonia and 4-methoxybenzylamine could be inserted giving appropriate 3-oxo-γ-carbolines. Condensation with 2-chlorobenzaldehyde under microwave irradiation gave a 4-(2-chlorobenzyl)-3-oxo-γ-carboline. N-methylation lead to a product with very promising antifungal and cytotoxic activities.

Synthesis and biological evaluation of novel 5,8-disubstituted-2-methyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b] indoles as 5-HT(6) and H(1) receptors antagonists

Synthesis, biological evaluation, and structure-activity relationships (SAR) for a series of novel gamma-carboline analogues of Dimebon are described. Among the studied compounds, tetrahydro-gamma-carboline 5b (2,8-dimethyl-5-[cis-2-pyridin-3-ylvinyl]-2,3,4,5-tetrahydro-carboline) has been identified as the most potent small molecule antagonist, in particular against histamine H(1) and serotonin 5-HT(6) receptors (IC(50) < 0.45 microM and IC(50) = 0.73 microM, respectively). A thorough comparative SAR study performed for the tested compounds has revealed significant correlations between the nature of side substituents and the related antagonistic activity.

Synthesis and biological activity of 5-styryl and 5-phenethyl-substituted 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indoles

Syntheses, biological evaluation, and structure-activity relationships for a series of novel 5-styryl and 5-phenethyl analogs of dimebolin are disclosed. The novel derivatives and dimebolin share a broad spectrum of activities against therapeutically relevant targets. Among all synthesized derivatives, 2,8-dimethyl-5-[(Z)-2-phenylvinyl]-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole and its 5-phenethyl analog are the most potent blockers of 5-HT(7), 5-HT(6), 5-HT(2C), Adrenergic alpha(2) and H(1) receptors. The general affinity rank order towards the studied receptors was Z-3(2)>4(2)4(3)>>dimebolin, all of them having highest affinities to 5-HT(7) receptors.