Enantioselective Synthesis of Tetrahydroquinolines from 2-Aminochalcones via a Consecutive One-Pot Reaction Catalyzed by Chiral Phosphoric Acid

Design, Synthesis, and Application of a Family of Chiral Non-C2-Symmetric NHCs with a Fused Sidechain

Although a considerable number of chiral nitrogen heterocyclic carbenes (NHCs) have been developed yet it is highly necessary to develop new NHCs bearing multiple sites for facile modifications of both electronic nature and steric hindrance. Herein, we uncover a new family of chiral non-C2-Symmetric NHCs with a fused sidechain, whose precursors are synthesized by a simple five-step route. The synthesis includes Pd-catalyzed cross-coupling or nucleophilic addition/oxidation, chiral phosphoric acid-catalyzed asymmetric reduction of 2-aryl-quinolines, bromination at C8, Buchwald-Hartwig amination, and cyclization with methyl orthoformate. Among nine prepared NHCs, YC-NHC8 is the optimal ligand for Cu(I)-catalyzed asymmetric SN2' silylation, YC-NHC3 works as the best ligand for Cu(I)-catalyzed enantioselective conjugate silylation of simple α,β-unsaturated amides, and YC-NHC9 serves as the most suitable ligand for copper(I)-catalyzed asymmetric silylation of azadienes. Remarkably, these three reactions are successfully run under a catalyst loading of 0.1 mol%, indicating that YC-NHCs may have the potential to be broadly used in efficient asymmetric transition metal catalysis.

Development of Chiral Bisphosphine Ligands with a Terminal Olefin Enables Asymmetric Orthogonal Auto-Tandem Catalysis

Auto-tandem catalysis (ATC) leverages the multiple roles of the catalyst, facilitating the direct synthesis of complex molecules from simple starting materials in a highly efficient and step-economical manner. However, in conventional ATC, employing a single catalyst imposes significant limitations on the variety of catalyzed reactions. Herein, we present a novel catalysis strategy termed "orthogonal auto-tandem catalysis (OATC)", which provides the unique opportunity to develop more efficient chemical transformations that cannot be achieved with existing catalytic modes. An unprecedented tandem annulation process involving four different catalytic cycles via asymmetric OATC is demonstrated. The enantio-determining step is a previously unrealized Pd(0)-catalyzed asymmetric allylic lactonization. The key to the whole success lies in the development of a new class of chiral bisphosphine ligands with a terminal olefin (TOPhos) that addressed not only the racemization issue of allylic ester products in classic π-allylmetal chemistry, but also the inherent compatibility challenges in transition-metal/organo dual catalytic system. Density functional theory (DFT) calculations unveil the vital role of the terminal olefin in TOPhos, enabling the differentiation of competitive enantioselective pathways.

Unlocking the Phosphoric Acid Catalyzed Asymmetric Transfer Hydrogenation of 2-Alkenyl Quinolines for Efficient Flow Synthesis of Hancock Alkaloids

Chiral tetrahydroquinolines are present in several bioactive molecules, such as the Hancock alkaloids. Although the organocatalytic asymmetric transfer hydrogenation of 2-aryl quinolines has emerged as a safer alternative to using hydrogen gas, analogous reactions with 2-alkenyl quinolines remain unexplored. Here we present a protocol to synthesize key enantioenriched intermediates to the Hancock alkaloids, providing a constant outcome for, at least, a 24 h continuous flow operation.

[4+2]-Cycloaddition of 2-Aminophenyl Enones with Cyclic N-Sulfimines to Access Enantioenriched Ring-Fused Aminals

Ring-fused aminal is an interesting structural skeleton in biologically active and pharmaceutically relevant compounds. A novel and efficient method for synthesizing benzosulfamidate-fused tetrahydroquinazolines is described. By employing the [4+2]-cycloaddition of 2-aminophenyl enones with cyclic N-sulfimines in the presence of DMAP as a base, the desired benzosulfamidate-fused tetrahydroquinazolines were obtained in good yields with high diastereoselectivities. Furthermore, an organocatalytic asymmetric [4+2]-cycloaddition was successfully achieved using a squaramide-based catalyst, enabling the enantioselective synthesis of chiral ring-fused tetrahydroquinazolines with high yields and enantio- as well as diastereoselectivities (up to 89 % yield, 94 % ee, and >30 : 1 dr).

Atroposelective Brønsted Acid-Catalyzed Photocyclization to Access Chiral N-Aryl Quinolones with Low Rotational Barriers

Herein, a novel route to atropisomeric N-aryl quinolones with low rotational barriers is demonstrated, leveraging a dual photochemical/organocatalytic approach to the required ring closure in up to 94% yield and up to >99% ee. The use of a continuous flow system allows for impurity suppression and enables rapid scale-up to a decagram scale.

