Gold-Catalyzed Cascade Reaction of β-(2-Aminophenyl)-α,β-ynones with Ynamides: A Sequential Route to Polysubstituted 2-Aminoquinolines

Gold catalysis in quinoline synthesis

Quinolines are biologically and pharmaceutically important N-heterocyclic aromatic compounds, which have broad applications in medicinal chemistry. Thus, their efficient synthesis has attracted extensive attention, and a broad range of synthetic strategies have been established. Of note, gold-catalyzed methodologies for the synthesis of quinolines have greatly advanced this field. Various gold-catalyzed intermolecular annulation reactions, such as annulations of aniline derivatives with carbonyl compounds or alkynes, annulations of anthranils with alkynes, and annulations based on A3-coupling reactions, as well as intramolecular cyclization reactions of azide-tethered alkynes, 1,2-diphenylethynes, and 2-ethynyl N-aryl indoles, have been developed. This review provides an overview of this exciting research area. Typical achievements in reaction methodologies and plausible reaction mechanisms are summarized.

TMSOTf-Catalyzed [4 + 2] Annulation of Ynamides and β-(2-Aminophenyl)-α,β-ynones for the Synthesis 2-Aminoquinolines

A metal-free TMSOTf-catalyzed [4 + 2] annulation of ynamides with β-(2-aminophenyl)-α,β-ynones enables the regiospecific and facile assembly of 2-aminoquinoline frameworks. The catalyst TMSOTf presented a remarkable advancement compared to previously reported transition-metal catalysts. A wide range of 3-aryl/alkyl-substituted 2-aminoquinolines were generated in moderate to excellent yields due to the mild conditions.

Copper/Zinc-Catalyzed Stitching of 2-Carbonylanilines with Bis(ynamides): Access to Pyrrolo[2,3-b]quinolines and Its Photophysical Studies

Herein, a one-pot desulfonylative protocol enabled by copper(II)/zinc(II) salts to access pyrrolo[2,3-b]quinolines in good to excellent yields from 2-carbonylanilines and ynamide-derived buta-1,3-diynes has been reported. Significantly, various 2-carbonylanilines carrying reactive functional groups are well tolerated. Moreover, a gram-scale synthesis and synthetic application highlight the practical utility of the current protocol. Notably, the fluorescence properties of pyrrolo[2,3-b]quinolines have been recorded, and their potential use as a fluorescent probe in the imaging of live cells has been demonstrated.

Rhodium(II)-catalyzed synthesis of 2-aminoquinoline derivatives from 2-quinolones and N-sulfonyl-1,2,3-triazoles

Herein, we disclose a rhodium(II) catalyzed efficient and convenient method for the synthesis of 2-aminoquinoline derivatives from 2-quinolones and N-sulfonyl-1,2,3-triazoles. The reaction provides rapid access to a series of 2-aminoquinolines with moderate to excellent yields. The reaction proceeds via quinolone-hydroxyquinoline tautomerization/O-H insertion to a rhodium(II)-aza vinyl carbene intermediate generated by denitrogenation of triazole followed by rearrangement to deliver the desired product. Furthermore, we demonstrated the iodine-mediated dealkylation of a 2-aminoquinoline derivative.

Leveraging Zn(II) Catalyst: Synthesis of Amidoquinolines via (3 + 3) Heteroannulation of Aromatic Amines and Ynamides

Herein, we present a Zn(II)-catalyzed (3 + 3) heteroannulation reaction between aromatic amines and 1,3-diynamides for the synthesis of amidoquinolines. A large number of aromatic amines are well tolerated, furnishing quinoline derivatives in up to excellent yield. Notably, various reactive functional groups have survived under the optimal reaction conditions, highlighting the mildness of the developed protocol. In addition, amines derived from bioactive molecules show modest reactivity.

Advancements in Gold-Catalyzed Cascade Reactions to Access Carbocycles and Heterocycles: An Overview

This review summarizes recent developments (from 2006 to 2022) in numerous important and efficient carbo- and heterocycle generations using gold-catalyzed cascade protocols. Herein, methodologies involve selectivity, cost-effectiveness, and ease of product formation being controlled by the ligand as well as the counter anion, catalyst, substrate, and reaction conditions. Gold-catalyzed cascade reactions covered different strategies through the compilation of various approaches such as cyclization, hydroarylation, intermolecular and intramolecular cascade reactions, etc. This entitled reaction is also useful for the synthesis of spiro, fused, bridged carbo- and heterocycles.

1,2-Difunctionalizations of alkynes entailing concomitant C–C and C–N bond-forming carboamination reactions

Vicinal carboamination of alkynes is a highly reliable and efficient practical strategy for the quick preparation of valuable and diverse amine derivatives starting from simple synthons. The last decade has witnessed numerous practical methods employing transition-metal-based/metal-free carboamination approaches using alkynes for the synthesis of these N-bearing entities. Driven by the renaissance of transition metal catalysis, intermolecular and intramolecular carboamination of alkynes comprising concomitant C–N and C–C bond formation has been studied extensively. In contrast to metal catalysis, though analogous metal-free approaches have been relatively less explored in the literature, they serve as alternatives to these expensive approaches. Despite this significant progress, reviews documenting such examples are sporadic; as a result, most reports of this type remained scattered throughout the literature, thereby hampering further developments in this escalating field. In this review, different conceptual approaches will be discussed and examples from the literature will be presented. Further, the reader will get insight into the mechanisms of different transformations.

