Iron-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Tertiary Anilines and Enamides for the Synthesis of Tetrahydroquinolines with Amido-Substituted Quaternary Carbon Centers

Palladium-Catalyzed Annulation of Tertiary Anilines with 3-Butenoic Acid via Dual C-H Bond Activation

Catalytic cyclization via dual C-H bond activation has evolved as a powerful strategy for building bi- and polycyclic molecules. Herein, a palladium-catalyzed annulation of tertiary anilines with 3-butenoic acid via N-α-C(sp3)-H and ortho-C(sp2)-H activation is described. The remarkable characteristics of this reaction include excellent diastereoselectivity, broad substrate scope, and good tolerance for some highly sensitive groups. In addition, the KIE experiment suggested that the C-H bond abscission is not the turnover-limiting step.

Electrochemical Oxidative (4 + 2) Cyclization of Anilines and o-Phenylenediamines for the Synthesis of Phenazines

Phenazines, crucial constituents of nitrogen-containing heterocycles, widely exist in functional compounds. Herein, we report an anodic oxidative (4 + 2) cyclization between anilines and o-phenylenediamines for the uniform construction of phenazines in a simple undivided cell. Dual C-H amination followed by oxidation represents an outstanding step and atom efficiency. A sequence of phenazines is produced with excellent functional group tolerance at room temperature.

Synthesis of 2,2-difunctionalized 2H-azirines via I2-mediated annulation of enamines

Various 2,2-difunctionalized 2H-azirines were synthesized via I2-mediated annulation reactions of readily accessible enamines in the presence of nitrogen or non-nitrogen nucleophiles. The features of the present synthesis process also include no use of transition metals, simple operation, mild reaction conditions, a broad substrate scope, and gram-scale synthesis.

Electrochemical trifluoroalkylation/annulation for the synthesis of CF3-functionalized tetrahydroquinolines and dihydroquinolinones

Tuning the electronic structure of protecting groups on the nitrogen atom of substrates has emerged as an effective strategy to achieve the tandem trifluoromethylation/C(sp2)-H annulation using Langlois' reagent as the CF3 source for the electrochemical synthesis of functionalized tetrahydroquinolines and dihydroquinolinones.

Gold-Catalyzed Intramolecular Hydroarylation and Transfer Hydrogenation of N-Aryl Propargylamines to Construct Tetrahydroquinolines and 5,6-Dihydro-4H-pyrrolo[3,2,1-ij]quinolines

A novel protocol of gold-catalyzed N-aryl propargylamines to construct tetrahydroquinolines and 5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinolines in moderate to good yields has been developed through the tandem reactions of intramolecular hydroarylation and transfer hydrogenation. The strategy has the advantages of easy access to raw materials, simple reaction conditions, good substrate compatibility, high efficiency, and excellent regioselectivity.

K2S2O8-Induced [4+2] Annulation of Tertiary Anilines and Alkenes toward Tetrahydroquinolines

Due to the unique physicochemical properties of heterocyclic compounds, their construction is one of the central issues in synthetic chemistry. Here, we report a K₂S₂O₈-induced protocol for constructing tetrahydroquinolines from bulk chemicals (alkenes and anilines). The merit of this method has been demonstrated by its operational simplicity, wide scope, mild conditions, and transition-metal-free system.

Borane-catalyzed cascade Friedel–Crafts alkylation/[1,5]-hydride transfer/Mannich cyclization to afford tetrahydroquinolines

We report a redox-neutral annulation reaction of tertiary amines with electron-deficient alkynes under metal-free and oxidant-free conditions.

Dehydrogenative Aza-[4 + 2] Cycloaddition of Amines with 1,3-Dienes via Dual Catalysis

We describe a synergistic utilization of copper catalysis and proton-transfer catalysis that enables an atom- and step-economical aza-[4 + 2] cycloaddition reaction of readily available N-arylamino carbonyl compounds with simple 1,3-dienes. The reaction proceeds smoothly under an air atmosphere and produces water as the sole side product. Whereas the amines can directly serve as the C- and N-atom donors, this operationally simple protocol provides green, rapid, and efficient access to 1,2,3,6-tetrahydropyridines with a broad scope.

Environmentally Benign Synthesis of Quinoline-Spiroquinazolinones by Iron-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Secondary/Tertiary Anilines and 4-Methylene-quinazolinones

We report an efficient iron-catalyzed cross-dehydrogenative coupling [4 + 2] annulation of secondary/tertiary anilines with quinazolinones to generate quinoline-spiroquinzolinones. The reaction proceeds smoothly with a relatively broad variety of functional groups, a cheap transition metal catalyst (FeCl₃), and environmentally friendly oxidant (H₂O₂/O₂) under mild reaction conditions. Creatively, N-methylanilines are employed for the first time for the cycloaddition as both methyl and methylene sources attached to the N atom of tetrahydroquinolines.

