A Modular Synthesis of 4-Aminoquinolines and [1,3] N-to-C Rearrangement to Quinolin-4-ylmethanesulfonamides

4-Aminoquinoline: a comprehensive review of synthetic strategies

4-Aminoquinoline is an important scaffold due to its variety of applications in medicinal, synthetic organic, and industrial chemistry. It has gained great relevance for the development of selective and potent leishmanicidal agents targeting parasite mitochondria, agonists and antagonists of Toll-like receptors (TLRs), antimalarials, and anticancer agents. As a consequence of the importance of 4-aminoquinoline as leishmanicidal, the present mini-review article aims to give comprehensive information about the different synthetic alternatives for the synthesis of 4-aminoquinolines, including (i) reactions based on nucleophilic aromatic substitution via conventional heating, microwave, and ultrasound; (ii) one-pot metal-free or metal-catalyzed reactions of inter- and intramolecular cyclization/annulation; (iii) miscellaneous reactions including the dehydrogenative amination of dihydroquinolin-4(1H)one and amination via Hartwig-Buchwald cross-coupling or rearrangement reactions.

Visible-Light-Induced Decarboxylative Annulation of α,β-Unsaturated Acids with Amines and α-Keto Acids for 2,4-Diarylquinoline Synthesis

An efficient and sustainable approach for the synthesis of 2,4-diarylquinolines has been developed via a visible-light-promoted metal-free three-component decarboxylative annulation pathway. This one-pot protocol combines readily available feed-stock α,β-unsaturated acids, aromatic amines, and α-keto acids in a cascade manner to access substituted quinolines under eco-benign conditions. Moreover, mechanistic insights suggest initial C-C cross coupling followed by decarboxylative 6π electrocyclic annulation to afford the desired products. The broad substrates scope and excellent functional group tolerance make this protocol more attractive and synthetically applicable toward the construction of complex N-heterocycles.

Intermediate Control: Unlocking Hitherto Unknown Reactivity and Selectivity in N-Conjugated Allenes and Alkynes

ConspectusControlling selectivity through manipulation of reaction intermediates remains one of the most enduring challenges in organic chemistry, providing novel solutions for selective C-C π-bond functionalization. This approach, guided by activation principles, provides an effective method for selective functional group installation, enabling direct synthesis of organic molecules that are inaccessible through conventional pathways. In particular, the selective functionalization of N-conjugated allenes and alkynes has emerged as a promising research focus, driven by advances in controlling reactive intermediates and activation strategies. In this regard, our group, alongside others, has established some new approaches that have emerged as a suitable platform for the synthesis of functionalized enamides. This Account reviews recent developments in the field, highlighting new modes of reactivity and selectivity, atom-economical functionalizations, and strategies for regio- and stereocontrol, while providing mechanistic insights into related transformations.Our study is systematically organized into two sections based on substrate type and chronological research progression. In the first section, we establish a platform by controlling allenamide-derived intermediates, enabling both allenamide-alkyne (AA) cross-coupling and a few novel electrophile-promoted hydrofunctionalization reactions. The unprecedented selectivity in Pd-catalyzed allenamide-alkyne cross-coupling is achieved through neighboring group chelation, with phosphine ligand selection controlling the reaction outcome. In parallel, the electrophile-promoted functionalizations─including haloalkynylation, hydrooxycarbonylation, hydrodifluoroalkylation, and intermolecular hydroamination─are achieved through strategic selection of electrophiles or their precursors.Additionally, our findings demonstrate how ynamides' reactivity toward both electrophiles and nucleophiles, controlled through activator modulation, expands the scope of accessible transformations. Key findings include: (1) chemoselective [2 + 2 + 2] annulation through efficient trapping of N-arylated nitrilium electrophiles by ynamides, (2) divergent C-H annulation of indole-derived vinylogous ynamides controlled by metal and ligand selection via intramolecular hydroarylation, (3) bromoalkynylation-enabled functional group migration through a novel 1,3-alkynyl shift.The final section explores how N-electron polarization in 1,3-enynes enables new chemoselectivity in metal-free inter- and intramolecular couplings with indole substrates. Our findings demonstrate that modulating N-electron conjugation within the enyne skeleton─through both linear and cross conjugation─can direct activation pathways and control product selectivity.This Account aims to stimulate broader research into the intermediate-controlled functionalization of activated π-systems. Future research directions include advanced activator design, novel functional group migration strategies, and deeper mechanistic studies to enable rational reaction development.