Applications of Hantzsch Esters in Organocatalytic Enantioselective Synthesis

Hantzsch esters (1,4-dihydropyridine dicarboxylates) have become, in this century, very versatile reagents for enantioselective organic transformations. They can act as hydride transfer agents to reduce, regioselectively, a variety of multiple bonds, e.g., C=C and C=N, under mild reaction conditions. They are excellent reagents for the dearomatization of heteroaromatic substances, and participate readily in cascade processes. In the last few years, they have also become useful reagents for photoredox reactions. They can participate as sacrificial electron and hydrogen donors and when 4-alkyl or 4-acyl-substituted, they can act as alkyl or acyl radical transfer agents. These last reactions may take place in the presence or absence of a photocatalyst. This review surveys the literature published in this area in the last five years.

Recent Advances in the Transition Metal-Free Synthesis of Heterocycles from α, β-Unsaturated Ketones

Heterocyclic compounds are an inevitable part of our life. These important classes of molecules have a wide range of applications starting from life-sustaining drugs to agrochemicals. Numerous methods, including metal...

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

Herein, recent developments in the field of organocatalytic asymmetric transfer hydrogenation (ATH) of C=N, C=O and C=C double bonds using chiral phosphoric acid catalysis are reviewed. This still rapidly growing area of asymmetric catalysis relies on metal-free catalysts in combination with biomimetic hydrogen sources. Chiral phosphoric acids have proven to be extremely versatile tools in this area, providing highly active and enantioselective alternatives for the asymmetric reduction of α,β-unsaturated carbonyl compounds, imines and various heterocycles. Eventually, such transformations are more and more often used in multicomponent/cascade reactions, which undoubtedly shows their great synthetic potential and the bright future of organocatalytic asymmetric transfer hydrogenations.

Synthesis of 2-trifluoromethylated quinolines from CF3-alkenes

α-CF₃-enamines can be prepared by the reaction of pyrrolidine with the corresponding haloalkenes. The prepared enamines react with 2-nitrobenzaldehydes to give ortho-nitro-substituted α,β-diaryl-CF₃-enones highly stereoselectively in up to 88% yield. Subsequent reduction of the nitro-group by an Fe-AcOH system promotes intramolecular cyclization to afford 2-CF₃-3-arylquinolines in up to 99% isolated yield. High synthetic utility of all synthetic steps of the sequence was shown. A one-pot procedure was developed to give the target trifluoromethylated quinolines directly from enamines or haloalkenes.

Diastereoselective Construction of Cyclopropane-Fused Tetrahydroquinolines via a Sequential [4 + 2]/[2 + 1] Annulation Reaction

A sequential [4 + 2]/[2 + 1] annulation of α-aryl vinylsulfoniums with 2-aminochalcones and 2-(2-aminobenzylidene)-1H-indene-1,3(2H)-dione is reported that affords a series of cyclopropane-fused tetrahydroquinolines. The salient features of this novel and practical transformation include high efficiency, transition-metal-free nature, operational simplicity, and outstanding functional group tolerance.

Regioselective synthesis of tetrahydroquinolines via syn- and anti-nucleopalladation-initiated cascade processes

Regioselective synthesis of tetrahydroquinolines via nucleopalladation-initiated cascade sequences was established.

P-Chiral, N-phosphoryl sulfonamide Brønsted acids with an intramolecular hydrogen bond interaction that modulates organocatalysis

Brønsted acids exemplified by OttoPhosa I (5c) were designed and evaluated in the asymmetric transfer hydrogenation of quinolines. Their catalytic properties are modulated by an intramolecular hydrogen bond that rigidifies their catalytic cavity, accelerates the reaction rate and improves enantioselectivity.

Visible-Light Induction/Brønsted Acid Catalysis in Relay for the Enantioselective Synthesis of Tetrahydroquinolines

An efficient method merging Brønsted acid catalysis with visible-light induction for the highly enantioselective synthesis of tetrahydroquinolines has been developed. This mild process directly transforms 2-aminoenones into 2-substituted tetrahydroquinolines with excellent enantioselectivities through a relay visible-light-induced cyclization/chiral phosphoric acid-catalyzed transfer hydrogenation reaction.

Stereoselective synthesis of all-cis boryl tetrahydroquinolines via copper-catalyzed regioselective addition/cyclization of o-aldiminyl cinnamate with B2Pin2

A copper catalyzed intramolecular 1,2-carboboration of o-aldiminyl cinnamate has been realized in both regio- and stereoselective fashions. This reaction provides a convenient entry to highly valuable and otherwise challenging cis-2,3,4-trisubstituted tetrahydroquinolines carrying a 4-boryl group. An unusual non-Michael addition intermediate or alternatively, a cyclic enolate is proposed to account for the intriguing all-cis configuration in the final products.