Synthesis of quinoline mimics via C-H bond functionalization of quinoline: a review on recent progress

Functionalization of the quinoline ring has emerged as a transformative strategy in modern synthetic chemistry because of the medicinal potential of quinoline-based scaffolds. The precise and selective introduction of diverse functional groups significantly expands the chemical space and enhances the pharmacological profile of quinoline derivatives. By carefully selecting catalysts, reaction conditions, and directing groups, researchers have unlocked novel pathways for the efficient synthesis of quinoline-based compounds with improved efficacy, target selectivity, and safety. This approach accelerates drug discovery and broadens the therapeutic potential of quinoline scaffolds for treating various diseases, including cancer, infectious diseases, and neurological disorders. Over the past two decades, this field has experienced exponential growth, as evidenced by the increasing number of research publications and comprehensive review articles. This surge in interest is driven by the potential of quinoline functionalization to generate novel drug candidates with enhanced bioactivity and reduced side effects. This review summarizes the key advancements from January 2021 to 2024, focusing on the latest methodologies, catalytic systems, and applications in drug development.

Rh(III)-catalyzed regioselective C(sp2)-H alkenylation of isoquinolones with methoxyallene: A facile access to aldehyde-bearing isoquinolones

A simple and rapid access to isoquinolone aldehyde scaffolds has been established by a rhodium-catalyzed reaction between isoquinolone and methoxyallene that forges alkenylation in an explicit regioselective manner. Herein, methoxyallene serving as an acrolein equivalent results in execution of this unique functionalization. Furthermore, the compatibility with complex molecules underscores the significance of this developed protocol. The mechanistic proposal for this regioselective transformation was consistent with kinetic studies and several control reactions.

Cobalt phthalocyanine (CoPc) anchored on Ti3C2 MXene nanosheets for highly efficient selective catalytic oxidation

Quinclorac is an important precursor for pharmaceutical, agricultural, and synthetic chemistry. The state-of-the-art synthesis of quinclorac via condensation, chlorination and oxidative hydrolysis often uses homogeneous catalysts and strong acid oxidant agents to promote the catalytic oxidation, which requires huge manpower input for the late-stage purification process and is usually environmentally unfriendly. In this work, we successfully fabricated a stable cobalt phthalocyanine (CoPc) Co-based composite (CoPc/Ti3C2) by anchoring CoPc on the surface of Ti3C2 nanosheets for the selective oxidation of 3,7-dichloro-8-dichloro methyl quinoline (3,7-D-8-DMQ) into quinclorac. More impressively, CoPc/Ti3C2-4.5%-Mn-Br exhibits a high selectivity of 91.8% for the catalytic oxidation of 3,7-D-8-DMQ to quinclorac in acetic acid, with a quinclorac yield of 87.5%, which is approximately 2.46 times higher than that of pristine CoPc-Mn-Br. The obtained heterogeneous catalytic system shows good reusability. Detailed mechanistic investigations reveal that the system works through the free radical mechanism via the formation of Co2+/Co3+ redox cycles. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the design of catalysts for selective catalytic oxidation.

Rh(III)-Catalyzed C7-Alkylation of Isatogens with Malonic Acid Diazoesters

Rh(III)-catalyzed C7-alkylation of isatogens (indolin-3-one N-oxides) with malonic acid diazoesters has been developed. This strategy utilizes oxygen anion on the N-oxide group of isatogens as a directing group and successfully achieves the synthesis of a series of C7-alkylated isatogens with moderate to good yields (48-86% yields). Moreover, the N-oxides of isatogens can not only serve as the simple directing group for C7-H bond cleavage but also be deoxidized for easy removal.

Directed C8-H allylation of quinoline N-oxides with vinylcyclopropanes via sequential C-H/C-C activation

The Rh(III)-catalyzed C8-allylation of quinoline N-oxides has been accomplished using vinylcyclopropanes as an allyl source with excellent diastereoselectivity at room temperature. The C-H/C-C activation, substrate scope and natural product mutation are the important practical features.

