Tautomerism and acidity in 4-quinolone-3-carboxylic acid derivatives

15.4.5 Quinolinones and Related Systems (Update 2022)

Quinolinones, of which the quinolin-4(1H)-one ring system can be highlighted, represent an exciting class of nitrogen heterocycles. The quinolinone motif can be found in many natural compounds and approved drugs for several diseases. This chapter is a comprehensive survey of the methods for the synthesis of quinolin-2(1H)-ones, quinolin-4(1H)-ones, and their thio- and amino derivatives, and is an update to the previous Science of Synthesis chapter (Section 15.4), covering the period between 2003 and 2020.

4-Oxoquinolines and monoamine oxidase: When tautomerism matters

4-Oxoquinoline derivatives have been often used in drug discovery programs due to their pharmacological properties. Inspired on chromone and 4-oxoquinoline chemical structure similarity, a small series of quinoline-based compounds was obtained and screened, for the first time, toward human monoamine oxidases isoforms. The data showed the N-(3,4-dichlorophenyl)-1-methyl-4-oxo-1,4-dihydroquinoline-3-carboxamide 10 was the most potent and selective MAO-B inhibitor (IC₅₀ = 5.30 ± 0.74 nM and SI: ≥1887). The data analysis showed that prototropic tautomerism markedly influences the biological activity. The unequivocal characterisation of the quinoline tautomers was performed to understand the attained data. To our knowledge, there have been no prior reports on the characterisation of quinolone tautomers by 2D NMR techniques, namely by ¹H-¹⁵N HSQC and ¹H-¹⁵N HMBC, which are proposed as expedite tools for medicinal chemistry campaigns. Computational studies on enzyme-ligand complexes, obtained after MM-GBSA calculations and molecular dynamics simulations, supported the experimental data.

Recent Advances in One-Pot Modular Synthesis of 2-Quinolones

It is known that 2-quinolones are broadly applicable chemical structures in medicinal and agrochemical research as well as various functional materials. A number of current publications about their synthesis and their applications emphasize the importance of these small molecules. The early synthetic chemistry originated from the same principle of the classical Friedländer and Knorr procedures for the preparation of quinolines. The analogous processes were developed by applying new synthetic tools such as novel catalysts, the microwave irradiation method, etc., whereas recent innovations in new bond forming reactions have allowed for novel strategies to construct the core structures of 2-quinolones beyond the bond disconnections based on two classical reactions. Over the last few decades, some reviews on structure-based, catalyst-based, and bioactivity-based studies have been released. In this focused review, we extensively surveyed recent examples of one-pot reactions, particularly in view of modular approaches. Thus, the contents are categorized as three major sections (two-, three-, and four-component reactions) according to the number of reagents that ultimately compose atoms of the core structures of 2-quinolones. The collected synthetic methods are discussed from the perspectives of strategy, efficiency, selectivity, and reaction mechanism.

Dimethyl Sulfoxide-Promoted Dehydrosulfonylation of 2-Aryl-3-Bromo-1-Methanesulfonyl-2,3-Dihydroquinolin-4(1H)-Ones

The 2-aryl-3-bromo-1-methanesulfonyl-2,3-dihydroquinolin-4(1H)-ones undergo dimethyl sulfoxide (DMSO)-promoted dehydrosulfonylation under reflux at 130–140°C to afford the corresponding 2-aryl-3-bromoquinolin-4(1H)-ones.

Structural assignment of the enol–keto tautomers of one-pot synthesized 4-hydroxyquinolines/4-quinolones

The one-pot preparation of 2,3-disubstituted 4-quinolones and the structural assignment of their tautomers are described.