Inverse Electron-Demand Aza-Diels-Alder Reaction of Cyclic Enamides with 1,2-Diaza-1,3-dienes in Situ Generated from α-Halogeno Hydrazones: Access to Fused Polycyclic Tetrahydropyridazine Derivatives

An efficient inverse electron-demand aza-Diels-Alder reaction of cyclic enamides and 1,2-diaza-1,3-dienes, which could be readily formed in situ from α-halogeno hydrazones and a base, has been successfully developed. With the developed approach, a wide range of fused polycyclic tetrahydropyridazines were smoothly obtained in up to 99% yield under benign reaction conditions. This reaction concept was also extended to acyclic enamide substrates for accessing 1,4,5,6-tetrahydropyridazines. A gram-scale experiment and further derivatizations of the polycyclic tetrahydropyridazine products were also conducted to verify the practicability of the methodology.

Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

An efficient rhodium(iii)-catalyzed C-H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

Selective Dehydrogenative Acylation of Enamides with Aldehydes Leading to Valuable β-Ketoenamides

We have presented a unique example of dehydrogenative acylation of enamides with aldehydes enabled by an earth-abundant iron catalyst. The protocol provides the straightforward access to valuable β-ketoenamides with ample substrate scope and excellent functional group tolerance. Notably, distinct C-H acylation of enamide rather than at N-H moiety site occurs with absolute Z-selectivity was observed. Late-stage modifications of complex molecules and versatile synthetic utility of β-ketoenamides further highlight the practicability of this transformation.

One-electron oxidative dehydrogenative annulation and cyclization reactions

This review focuses on the recent advances in one-electron oxidation involved oxidative dehydrogenative annulations and cyclizations for the intermolecular and intramolecular construction of valuable ring structures.

NHC-Catalyzed Chemoselective Reactions of Enals and Aminobenzaldehydes for Access to Chiral Dihydroquinolines

An N-heterocyclic carbene (NHC)-catalyzed reaction between α-bromoenals and 2-aminoaldehydes has been developed. Key steps include chemoselective reaction of the NHC catalyst with one of the aldehyde substrates (the bromoenal) to eventually generate an α,β-unsaturated acylazolium intermediate. Addition of the nitrogen atom of aminoaldehyde to the unsaturated azolium ester intermediate followed by intramolecular aldol reaction, β-lactone formation, and decarboxylation leads to chiral dihydroquinolines with high optical purity. The dihydroquinoline products, which are quickly prepared by using this method, can be readily transformed into a diverse set of functional molecules such as pyridines and chiral piperidines.

Redox-Selective Iron Catalysis for α-Amino C-H Bond Functionalization via Aerobic Oxidation

Single-electron oxidation and α-deprotonation of tertiary anilines using Fe(phen)₃(PF₆)₃ afford α-aminoalkyl radicals, which can be coupled with electrophilic partners to afford various tetrahydroquinolines. Mechanistically, the Fe(phen)_n^2+/3+ catalytic cycle is maintained by O₂ or a TBHP oxidant, and the presence of the oxygen bound iron complex, Fe(III)-OO(H), was elucidated by electron paramagnetic resonance and electrospray ionization mass spectrometry. This redox-selective nonheme iron catalyst behaves similarly to bioinspired heme iron catalysts.

Synthesis of Functionalized Imidazolium Salts via Iodine-Mediated Annulations of Enamines

A novel annulation reaction of two enamine molecules with iodine under basic conditions to form 4-functionalized imidazolium salts has been established. In this reaction, iodine acts as both an iodinating reagent and a Lewis acid catalyst. Features of this synthetic method include facilitative preparation of substrates, no use of transition metals, mild reaction conditions, simplicity of operation, and gram scale synthesis.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Synthesis of D-Ring Annulated Pyridosteroids from β-Formyl Enamides and Their Biological Evaluations

Herein, we report the synthesis of a novel class of substituted androst[17,16- b]pyridines (pyridosteroids) from the reaction of β-formyl enamides with alkynes in high yields. The optimized reaction protocol was extended to acyclic and cyclic β-formyl enamides to afford nonsteroidal pyridines. Cell survival assay of all compounds were carried against prostate cancer PC-3 cells wherein 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine showed the highest cytotoxic activity. Phase contrast microscopy and flow cytometry studies exhibited marked morphological features characteristic of apoptosis in 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine and abiraterone treated PC-3 cells. The treatment of 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine induces G₂/M phase cell cycle arrest in prostate cancer PC-3 cells. Enhancement of apoptotic inductions of PC-3 cells by 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine and abiraterone through the activation of caspases-6, -7, and -8 pathways were supported by qRT-PCR. In silico study of the compound 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine showed stable and promising interaction with the key caspase proteins. Our studies revealed that the pyridosteroid 3-hydroxy-5-en-2',3'-dicarbethoxy-androst[17,16- b]pyridine, bearing pyridine-2,3-dicarbethoxy pharmacophore, facilitated initiation of caspase-8 and activates downstream effectors caspase-6 and caspase-7 and thereby triggering apoptosis of PC-3 cancer cells.

