Step-Economical Synthesis of Tetrahydroquinolines by Asymmetric Relay Catalytic Friedländer Condensation/Transfer Hydrogenation

Asymmetric magnesium catalysis for important chiral scaffold synthesis

Magnesium catalysts are widely used in catalytic asymmetric reactions, and a series of catalytic strategies have been developed in recent years. Herein, in this review, we have tried to summarize asymmetric magnesium catalysis for the synthesis of important chiral scaffolds. Several important optically active motifs that are present in classic chiral ligands or natural products synthesized by Mg(II) catalytic methods are briefly discussed. Moreover, the representative mechanisms for different magnesium catalytic strategies, including in situ generated magnesium catalysts, are also shown in relation to synthetic routes for obtaining these important chiral scaffolds.

General, Modular Access toward Immobilized Chiral Phosphoric Acid Catalysts and Their Application in Flow Chemistry

Chiral phosphoric acids (CPAs) are among the most frequently used organocatalysts, with an ever-increasing number of applications. However, these catalysts are only obtained in a multistep synthesis and are poorly recyclable, which significantly deteriorates their environmental and economic performance. We herein report a conceptually different, general strategy for the direct immobilization of CPAs on a broad scope of solid supports including silica, polystyrene, and aluminum oxide. Solid-state catalysts were obtained in high yields and thoroughly characterized with elemental analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES), nitrogen sorption measurements, thermogravimetric analysis, scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) images, and solid-state NMR spectroscopy. Further, the immobilized catalysts were applied to a variety of synthetically valuable, highly stereoselective transformations under batch and flow conditions including transfer hydrogenations, a Friedländer condensation/transfer hydrogenation sequence, and Mannich reactions under cryogenic flow conditions. Generally, high yields and stereoselectivities were observed along with robust catalyst stability and reusability. After being used for 10 runs under batch conditions, no loss of selectivity or catalytic activity was observed. Under continuous-flow conditions, the heterogeneous system was in operation for 19 h and the high enantioselectivity remained unchanged throughout the entire process. We expect our approach to extend the applicability of CPAs to a higher level, with a focus on flow chemistry and a more environmentally friendly and resource-efficient use of these powerful catalysts.

Charting the Evolution of Chemoenzymatic Strategies in the Syntheses of Complex Natural Products

Bolstered by recent advances in bioinformatics, genetics, and enzyme engineering, the field of chemoenzymatic synthesis has enjoyed a rapid increase in popularity and utility. This Perspective explores the integration of enzymes into multistep chemical syntheses, highlighting the unique potential of biocatalytic transformations to streamline the synthesis of complex natural products. In particular, we identify four primary conceptual approaches to chemoenzymatic synthesis and illustrate each with a number of landmark case studies. Future opportunities and challenges are also discussed.

Transfer-catalyst-free biomimetic asymmetric reduction of 3-sulfonyl coumarins with a regenerable NAD(P)H model

A novel transfer-catalyst-free biomimetic reduction of the tetrasubstituted olefins 3-sulfonyl coumarins with the chiral and regenerable [2.2]paracyclophane-based NAD(P)H model CYNAM has been developed, affording chiral 3-sulfonyl dihydrocoumarins with excellent enantioselectivities.

Enantioselective construction of spiro-tetrahydroquinoline scaffolds through asymmetric catalytic cascade reactions

An efficient and concise strategy has been successfully developed for merging spiro-tetrahydroquinoline with spiro-benzofuranone into a single new skeleton through asymmetric catalytic cascade reactions catalyzed by quinine-derived chiral bifunctional squaramide organocatalysts. In this approach, differently substituted spiro-tetrahydroquinoline derivatives were smoothly obtained with high yields, and excellent diastereoselectivities and enantioselectivities (up to 99% yield, up to >20 : 1 dr, up to >99% ee, 40 examples) under mild reaction conditions.

Enantioselective Total Synthesis of (-)-Finerenone Using Asymmetric Transfer Hydrogenation

(-)-Finerenone is a nonsteroidal mineralocorticoid receptor antagonist currently in phase III clinical trials for the treatment of chronic kidney disease in type 2 diabetes. It contains an unusual dihydronaphthyridine core. We report a 6-step synthesis of (-)-finerenone, which features an enantioselective partial transfer hydrogenation of a naphthyridine using a chiral phosphoric acid catalyst with a Hantzsch ester. The process is complicated by the fact that the naphthyridine exists as a mixture of two atropisomers that react at different rates and with different selectivities. The intrinsic kinetic resolution was converted into a kinetic dynamic resolution at elevated temperature, which enabled us to obtain (-)-finerenone in both high yield and high enantioselectivity. DFT calculations have revealed the origin of selectivity.

Recent advances in alkaline earth metal-enabled syntheses of heterocyclic compounds

This review summarizes recent progress in alkaline earth metal enabled syntheses of heterocyclic compounds.

Highly Diastereo- and Enantioselective Ir-Catalyzed Hydrogenation of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned Phosphine-Phosphoramidite Ligands

A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a structurally fine-tuned chiral phosphine-phosphoramidite ligand with a (S_a)-3,3'-dimethyl H₈-naphthyl moiety and (R_c)-1-phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.