Metal-free regioselective formation of C-N and C-O bonds with the utilization of diaryliodonium salts in water: facile synthesis of N-arylquinolones and aryloxyquinolines

Regioselective construction of crucial C-N and C-O bonds leading to N-arylquinolones and aryloxyquinolines has been accomplished by employing easily accessible diaryliodonium salts and quinolones in water under metal- and ligand-free conditions. This operationally simple strategy is significant due to mild reaction conditions, high product yields, recyclability of released iodoarenes and scalability to the gram level. The practical utility of the developed protocol was proved by the arylation of medicinally important heterocycles like acridin-9(10H)-one, 3-methylquinoxalin-2(1H)-one and 1H-benzo[d]imidazol-2(3H)-one.

FRET Studies of Quinolone-Based Bitopic Ligands and Their Structural Analogues at the Muscarinic M1 Receptor

Aiming to design partial agonists as well as allosteric modulators for the M₁ muscarinic acetylcholine (M₁AChR) receptor, two different series of bipharmacophoric ligands and their structural analogues were designed and synthesized. The hybrids were composed of the benzyl quinolone carboxylic acid (BQCA)-derived subtype selective allosteric modulator 3 and the orthosteric building block 4-((4,5-dihydroisoxazol-3-yl)oxy)-N,N-dimethylbut-2-yn-1-amine (base of iperoxo) 1 or the endogenous ligand 2-(dimethylamino)ethyl acetate (base of acetylcholine) 2, respectively. The two pharmacophores were linked via alkylene chains of different lengths (C4, C6, C8, and C10). Furthermore, the corresponding structural analogues of 1 and 2 and of modified BQCA 3 with varying alkyl chain length between C2 and C10 were investigated. Fluorescence resonance energy transfer (FRET) measurements in a living single cell system were investigated in order to understand how these compounds interact with a G protein-coupled receptor (GPCR) on a molecular level and how the single moieties contribute to ligand receptor interaction. The characterization of the modified orthosteric ligands indicated that a linker attached to an orthoster rapidly attenuates the receptor response. Linker length elongation increases the receptor response of bitopic ligands, until reaching a maximum, followed by a gradual decrease. The optimal linker length was found to be six methylene groups at the M₁AChR. A new conformational change is described that is not of inverse agonistic origin for long linker bitopic ligands and was further investigated by exceptional fragment-based screening approaches.

Tautomeric switching and metal-cation sensing of ligand-equipped 4-hydroxy-/4-oxo-1,4-dihydroquinolines

Novel 4-hydroxyquinoline (4HQ) based tautomeric switches are reported. 4HQs equipped with coordinative side arms (8-arylimino and 3-piperidin-1-ylmethyl groups) were synthesized to access O- or N-selective chelation of Zn(2+) and Cd(2+) ions by 4HQ. In the case of the monodentate arylimino group, O chelation of metal ions induces concomitant switching of phenol tautomer to the keto form in nonpolar or aprotic media. This change is accompanied by selective and highly sensitive fluorometric sensing of Zn(2+) ions. In the case of the bidentate 8-(quinolin-8-ylimino)methyl side arm, NMR studies in CD(3) OD indicated that both Cd(2+) and Zn(2+) ions afford N chelation for 4HQ, coexisting with tautomeric switching from quinolin-4(1H)-one to quinolin-4-olate. In corroboration, UV/Vis-monitored metal-ion titrations in toluene and methanol implied similar structural changes. Additionally, fluorescence measurements indicated that the metal-triggered tautomeric switching is associated with compound signaling properties. The results are supported by DFT calculations at the B3LYP 6-31G* level. Several X-ray structures of metal-free and metal-chelating 4HQ are presented to support the solution studies.

Anodic oxidation of selenadiazoloquinolones in alkaline media

Newly synthesized derivatives of 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinoline variously substituted at position 7 (R = H, COOH, COCH(3), CN, COOC(2)H(5) and COOCH(3)) are established in strongly alkaline aqueous solutions (0.1 M NaOH; pH ∼ 13) as N(9)-deprotonated structures, but in less alkaline solutions (0.001 M NaOH; pH ∼ 11) the N(9)-protonated oxo tautomeric forms dominate. Upon their anodic oxidation in alkaline solutions, the selenadiazole ring is replaced, forming instead the paramagnetic species analogous to the ortho semiquinone radical anions as monitored by in situ EPR spectroscopy. The quantum chemical calculations for two representative selenadiazoloquinolones (R = H and COOH) and their anodic oxidation products presented are in agreement with experiments.