Rhodium(III)-Catalyzed Conjugate Addition of β-CF3-Enones with Quinoline N-Oxides

The site-selective incorporation of a trifluoromethyl group into biologically active molecules and pharmaceuticals has emerged as a central topic in medicinal chemistry and drug discovery. Herein, we demonstrate the rhodium(III)-catalyzed conjugate addition of β-trifluoromethylated enones with quinoline N-oxides, which result in the generation of β-trifluoromethyl-β'-quinolinated ketones. The reaction proceeds under mild conditions with complete functional group tolerance. The synthetic applicability was showcased by successful gram-scale experiments and valuable synthetic transformations of coupling products.

Synthesis and spectroscopic and structural characterization of three new 2-methyl-4-styrylquinolines formed using Friedländer reactions between (2-aminophenyl)chalcones and acetone

Three new 2-methyl-4-styrylquinoline derivatives have been synthesized in high yields using Friedländer reactions between chalcones [1-(2-aminophenyl)-3-arylprop-2-en-1-ones] and acetone, and characterized using IR, 1H and 13C NMR spectroscopy, and mass spectrometry, and by crystal structure analysis. In (E)-4-(4-fluorostyryl)-2-methylquinoline, C18H14FN, (I), the molecules are joined into cyclic centrosymmetric dimers by C-H...N hydrogen bonds and these dimers are linked into sheets by π-π stacking interactions. The molecules of (E)-2-methyl-4-[4-(trifluoromethyl)styryl]quinoline, C19H14F3N, (II), are linked into cyclic centrosymmetric dimers by C-H...π hydrogen bonds and these dimers are linked into chains by a single π-π stacking interaction. There are no significant hydrogen bonds in the structure of (E)-4-(2,6-dichlorostyryl)-2-methylquinoline, C18H13Cl2N, (III), but molecules related by translation along [010] form stacks with an intermolecular spacing of only 3.8628 (2) Å. Comparisons are made with the structures of some related compounds.

Cp*Co(III)-Catalyzed Selective C8-Olefination and Oxyarylation of Quinoline N-Oxides with Terminal Alkynes

Herein we report Cp*Co(III)-catalyzed site-selective (C8)-H olefination and oxyarylation of quinoline N-oxides with terminal alkynes. The selectivity for C8-olefination and oxyarylation is sterically and electronically controlled. In the case of quinoline N-oxides (unsubstituted at the C2 position), only the olefination product was obtained irrespective of the nature of the alkynes. In contrast, oxyarylation was observed exclusively when 2-substituted quinoline N-oxides were reacted with 9-ethynylphenanthrene. However, alkynes with electron-withdrawing groups provided only olefination products with 2-substituted quinoline N-oxides. The developed strategy allowed a facile functionalization of quinoline N-oxides bearing natural molecules and an estrone-derived terminal alkyne to deliver the corresponding olefinated and oxyarylated products. To understand the reaction mechanism, control experiments, deuterium-labeling experiments, and kinetic isotope effect (KIE) studies were performed.

Recent Advances in the High-Valent Cobalt-Catalyzed C-H Functionalization of N-Heterocycles

Direct functionalization of heterocycles using C-H activation widely relies on the precious metal complexes. In past decade, the use of earth abundant and inexpensive transition metal to functionalize heterocycles has become an attractive alternate strategy. This concept is also interesting due to the unique reactivity pattern of these inexpensive metals. In this context we and other research groups have utilized the high-valent cobalt complexes as an inexpensive and readily available catalyst for the functionalization of heterocycles. In this review, we intend to brief recent progress made in the area of high-valent cobalt complexes catalyzed C-H functionalization of N-containing heterocycles.

Site-selective C-H functionalization to access the arene backbone of indoles and quinolines

The site-selective C-H bond functionalization of heteroarenes can eventually provide chemists with great techniques for editing and building complex molecular scaffolds. During the past decade, benzo-fused N-heterocycles such as indoles and quinolines have been among the most widely investigated organic templates. Early developments have led to site-selective C-H bond functionalization on the pyrrole and pyridine cores of indoles and quinolines; however, C-H functionalization on the benzenoid ring has remained a great challenge in catalysis. In this review, we elaborate on recent developments in the highly challenging functionalization of C-H bonds on the less-reactive benzenoid core of indoles and quinolines. These findings are mainly described as selective directing group assisted strategies, remote C-H functionalization techniques and their reaction mechanisms. The underlying principle in each strategy is elucidated, which aims to facilitate the design of a more advanced structure of heterocycles based on bioactive molecules, synthetic drugs, and material aspects. Moreover, the challenges and perspectives for catalytic C-H functionalization to access the arene backbone of indoles and quinolines are also proposed in the conclusion section.

