Rh(III)-Catalyzed C(8)–H Functionalization of Quinolines via Simultaneous C–C and C–O Bond Formation: Direct Synthesis of Quinoline Derivatives with Antiplasmodial Potential

2D/3D-QSAR Model Development Based on a Quinoline Pharmacophoric Core for the Inhibition of Plasmodium falciparum: An In Silico Approach with Experimental Validation

Malaria is an infectious disease caused by Plasmodium spp. parasites, with widespread drug resistance to most antimalarial drugs. We report the development of two 3D-QSAR models based on comparative molecular field analysis (CoMFA), comparative molecular similarity index analysis (CoMSIA), and a 2D-QSAR model, using a database of 349 compounds with activity against the P. falciparum 3D7 strain. The models were validated internally and externally, complying with all metrics (q2 > 0.5, r2test > 0.6, r2m > 0.5, etc.). The final models have shown the following statistical values: r2test CoMFA = 0.878, r2test CoMSIA = 0.876, and r2test 2D-QSAR = 0.845. The models were experimentally tested through the synthesis and biological evaluation of ten quinoline derivatives against P. falciparum 3D7. The CoMSIA and 2D-QSAR models outperformed CoMFA in terms of better predictive capacity (MAE = 0.7006, 0.4849, and 1.2803, respectively). The physicochemical and pharmacokinetic properties of three selected quinoline derivatives were similar to chloroquine. Finally, the compounds showed low cytotoxicity (IC50 > 100 µM) on human HepG2 cells. These results suggest that the QSAR models accurately predict the toxicological profile, correlating well with experimental in vivo data.

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.

Cooperative Lewis Acid Catalysis for the Enantioselective C(sp3)-H Bond Functionalizations of 2-Alkyl Azaarenes

Herein, we describe the enantioselective C(sp³)-H bond functionalizations of 2-alkyl azaarenes using a cooperative dual Lewis acid catalysis. An achiral Lewis acid activates the unactivated azaarene partner without the need for a strong base. Orthogonally, a chiral-at-metal Lewis acid catalyst enables LUMO lowering and induces chirality. This method tolerates a range of complex molecular scaffolds and exhibits good to excellent yields and selectivity while accepting a wide variety of functional groups.

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.

Metal-free C8-H functionalization of quinoline N-oxides with ynamides

The metal-free C8-H functionalization of quinoline N-oxides with ynamides is unveiled for the first time by the intramolecular Friedel-Crafts-type reaction of quinolyl enolonium intermediates generated from Brønsted acid-catalyzed addition of quinoline N-oxides to ynamides. Various quinoline N-oxides and terminal ynamides prove to be suitable substrates for this method. A one-pot protocol was then developed for the metal-free direct C8-H functionalization of quinolines.

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.

Regioselective C-H Functionalization of Heteroarene N-Oxides Enabled by a Traceless Nucleophile

Although N-alkenoxyheteroarenium salts have been widely used as umpoled synthons with nucleophilic (hetero)arenes, the use of electron-poor heteroarenes has remained unexplored. To overcome the inherent electron deficiency of quinolinium salts, a traceless nucleophile-triggered strategy was designed, wherein the quinolinium segment is converted into a dearomatized intermediate, thereby allowing simultaneous C8-functionalization of quinolines at room temperature. Experimental and computational studies support the traceless operation of a nucleophile, which enables the previously inaccessible transformation of N-alkenoxyheteroarenium salts. Remarkably, the generality of this strategy has been further demonstrated by broad applications in the regioselective C-H functionalization of other electron-deficient heteroarenes such as phenanthridine, isoquinoline, and pyridine N-oxides, offering a practical tool for the late-stage functionalization of complex biorelevant molecules.

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.

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.