Synthesis and structure-activity relationships of antimalarial 4-oxo-3-carboxyl quinolones

Malaria is endemic in tropical and subtropical regions of Africa, Asia, and the Americas. The increasing prevalence of multi-drug-resistant Plasmodium falciparum drives the ongoing need for the development of new antimalarial drugs. In this light, novel scaffolds to which the parasite has not been exposed are of particular interest. Recently, workers at the Swiss Tropical Institute discovered two novel 4-oxo-3-carboxyl quinolones active against the intra-erythrocytic stages of P. falciparum while carrying out rationally directed low-throughput screening of potential antimalarial agents as part of an effort directed by the World Health Organization. Here we report the design, synthesis, and preliminary pharmacologic characterization of a series of analogues of 4-oxo-3-carboxyl quinolones. These studies indicate that the series has good potential for preclinical development.

Accelerated dereplication of crude extracts using HPLC-PDA-MS-SPE-NMR: quinolinone alkaloids of Haplophyllum acutifolium

Direct hyphenation of analytical-scale high-performance liquid chromatography, photo-diode array detection, mass spectrometry, solid-phase extraction and nuclear magnetic resonance spectroscopy (HPLC-PDA-MS-SPE-NMR) has been used for accelerated dereplication of crude extract of Haplophyllum acutifolium (syn. Haplophyllum perforatum). This technique allowed fast on-line identification of six quinolinone alkaloids, named haplacutine A-F, as well as of acutine, haplamine, eudesmine, and 2-nonylquinolin-4(1H)-one. Acutine and haplacutine E, isolated by preparative-scale HPLC, showed moderate antiplasmodial activity with IC(50) values of 2.17+/-0.22 microM and 3.79+/-0.24 microM, respectively (chloroquine-sensitive Plasmodium falciparum 3D7 strain).

Studies on anti-HIV quinolones: new insights on the C-6 position

The 6-desfluoroquinolones which have been developed by our group represent a promising class of compounds for the treatment of HIV infection since they act on transcriptional regulation, a crucial step in the replication cycle that has not been clinically exploited, yet. Focussing attention on the N-1 and C-6 positions, a novel series of quinolones has been synthesized. New SAR insights have been obtained, in particular, the hydroxyl group emerged as a suitable C-6 substituent when coupled with the appropriate arylpiperazine at the neighboring C-7 position.

Structure-activity relationship study on anti-HIV 6-desfluoroquinolones

On the basis of our recent findings that 6-aminoquinolones inhibit the HIV Tat-mediated transactivation, we have designed a broad series of derivatives identifying novel potent agents such as the 6-desfluoroquinolones 24 (HM12) and 27 (HM13), which showed pronounced anti-HIV activity in acutely, chronically, and latently HIV-1 infected cell cultures. We demonstrate here that highly potent molecules can be obtained by optimizing the substituent in the various positions of the quinolone nucleus.

Synthesis of novel anilinoquinolines as c-fms inhibitors

A novel series of potent substituted anilinoquinolines were discovered as c-fms inhibitors. The potency could be manipulated upon modification of the C4 aniline and C7 aryl functionality. Pharmacokinetic analysis identified a metabolically stable analog suitable for further investigative work.