Regioselective Functionalization of Quinolines through C-H Activation: A Comprehensive Review

Quinoline is a versatile heterocycle that is part of numerous natural products and countless drugs. During the last decades, this scaffold also became widely used as ligand in organometallic catalysis. Therefore, access to functionalized quinolines is of great importance and continuous efforts have been made to develop efficient and regioselective synthetic methods. In this regard, C-H functionalization through transition metal catalysis, which is nowadays the Graal of organic green chemistry, represents the most attractive strategy. We aim herein at providing a comprehensive review of methods that allow site-selective metal-catalyzed C-H functionalization of quinolines, or their quinoline N-oxides counterparts, with a specific focus on their scope and limitations, as well as mechanistic aspects if that accounts for the selectivity.

Site-Selective C8-Alkylation of Quinoline N-Oxides with Maleimides under Rh(III) Catalysis

The site-selective modification of quinolines and their analogs has emerged as a pivotal topic in medicinal chemistry and drug discovery. Herein, we describe the rhodium(III)-catalyzed C8-alkylation of quinoline N-oxides with maleimides as alkylating agents, resulting in the formation of bioactive succinimide-containing quinoline derivatives. The reaction proceeds under mild conditions with complete functional group tolerance.

Rh(III)-Catalyzed Regioselective C8-Alkylation of Quinoline N-Oxides with Maleimides and Acrylates

Herein, we disclose the Rh(III)-catalyzed selective C8-alkylation of quinoline N-oxides with maleimides and acrylates. The main features of the reaction include complete C8-selectivity and broad substrate scope with good to excellent yields. The reaction also proceeded well with unprotected maleimide. The applicability of the developed methodology is demonstrated with gram-scale synthesis and post-modification of the alkylated product. Preliminary mechanistic study revealed that the reaction proceeds through a five-membered rhodacycle and involves proto-demetalation step.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Cobalt-Catalyzed C8-Dienylation of Quinoline-N-Oxides

An efficient Cp*Co^III -catalyzed C8-dienylation of quinoline-N-oxides was achieved by employing allenes bearing leaving groups at the α-position as the dienylating agents. The reaction proceeds by Co^III -catalyzed C-H activation of quinoline-N-oxides and regioselective migratory insertion of the allene followed by a β-oxy elimination, leading to overall dienylation. Site-selective C-H activation was achieved with excellent selectivity under mild reaction conditions, and 30 mol % of a NaF additive was found to be crucial for the efficient dienylation. The methodology features high stereoselectivity, mild reaction conditions, and good functional-group tolerance. C8-alkenylation of quinoline-N-oxides was achieved in the case of allenes devoid of leaving groups as coupling partners. Furthermore, gram-scale preparation and preliminary mechanistic experiments were carried out to gain insights into the reaction mechanism.

4-Styrylquinolines from cyclocondensation reactions between (2-aminophenyl)chalcones and 1,3-diketones: crystal structures and regiochemistry

Structures are reported for two matched sets of substituted 4-styrylquinolines which were prepared by the formation of the heterocyclic ring in cyclocondensation reactions between 1-(2-aminophenyl)-3-arylprop-2-en-1-ones with 1,3-dicarbonyl compounds. (E)-3-Acetyl-4-[2-(4-methoxyphenyl)ethenyl]-2-methylquinoline, C21H19NO2, (I), (E)-3-acetyl-4-[2-(4-bromophenyl)ethenyl]-2-methylquinoline, C20H16BrNO, (II), and (E)-3-acetyl-2-methyl-4-{2-[4-(trifluoromethyl)phenyl]ethenyl}quinoline, C21H16F3NO, (III), are isomorphous and in each structure the molecules are linked by a single C-H...O hydrogen bond to form C(6) chains. In (I), but not in (II) or (III), this is augmented by a C-H...π(arene) hydrogen bond to form a chain of rings; hence, (I)-(III) are not strictly isostructural. By contrast with (I)-(III), no two of ethyl (E)-4-[2-(4-methoxyphenyl)ethenyl]-2-methylquinoline-3-carboxylate, C22H21NO3, (IV), ethyl (E)-4-[2-(4-bromophenyl)ethenyl]-2-methylquinoline-3-carboxylate, C21H18BrNO2, (V), and ethyl (E)-2-methyl-4-{2-[4-(trifluoromethyl)phenyl]ethenyl}quinoline-3-carboxylate, C22H18F3NO2, (VI), are isomorphous. The molecules of (IV) are linked by a single C-H...O hydrogen bond to form C(13) chains, but cyclic centrosymmetric dimers are formed in both (V) and (VI). The dimer in (V) contains a C-H...π(pyridyl) hydrogen bond, while that in (VI) contains two independent C-H...O hydrogen bonds. Comparisons are made with some related structures, and both the regiochemistry and the mechanism of the heterocyclic ring formation are discussed.