Halogen bonding accelerated aerobic dehydrogenative aromatization for 4-aminoquinoline preparation

This study presents a highly efficient method for 4-aminoquinoline derivative preparation under transition metal-free conditions. The process involves an aerobic oxidative dehydrative coupling of 2,3-dihydroquinolin-4(1H)-ones with various amines, including ammonia, resulting in high yields of the desired products. The method is also applicable to substituted 4-aminoquinoline derivative construction through a cyclization/dehydrative coupling cascade process starting from 2'-amino chalcones. Mechanistic studies reveal that iodine (I2) is consumed to produce 3-iodoquinolin-4-ol, which acts as a true catalyst with high catalytic efficacy (as low as 0.5 mol%). The presence of halogen bonding is critical in the inter-molecular transfer hydrogenation process to generate inactive quinolin-4-ol. Subsequently, using air/oxygen as the terminal oxidant, the iodine anion was oxidized to I2 to regenerate the 3-iodoquinolin-4-ol from quinolin-4-ol in the catalytic cycle. Key benefits of this methodology include its simplicity, transition metal-free conditions, environmentally-benign oxidant, and high atom economy, making it a valuable approach for synthesizing medicinally significant 4-aminoquinoline derivatives.

Aerobic Dehydrogenative Aromatization in the Preparation of 4-Aminoquinoline Derivatives by Synergistic Pd/Cu Catalysis

The 4-aminoquinoline moiety is widely present in various bioactive compounds and marketed drugs, while the preparation of this target structure relies heavily on the amination of 4-chloroquinolines. Herein, an atom and step economic procedure was developed based on an aerobic dehydrogenative aromatization strategy. Unlike the well-known palladium-catalyzed dehydrogenative aromatization of cyclohexanones with amines, synergistic Pd/Cu catalysis is crucial for 2,3-dihydroquinolin-4(1H)-one type of substrates. Under the optimized conditions, a range of aromatic/aliphatic amines and 2,3-dihydroquinolin-4(1H)-ones were coupled to give the corresponding 4-aminoquinoline products in moderate to high yields, and the application of the current methodology for the preparation and late-stage diversification of marketed drugs was also demonstrated.

Diaryliodonium(III) Salts in One-Pot Double Functionalization of C–IIII and ortho C–H Bonds

Since the 1950s, diaryliodonium(III) salts have been demonstrated to participate in various arylation reactions, forming aryl–heteroatom and aryl–carbon bonds. Incorporating the arylation step into sequential transformations would provide access to...

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.

Triflic acid-mediated N-heteroannulation of β-anilino-β-(methylthio)acrylonitriles: a facile synthesis of 4-amino-2-(methylthio)quinolines

Various functionalised 4-amino-2-(methylthio)quinolines are synthesised through triflic acid-mediated N-heteroannulation of α-functionalized-β-anilino-β-(methylthio)acrylonitriles for the first time. The N-heteroannulation process is highly chemoselective and has mild reaction conditions. However, this process fails in the absence of the β-methylthio group in the acrylonitriles. In addition, a new double N-heteroannulation process is demonstrated to synthesise indolo[3,2-c]quinolines from non-heterocyclic precursors. Natural product isocryptolepine is synthesised in four steps from an acyclic precursor.