Manganese(iii)-mediated selective phosphorylation of enamides: direct synthesis of β-phosphoryl enamides

Mn(iii)-Mediated radical oxidative selective phosphorylation ofN-styrylamides to afford β-phosphoryl enamides, which could be easily transformed to β-amino phosphonic acids, is described.

Construction of N-Heterocycles through Cyclization of Tertiary Amines

N-Heterocycles have been found in a large number of natural products, drug molecules, and bioactive compounds, and they thereby play a vital role in diverse research disciplines including drug discovery, organic synthesis, chemical biology, and material science. To this end, the development of new methods and strategies for the construction of N-heterocyclic frameworks is arguably one of the most dynamic and significant research areas in organic synthesis. One of these powerful approaches to the synthesis of N-heterocycles is to establish cyclization reactions based on the transformation of tertiary amines, which has emerged as an attractive research topic. In this Minireview, the significant achievements in the construction of N-heterocycles through cyclization of tertiary amines are highlighted and a comprehensive overview of the rational design, development, and application of these synthetic methods is presented.

Organocatalytic Aza-Friedel-Crafts/Lactonization Domino Reaction of Naphthols and Phenols with 2-Acetamidoacrylate to Naphtho- and Benzofuranones Bearing a Quaternary Center at the C3 Position

N-Acetyl ketimine generated from methyl 2-acetamidoacrylate was explored to develop an unprecedented domino aza-Friedel-Crafts/lactonization reaction with naphthols and phenols (including 5-hydroxyindoles). This novel method requires a catalyst loading of only 5 mol % of a phosphoric acid catalyst and provides a new series of 3-NHAc-naphtho- and benzofuranone derivatives bearing tetra-substituted stereogenic centers in moderate-to-good yields. The enantioselective variant using BINOL-derived phosphoric acids was also explored with 1-naphthol, providing the desired product with moderate enantioselectivities (up to 99:1 following recrystallization).

Oxidative Asymmetric Formal Aza-Diels⁻Alder Reactions of Tetrahydro-β-carboline with Enones in the Synthesis of Indoloquinolizidine-2-ones

Ru-catalyzed tandem amine oxidative dehydrogenation/formal aza-Diels⁻Alder reaction for enantio- and diastereoselective synthesis of indoloquinolizidine-2-ones from tetrahydro-β-carbolines and α,β-unsaturated ketones is described. The reaction proceeds via tandem ruthenium-catalyzed amine dehydrogenation using tert-butyl hydroperoxide (TBHP) as the oxidant and a chiral thiourea-catalyzed formal aza-[4 + 2] cycloaddition, providing a step-economical strategy for the synthesis of these valuable heterocyclic products.

Transformation of N, N-Dimethylaniline N-Oxides into Diverse Tetrahydroquinoline Scaffolds via Formal Povarov Reactions

A one-pot protocol for the assembly of diversely functionalized tetrahydro-, hexahydrofuro-, hexahydropyrano-, and tetrahydrobenzofuroquinolines from N, N-dimethylaniline N-oxides and various electron-rich olefins in a tandem Polonovski-Povarov sequence is reported. Following activation of the N-O bond with Boc₂O, an exocyclic iminium ion is unveiled upon exposure to tin(IV) chloride. A formal inverse-electron-demand aza-Diels-Alder cyclization generates the tetrahydroquinoline core of 29 examples in up to 92% yield.

Highly efficient asymmetric construction of novel indolines and tetrahydroquinoline derivatives via aza-Barbier/C–N coupling reaction

An efficient approach towards stereoselective synthesis of chiral indolines and tetrahydroquinolines via Zn-mediated allylation of chiral-N-tert-butanesulfinyl imines and intramolecular C–N cross-coupling is described.

Iron-catalyzed C(sp3)–H functionalization ofN,N-dimethylanilines with isocyanides

Iron salts as cost-efficient catalysts for the assembly of privileged α-amino amides and short peptides through Ugi-type reactions.

Iron-catalyzed oxidative sp3 carbon–hydrogen bond functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones

Iron-catalyzed sp³ C–H bond functionalization of benzoxazinones has been developed and applied for the synthesis of natural product cephalandole A.