The 1,2-difunctionalization of alkynes happening through concomitant C–C and C–N bond formation strategies have provide an unified access to diversely functionalized N-bearing heterocycles.

Gold-Catalyzed Reactions Towards Diversity: From Simple Substrates to Functionalized Carbo- and Heterocycles

The field of gold catalysis has been in constant expansion during the last twenty years. Based on the precept of π-activation of unsaturated simple substrates, several new rearrangements have been discovered, implying aryl, alkyne, alkene or keto derivatives as key partners. In this personal account, the main contributions in the field of gold catalysis from our group will be highlighted, emphasizing the recent reports, starting from 1,6- and 1,5-enynes and then moving to keto-ynes derivatives. The gold-catalyzed reactions will be presented starting from classical skeletal rearrangements (cycloisomerization) and then domino processes. In each part, the presentation of asymmetric versions will be highlighted.

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.

Gold-Catalyzed Transformation of Ynamides

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N-bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom-transfer reagents, like nitrones, sulfoxides, and pyridine N-oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed.

Indium-Catalyzed Cycloisomerization of 1,6-Cyclohexenylalkynes

Efficient four- and five-step routes to access functionalized bicyclo[3.2.1]oct-2-ene and bicyclo[3.3.1]nonadiene via indium-mediated cycloisomerization of 1,6-enynes has been developed. This atom-economical catalytic process was optimized and relied on the efficiency of InCl3 leading to the preparation of functionalized bicyclic adducts in up to 99% isolated yield. The cyclization occurred on two different processes (5-exo versus 6-endo pathway) and were influenced by the substitution of the alkynyl moiety. The exo process was favored for non-substituted alkynes whereas the endo pathway was generally observed for substituted alkynes. Then, the presence of electron-withdrawing groups on the aryl substituted alkyne increased the ratio of the exo isomer. DFT calculations were performed on stability of intermediates and corroborated the intervention of InCl3.

Zn(OTf)2-catalyzed hydroamination of ynamides with aromatic amines

The Zn(OTf)₂-catalyzed hydroamination of ynamides 2a-2l with aromatic amines 1a-1r was developed. This protocol features broad substrate scope of aromatic amines, good functional group tolerance for ynamides, and excellent regioselectivities. As a result, a variety of substituted amidine compounds 3aa-3oa, 3ab-3al and 3pa-3rk were prepared in moderate to excellent yields and with high regioselectivities.

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.

Nickel-Catalyzed β-Regioselective Amination/Cyclization of Ynamide-Nitriles with Amines: Synthesis of Functionalized 3-Aminoindoles and 4-Aminoisoquinolines

A highly regioselective nickel/Lewis acid catalyzed amination/cyclization of ynamide-nitriles with amines involving β-addition has been developed. The reaction offers an attractive and efficient route for the synthesis of 3-aminoindoles and 4-aminoisoquinoline derivatives. The Ts-group on the ynamide acts as a directing group to produce the alkenyl nickel species with high regioselectivity.

Sequential condensation/biannulation reactions of β-(2-aminophenyl)-α,β-ynones with 1,3-dicarbonyls

A divergent domino condensation/biannulation reaction of β-(2-aminophenyl) α,β-ynones with 1,3-dicarbonyls to construct a polycyclic 4H-pyrano[3,4-c]quinoline core has been developed. The p-TsOH·H2O catalyzed reaction of β-(2-aminophenyl) α,β-ynones with β-ketoesters in ethanol proceeds with good to excellent yields to provide a simple and effective method for the synthesis of functionalized 4H-pyrano[3,4-c]quinolinones. Further elaboration of these latter derivatives with an excess of 20% NH4OH in EtOH at 50 °C helps achieve the synthesis of the perlodinine analogues benzo[c][2,7]naphthyridin-4(3H)-one derivatives in high yields. Moreover, the p-TsOH·H2O mediated reaction of β-(2-aminophenyl) α,β-ynones with β-di-ketones leads to the formation of a variety of structurally diverse 4H-pyrano[3,4-c]quinoline polycyclic ketals by the incorporation of an alcohol solvent molecule in a cascade fashion.