Media-Driven Pd-Catalyzed Reaction Cascades with 1,3-Diynamides Leading Selectively to Either Indoles or Quinolines

Divergent Pd-catalyzed reaction cascades with various 1,3-diynamides yielding either 2-amino-3-alkynylindoles or 2-amino-4-alkenylquinolines were established. Omitting or adding TBAF (tetrabutylammonium fluoride) to the reaction of N,N-(2-iodophenyl)(4-toluenesulfonyl)-1,3-diynamides with secondary or primary amines in the presence of KOH in THF and catalytic amounts of Pd(PPh₃ )₄ completely changed the outcome of the reaction. In the absence of TBAF, 2-amino-3-alkynylindoles were the sole products, while the presence of TBAF switched the product formation to 2-amino-4-alkenylquinolines. Deuterium labeling proceeded selectively at the C3 and C11 positions of the 2-amino-4-alkenylquinoline products and this suggests the unprecedented formation of [4]cumulenimines from 1,3-diynamides as reactive key intermediates.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Preparation of isoquinazolines via metal-free [4 + 2] cycloaddition of ynamides with nitriles

TfOH-mediated [4 + 2] cycloaddition of ynamides with nitriles to construct 1,2-dihydroquinazolines is realized by a direct reaction in moderate to excellent yields (up to 93%) in a stereospecific manner. A rapid and efficient strategy has been employed for the syntheses of alkyl-substituted 1,2-dihydroquinazoline derivatives, and it exhibits good functional group tolerance, has a short reaction time, shows excellent diastereoselectivity, and is a simple and high-yielding reaction.

Synthesis of poly-functionalized benzofurans via one-pot domino oxidation/[3+2] cyclization reactions of a hydroquinone ester and ynamides

Densely substituted amino-functionalized benzofurans were concisely accessed via the first one-pot domino oxidation/[3+2] cyclization of a hydroquinone ester and easily accessible ynamides under mild conditions in a short time. The complex benzofurans were able to be efficiently synthesized all from simple and inexpensive starting materials in two steps.

Efficient method for the construction of poly-functionalized benzofurans has been developed via one-pot domino oxidation/[3+2] cyclization reactions.

[Cp*RhIII ]/Ionic Liquid as a Highly Efficient and Recyclable Catalytic Medium for C-H Amidation

A [Cp*Rh^III ]-catalyzed direct C-H amidation is carried out in ionic liquid. Both C(sp² )-H bonds of (hetero)arenes and alkenes and unactivated C(sp³ )-H bonds can be easily amidated with high functional-group tolerance and excellent yields under these conditions. Notably, using [Cp*Rh^III ]/[BMIM]BF₄ (BMIM=1-butyl-3-methylimidazolium) as the green and recyclable medium is environmentally benign, in light of characteristics such as the reusability of the expensive rhodium catalyst, avoidance of highly toxic organic solvents, and mild reaction conditions, as well as a short reaction time.

DFT calculations on the mechanism of copper-catalysed tandem arylation-cyclisation reactions of alkynes and diaryliodonium salts

We present a computational mechanistic study on the copper(III)-catalysed carboarylation-ring closure reactions leading to the formation of functionalised heterocycles. We have performed DFT calculations along selected routes and compared their free energy profiles. The calculations considered two viable options for the underlying mechanism which differ in the order of the oxazoline ring formation and the aryl transfer steps. In our model transformation, it was found that the reaction generally features the aryl transfer-ring closing sequence and this sequence shows very limited sensitivity to the variation of the substituent of the reactants. On the basis of the mechanism the origin of the stereoselectivity is ascribed to the interaction of the Cu ion with the oxazoline oxygen driving the ring-closure step selectively.

Modular Three-Component Synthesis of 4-Aminoquinolines via an Imidoylative Sonogashira/Cyclization Cascade

We developed a one-pot, two-stage synthetic route to substituted 4-aminoquinolines involving an imidoylative Sonogashira coupling followed by acid-mediated cyclization. This three-component reaction affords pharmaceutically valuable 4-aminoquinolines in a one-pot procedure from readily available starting materials. The reaction tolerates various substituents on the arene as well as the use of secondary and even primary isocyanides. Additionally, the wide tolerance for functionalized isocyanides allows for the one-pot synthesis of various substituted chloroquine analogues as well as other medicinally relevant products.

Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water: access to oxygen-bridged tetrahydro-1,4-oxazepines

The unique property of the Brønsted acid, as well as introducing a small amount of water, shows distinct chemoselectivity from the corresponding gold-catalyzed cycloadditions.