Co(III)-Catalyzed C-H Amidation of Nitrogen-Containing Heterocycles with Dioxazolones under Mild Conditions

A cobalt(III)-catalyzed C-8 selective C-H amidation of quinoline N-oxide using dioxazolone as an amidating reagent under mild conditions is disclosed. The reaction proceeds efficiently with excellent functional group compatibility. The utility of the current method is demonstrated by gram scale synthesis of C-8 amide quinoline N-oxide and by converting this amidated product into functionalized quinolines. Furthermore, the developed catalytic method is also applicable for C-7 amidation of N-pyrimidylindolines and ortho-amidation of benzamides.

Regioselective Arylation of Quinoline N-Oxides (C8), Indolines (C7) and N-tert-Butylbenzamide with Arylboronic Acids

Herein, we disclose Ru(II)-catalyzed regioselective distal C(sp2)-H arylation of quinoline N-oxide with arylboronic acids to 8-arylquinolines. In the developed method, the Ru(II)-catalyst shows dual activity, that is, distal C-H activation of quinoline N-oxides followed by in situ deoxygenation of arylated quinoline N-oxide in the same pot. The current catalytic method features use of Ru metal as the catalyst and arylboronic acids as the arylating source under mild reaction conditions. Use of the Rh(III)-catalyst in place of Ru(II) under the same conditions afforded 8-arylquinoline N-oxides with excellent regioselectivity. Furthermore, the developed Ru(II) catalytic system is also extended for the C(sp2)-H arylation of indolines, N-tert-butylbenzamide, and 6-(5H)-phenanthridinone. Formation of the quinoline N-oxide coordinated ruthenium adduct is found to be the key reaction intermediate, which has been characterized by single crystal X-ray diffraction and NMR spectroscopy.

The direct C–H alkenylation of quinoline N-oxides as a suitable strategy for the synthesis of promising antiparasitic drugs

This review highlights the direct C–H alkenylation of quinoline N-oxides covering the metal-free and transition-metal catalysed protocols, and the regioselectivity during the synthesis of antiparasitic drugs based on quinoline scaffold.

Catalyst Control in Switching the Site Selectivity of C-H Olefinations of 1,2-Dihydroquinolines: An Approach to Positional-Selective Functionalization of Quinolines

A unique approach to achieve site-selective C-H olefinations exclusively at the C-3- or C-8-positions in the quinoline framework has been developed by catalyst control. Distal C(3)-H functionalization is achieved by using palladium catalysis, whereas proximal C(8)-H functionalization is obtained by employing ruthenium catalysis. Switching the site selectivity within a single substrate directly indicates two diverse pathways, which are operating under the palladium- and ruthenium-catalyzed reaction conditions.

Evaluation of Antiplasmodial Potential of C2 and C8 Modified Quinolines: in vitro and in silico Study

BACKGROUND

Malaria remains a common life-threatening infectious disease across the globe due to the development of resistance by Plasmodium parasite against most antimalarial drugs. The situation demands new and effective drug candidates against Plasmodium.

OBJECTIVES

The objective of this study is to design, synthesize and test novel quinoline based molecules against the malaria parasite.

METHODS

C2 and C8 modified quinoline analogs obtained via C-H bond functionalization approach were synthesized and evaluated for inhibition of growth of P. falciparum grown in human red blood cells using SYBR Green microtiter plate based screening. Computational molecular docking studies were carried out with top fourteen molecules using Autodoc software.