Experimental and computational evidence on gold-catalyzed regioselective hydration of phthalimido-protected propargylamines: an entry to β-amino ketones

The results of our investigations on the Au-catalyzed regioselective hydration reaction of both alkyl- and aryl-substituted N-propargyl phthalimides directed to the selective formation of the corresponding β-phthalimido ketones are described. Experimental data, in particular the observed regioselectivity, have been qualitatively supported by quantum-chemical calculations carried out on model systems in the framework of Density Functional Theory (DFT) followed by quantum theory of atoms in molecules (QTAIMS). Our results suggest that the electronic features of the initial adduct between the propargyl triple bond and the Au(i) catalyst, in particular the character of the gold-triple bond interaction, are essential for the observed regioselectivity. Other effects, such as the presence of the solvent and the formation of a H-bond between the water molecule and the phthalimido moiety, although apparently irrelevant for the regioselectivity, have proven to be kinetically and catalytically rather important.

Gold-Catalyzed Cycloisomerization of 1,6-Cyclohexenylalkyne: An Efficient Entry to Bicyclo[3.2.1]oct-2-ene and Bicyclo[3.3.1]nonadiene

An efficient and mild synthetic route for the preparation of functionalized bicyclo[3.2.1]oct-2-ene and bicyclo[3.3.1]nonadiene via gold-mediated cycloisomerization of 1,6-enynes has been developed. This atom-economical catalytic process was optimized and relied on the efficiency of IPrAuNTf₂ allowing the formation of functionalized bicyclic adducts in 55-91% isolated yields (18 products). The reliable access to bicyclic derivatives was demonstrated on a 3 g scale with a low catalyst loading. The process occurred on a 5-exo versus 6-endo pathway depending on the substitution of the alkynyl moiety. Density functional theory (DFT) calculations were performed on the stability of intermediates, and this study corroborated the endo/exo ratio and the mechanistic pathway with key intermediates. Reduction of the ester moiety and hydrogenation of the exo-methylene double bond of the bicyclo[3.2.1]oct-2-ene adduct illustrated the potential postfunctionalization of bicyclic derivatives.

Facile one-pot synthesis of 2-aminoindoles from simple anilines and ynamides

A highly effective and facile one-pot reaction has been developed for the synthesis of 2-aminoindoles directly from anilines and ynamides.

Experimental results and computational insight into sequential reactions of β-(2-aminophenyl)-α,β-ynones with aryl isocyanates/benzoyl isothiocyanate

The selective formation of quinazoline vs. benzoxazine  and benzothiazine derivatives from β-(2-aminophenyl)-α,β-ynones and aryl isocyanates/benzoyl isothiocyanate was explored. DFT calculations provide a plausible rationale.

Copper-Catalyzed Amino-oxymethylation of Ynamides with N,O-Acetals

A copper-catalyzed 1,2-difunctionalization of ynamides has been developed by using readily available N,O-acetals as the bifunctional reagents. Importantly, contrary to the initially expected 1,2-oxy-aminomethylation reaction, an unprecedented 1,2-amino-oxymethylation has been observed. This reaction is atom-economical and tolerates a broad substrate scope, affording the difunctionalization products in good yield with unique Z configuration.

Juniperanol: First total synthesis and evaluation in Type 2 Diabetes disease

The first total synthesis of juniperanol, the tricyclic sesquiterpenoid enantiomer of α-cedrol is described. The synthesis relies on stereoselective gold-catalyzed Ohloff-type propargylic ester rearrangement performed on a 10 g scale, and a carbocationic cascade in the presence of acetyl methanesulfonate. The ability of juniperanol to interfere in glucose processes in different cell types is described.

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.

Preparation and Utility of N-Alkynyl Azoles in Synthesis

Heteroatom-substituted alkynes have attracted a significant amount of interest in the synthetic community due to the polarized nature of these alkynes and their utility in a wide range of reactions. One specific class of heteroatom-substituted alkynes combines this utility with the presence of an azole moiety. These N-alkynyl azoles have been known for nearly 50 years, but recently there has been a tremendous increase in the number of reports detailing the synthesis and utility of this class of compound. While much of the chemistry of N-alkynyl azoles mirrors that of the more extensively studied N-alkynyl amides (ynamides), there are notable exceptions. In addition, as azoles are extremely common in natural products and pharmaceuticals, these N-alkynyl azoles have high potential for accessing biologically important compounds. In this review, the literature reports of N-alkynyl azole synthesis, reactions, and uses have been assembled. Collectively, these reports demonstrate the growth in this area and the promise of exploiting N-alkynyl azoles in synthesis.

Preparation of 2-arylquinolines from β-arylpropionitriles with aryllithiums and NIS through iminyl radical-mediated cyclization

Treatment of β-arylpropionitriles with aryllithiums, followed by the reaction with water and then with NIS under irradiation with a tungsten lamp gave 2-arylquinolines in good to moderate yields.

Palladium-Catalyzed Cyclization of N-Acyl- o-alkynylanilines with Isocyanides Involving a 1,3-Acyl Migration: Rapid Access to Functionalized 2-Aminoquinolines

A simple and expedient approach for the synthesis of functionalized 2-aminoquinolines via palladium-catalyzed annulation of N-acyl- o-alkynylanilines with isocyanides has been developed with high atom economy, in which an unconventional 6- endo-dig cyclization process is observed. Further investigations of the mechanism demonstrated that an intramolecular acyl migration of the N-protecting groups was involved in this transformation.