Synthesis of novel quinolone and quinoline-2-carboxylic acid (4-morpholin-4-yl-phenyl)amides: A late-stage diversification approach to potent 5HT1B antagonists

Multiparallel amenable syntheses of 6-methoxy-8-amino-4-oxo-1,4-dihydroquinoline-2-carboxylic acid-(4-morpholin-4-yl-phenyl)amides (I) and 4-amino-6-methoxy-8-(4-methyl-piperazin-1-yl)-quinoline-2-carboxylic acid (4-morpholin-4-yl-phenyl)amides (II) which facilitate late-stage diversification at the 8-position of (I) and at the 4- and 8-positions of (II) are described. The resulting novel series were determined to contain potent 5HT(1B) antagonists. Preliminary SAR data are presented.

Sulfamoyl-4-oxoquinoline-3-carboxamides: novel potentiators of defective DeltaF508-cystic fibrosis transmembrane conductance regulator chloride channel gating

The synthesis of a small collection of sulfamoyl-4-oxoquinoline-3-carboxamides is described for use as correctors of defective gating of the DeltaF508-cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. Several compounds with submicromolar potency were obtained. N-Ethyl 6-(ethylphenylsulfamoyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide (7b) was found to be the most effective sulfonamide corrector of defective DeltaF508-CFTR gating.

Interactions of fluoroquinolone antibacterial agents with aqueous chlorine: reaction kinetics, mechanisms, and transformation pathways

Kinetics, products, and mechanistic aspects of reactions between free available chlorine (HOCl/OCl-), ciprofloxacin (CF), and enrofloxacin (EF) were extensively investigated to elucidate the behavior of fluoroquinolone antibacterial agents during water chlorination processes. Although the molecular structures of these two substrates differ only with respect to degree of N(4) amine alkylation, CF and EF exhibit markedly different HOCl reaction kinetics and transformation pathways. HOCI reacts very rapidly at CF's secondary N(4) amine, forming a chloramine intermediate that spontaneously decays in aqueous solution by concerted piperazine fragmentation. In contrast, HOCl reacts relatively slowly at EF's tertiary N(4) amine, apparently forming a highly reactive chlorammonium intermediate (R3N-(4)Cl+) that can catalytically halogenate EF or other substrates present in solution. Flumequine, a fluoroquinolone that lacks the characteristic piperazine ring, exhibits no apparent reactivity toward HOCI but appears to undergo facile halodecarboxylation in the presence of R3N(4)-Cl+ species derived from EF. Measured reaction kinetics were validated in real water matrixes by modeling CF and EF losses in the presence of free chlorine residuals. Combined chlorine (CC) kinetics were determined under selected conditions to evaluate the potential significance of reactions with chloramines. CF's rapid kinetics in direct reactions with HOCl, and relatively high reactivity toward CC, indicate that secondary amine-containing fluoroquinolones should be readily transformed during chlorination of real waters, whether applied chlorine doses are present as free or combined residuals. However, EF's slower HOCl reaction kinetics, recalcitrance toward CC, and participation in the catalytic halogenation cycle described herein suggest that tertiary amine-containing fluoroquinolones will be comparatively stable during most full-scale water chlorination processes.

Synthesis of acyclic 6,7-dihaloquinolone nucleoside analogues as potential antibacterial and antiviral agents

Reaction of the quinolone carboxylic acids 1 and 2 with (2-acetoxyethoxy)methyl chloride 3 in the presence of n-Bu4NI afforded the N-alkylated products 4 and 6, which could be deblocked to the free nucleoside analogues 5 and 7, respectively. The alkylated quinolone carboxylic acids 9 and 10 were obtained by condensation of I and 2 with 1,4-dichlorobut-2-ene 8 in the presence of NaH. Hydrolysis of 9 gave the alcohol 11. Similar treatment of 1 with 8 in the presence of K2CO3 at relatively high temperature furnished 12. Prolonged heating of the ester 13 with 8 in NaH/DMF afforded the conjugated-diene 15. Treatment of 1 and 2 with dimethyl acetylenedicarboxylate 16 furnished the pyrano[4,3-b]quinolones 17 and 18, respectively. Antibacterial and antiviral evaluations of the new products are reported.