RESULTS

The biological evaluation results revealed good activity of quinoline-8-acrylate 3f (IC50 14.2 µM), and the 2-quinoline-α-hydroxypropionates 4b (IC50 6.5 µM), 4j (IC50 5.5 µM) and 4g (IC50 9.5 µM), against chloroquine sensitive Pf3D7 strain. Top fourteen molecules were screened also against chloroquine resistant Pf INDO strain and the observed resistant indices were found to lie between 1 and 7.58. Computational molecular docking studies indicated a unique mode of binding of these quinolines to Falcipain-2 and heme moiety, indicating these to be the probable targets of their antiplasmodial action.

CONCLUSION

An important finding of our work is the fact that unlike Chloroquine which shows a resistance Index of 15, the resistance indices for the most promising molecules studied by us were about one indicating equal potency against drug sensitive and resistant strains of the malaria parasite.

Rhodium-Catalyzed C(sp2 )- or C(sp3 )-H Bond Functionalization Assisted by Removable Directing Groups

In recent years, transition-metal-catalyzed C-H activation has become a key strategy in the field of organic synthesis. Rhodium complexes are widely used as catalysts in a variety of C-H functionalization reactions because of their high reactivity and selectivity. The availability of a number of rhodium complexes in various oxidation states enables diverse reaction patterns to be obtained. Regioselectivity, an important issue in C-H activation chemistry, can be accomplished by using a directing group to assist the reaction. However, to obtain the target functionalized compounds, it is also necessary to use a directing group that can be easily removed. A wide range of directed C-H functionalization reactions catalyzed by rhodium complexes have been reported to date. In this Review, we discuss Rh-catalyzed C-H functionalization reactions that are aided by the use of a removable directing group such as phenol, amine, aldehyde, ketones, ester, acid, sulfonic acid, and N-heteroaromatic derivatives.

Catalyst and Additive-Free Diastereoselective 1,3-Dipolar Cycloaddition of Quinolinium Imides with Olefins, Maleimides, and Benzynes: Direct Access to Fused N,N'-Heterocycles with Promising Activity against a Drug-Resistant Malaria Parasite

A convenient and eco-friendly synthesis of various fused N-heterocyclic compounds through catalyst and additive-free 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes in excellent yields and diastereoselectivities is reported. The thermally controlled diastereoselective [3 + 2] cycloaddition reaction between quinolinium imides and olefins provided cis-isomers at low temperature and trans-isomers at high temperature. A reaction between quinolinium imides with substituted maleimides gave four-ring-fused N-heterocyclic compounds in high yields as a single diastereomer. The aryne precursors also provided four-ring-fused N,N'-heterocyclic compounds in high yields. The in vitro antiplasmodial activity of selected molecules revealed that this class of molecules possesses potential for ongoing studies against malaria.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

RhIII -Catalyzed Direct C8-Arylation of Quinoline N-Oxides using Diazonaphthalen-2(1H)-ones: A Practical Approach towards 8-aza BINOL

An efficient Rh^III -catalyzed redox-neutral method for the direct C8-arylation of quinoline N-oxides using diazonaphthalen-2(1H)-one as coupling partner has been demonstrated. The developed method is simple, scalable and straightforward with a wide range of substrate scope. The applicative potential was extended with a late-stage functionalization and straightforward synthesis of 8-azaBINOL derivative. A plausible reaction pathway was proposed after carrying out preliminary control studies.

Heterobicyclic Core Retained Hydroarylations through C-H Activation: Synthesis of Epibatidine Analogues

The heterobicyclic core retained hydroarylation of oxa/azabenzonorbornadienes with quinoline N-oxides has been achieved under rhodium catalysis, giving quinoline N-oxide substituted heterobicyclic structures with excellent regioselectivity and in good yields. As the first example of the direct introduction of quinoline N-oxides onto heterobicyclic structures, the strained heterobicyclic core was well retained in the reaction. The products could be successfully transformed into a series of useful compounds, including epibatidine analogues.

Weakly-coordinating N-oxide and carbonyl groups for metal-catalyzed C–H activation: the case of A-ring functionalization

This feature review is focused on recent challenges based on the functionalizations at C-8 and C-5 positions of heterocyclic and quinoidal compounds – a topic that is still rarely explored in the literature.