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

Ru(II)-Catalyzed Sustainable C-H Methylation of Indolines with Organoboranes in Ethanol

A sustainable protocol for Ru(II)-catalyzed regioselective C(sp2)-H methylation of indolines in the presence of ethanol has been explored. A wide array of substituted indolines were successfully methylated via the developed protocol with good to excellent yields. Deuterium labeling experiments suggested the reversible nature of the C-H activation step. Kinetic isotope effect studies revealed that C-H activation might be the rate-determining step. Gram scale reaction and post-transformation reactions of the methylated product demonstrated the potential of the developed protocol.

Selective Construction of C-S/S-N Bonds from N-Substituted O-Thiocarbamates and Indoles under Transition-Metal-Free Conditions

A method for the selective construction of S-N/C(sp2)-S bonds using N-substituted O-thiocarbamates and indoles as substrates is reported. This protocol features good atom utilization, mild conditions, short reaction time, and wide substrate scope, which can provide a convenient path for the functionalization of indoles. In addition, the reaction could be scaled up on gram scale, showing potential application value in industry synthesis.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Dearomatization of Pyridines: Photochemical Skeletal Enlargement for the Synthesis of 1,2-Diazepines

In this report, we developed a unified and standardized one-pot sequence that converts pyridine derivatives into 1,2-diazepines by inserting a nitrogen atom. This skeletal transformation capitalizes on the in situ generation of 1-aminopyridinium ylides, which rearrange under UV light irradiation. A thorough evaluation of the key parameters (wavelength, reaction conditions, activating agent) allowed us to elaborate on a simple, mild, and user-friendly protocol. The model reaction was extrapolated to more than 40 examples, including drug derivatives, affording unique 7-membered structures. Mechanistic evidence supports the transient presence of a diazanorcaradiene species. Finally, pertinent transformations of the products, including ring contraction reactions to form pyrazoles, were conducted and paved the way to a broad application of the developed protocol.

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 of Dihydroquinolinone Derivatives via the Cascade Reaction of o-Silylaryl Triflates with Pyrazolidinones

Presented herein is a novel synthesis of dihydroquinolinone derivatives through an unprecedented cascade reaction of o-silylaryl triflates with pyrazolidinones. Mechanistically, the formation of the title products is believed to involve a cascade procedure including in situ formation of aryne and its addition with pyrazolidinone followed by N-N bond cleavage and intramolecular C-C bond formation/annulation. Compared with literature methods for the synthesis of dihydroquinolinones, this protocol has advantages such as multistep transformations accomplished in one pot, broad substrate scope, mild reaction conditions, and good tolerance of diverse functional groups. In addition, the products thus obtained demonstrated significant in vitro antiproliferative activity in selected human cancer cell lines.

Facile syntheses of tetrahydroquinolines and 1,2-dihydroquinolines via vinylogous cascade hydride transfer/cyclization

The medicinally significant 3-monosubstituted tetrahydroquinolines and 1,2-dihydroquinolines were controllably constructed via redox-neutral vinylogous cascade condensation/[1,5]-hydride transfer/cyclization in EtOH.

DFT mechanistic studies on the regio-, stereo-, and enantio-selectivity of 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes for the synthesis of fused N,N'-heterocycles

This work investigated computationally the regio-, stereo-, and enantio-selectivity of the reactions of azomethine imines with olefins, maleimides, and benzynes, important reactions towards the synthesis of heteropolycyclic, N,N'-fused, spirocyclic systems, which serve as building blocks for the synthesis of many pharmaceuticals, agrochemicals, and biologically active compounds. The results show that the thermally controlled diastereoselective [3 + 2] cycloaddition reaction between quinolinium imide and methyl acrylate provides two regio-isomers: 1,4-regioisomer (N-C1, C-C2) and 1,3-regioisomer (N-C2, C-C1). The 1,4-regioisomer has cis and trans-stereoisomers while the 1,3-regioisomer has R-enantiomer and S-enantiomer, and the barriers for the formation of these isomers are 5.1, 19.1, 10.7, and 10.5 kcal/mol, respectively. The reaction between quinolinium imide and maleimide leads to the formation of two stereoisomers; cis-isomer and trans-isomer in which the cis-isomer is kinetically and thermodynamically favoured by 6.1 kcal/mol and 8.7 kcal/mol, respectively. The reaction between quinolinium imide and benzyne also leads to the formation of two stereoisomers through one transition state, with a barrier of 3.0 kcal/mol. Global electrophilicity index calculations show that the dipole acts as a good electrophile in the reaction and decreasing or increasing electrophilicity index has no correlation with the activation barriers. Graphical abstractPathways of the 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes for the synthesis of fused N,N'-heterocycles.

Diverse reactivity of isatin-based N,N′-cyclic azomethine imine dipoles with arynes: synthesis of 1′-methyl-2′-oxospiro [indene-1,3′-indolines] and 3-aryl-3-pyrazol-2-oxindoles

Reaction of aryne with isatin based N,N′-cyclic AMI 1,3-dipole afforded 3,3-disubstituted oxindole while methyl substitution on the pyrrolidine ring of 1,3-dipole directed [3+2] cycloaddition products.

Palladium-catalyzed [4 + 3] dearomatizing cycloaddition reaction of N-iminoquinolinium ylides

An efficient approach to obtain saturated seven-membered ring containing three heteroatoms has been developed by using a palladium-catalyzed [4 + 3] dearomatizing cycloaddition. This approach features mild conditions and good functional group tolerance.