Recent Advances in First-Row Transition Metal/Chiral Phosphoric Acid Combined Catalysis

Since the pioneering independent reports of Akiyama and Terada, the use of chiral phosphoric acids (CPAs) and derivatives as a versatile tool for asymmetric synthesis with good reactivity, regioselectivity, diastereoselectivity and enantioselectivity has emerged, forming an important part of the implementation of asymmetric counteranion-directed catalysis reported to date. In these achievements, the combination of metals with CPAs has enabled various catalytic modes beyond the scope of typical acid catalysis, such as relay catalysis, ion-pairing catalysis, and binary acid catalysis. The first-row transition metals (Sc-Zn) are considered to be sustainable transition metals and have received a great deal of attention. These naturally abundant metals display excellent Lewis acidity and function as powerful redox catalysts in synthesis involving both one and two-electron transfers. Hence, in this chapter, we summarize recent advances in the development of asymmetric reactions using a combination of first-row transition metals and CPAs. Furthermore, we provide a detailed discussion of the mechanisms involved in order to understand the interaction of the metal/phosphate and the origins of the asymmetric control of the transformations.

Enantioselective Synthesis of 4-Aminotetrahydroquinolines via 1,2-Reductive Dearomatization of Quinolines and Copper(I) Hydride-Catalyzed Asymmetric Hydroamination

A 1,2-reductive dearomatization of quinolines and copper(II) acetate monohydrate/( R, R)-Ph-BPE/P( p-tolyl)₃-catalyzed enantioselective hydroamination sequence was developed, affording diverse 4-amino-1,2,3,4-tetrahydroquinolines with high levels of enantioselectivity in either a stepwise or one-pot fashion. Pleasingly, internal cis-cyclic alkenes, which are challenging substrates in copper hydride-catalyzed enantioselective hydroamination reactions, were transformed efficiently under mild conditions.

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.

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.

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.

Catalytic Biomimetic Asymmetric Reduction of Alkenes and Imines Enabled by Chiral and Regenerable NAD(P)H Models

The development of biomimetic chemistry based on the NAD(P)H with hydrogen gas as terminal reductant is a long-standing challenge. Through rational design of the chiral and regenerable NAD(P)H analogues based on planar-chiral ferrocene, a biomimetic asymmetric reduction has been realized using bench-stable Lewis acids as transfer catalysts. A broad set of alkenes and imines could be reduced with up to 98 % yield and 98 % ee, likely enabled by enzyme-like cooperative bifunctional activation. This reaction represents the first general biomimetic asymmetric reduction (BMAR) process enabled by chiral and regenerable NAD(P)H analogues. This concept demonstrates catalytic utility of a chiral coenzyme NAD(P)H in asymmetric catalysis.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

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.

Catalytic Asymmetric Cascade Using Spiro-Pyrrolidine Organocatalyst: Efficient Construction of Hydrophenanthridine Derivatives

A newly developed SPD (spiro-pyrrolidine) organocatalyst has been demonstrated to enable an asymmetric aza-Michael/Michael/aldol cyclization cascade, in which two six-membered rings (B/C) and three stereocenters have been constructed in a catalytic one-step process. It is so far the most efficient method for construction of hydrophenanthridine derivatives featuring high enantioselectivity. The trans- or cis-fused B/C-rings can be selectively assembled in a substrate-controlled manner. Moreover, this cascade could magnify to gram scale without loss of enanioselectivity.

Enantioselective Synthesis of Tetrahydroquinolines by Borrowing Hydrogen Methodology: Cooperative Catalysis by an Achiral Iridacycle and a Chiral Phosphoric Acid

We report herein the highly enantioselective synthesis of 2-substituted tetrahydroquinolines through borrowing hydrogen, a process recognized for its environmentally benign and atom-economical nature. The use of an achiral iridacycle complex in combination with a chiral phosphoric acid as catalysts was the key to the development of this highly efficient and enantioselective transformation.

Recent advances in organocatalytic enantioselective transfer hydrogenation

Robust, environmentally friendly reductants enable highly enantioselective reactions in the presence of chiral catalysts.

Enantioselective magnesium-catalyzed transformations

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these green catalysts to promote many types of reactions.

Efficient one-pot per-O-acetylation–thioglycosidation of native sugars, 4,6-O-arylidenation and one-pot 4,6-O-benzylidenation–acetylation of S-/O-glycosides catalyzed by Mg(OTf)2

Sequential one-pot per-O-acetylation–S-/O-glycosidation under neat condition, regioselective 4,6-O-arylidenation and sequential one-pot benzylidenation–acetylation of Mg(OTf)₂as non-hygroscopic, recyclable catalyst are reported.

Aryne-induced dearomatized phosphonylation of electron-deficient azaarenes

Aryne-induced dearomatic phosphonylation of electron-deficient azaarenes.

The catalytic enantioselective synthesis of tetrahydroquinolines containing all-carbon quaternary stereocenters via the formation of aza-ortho-xylylene with 1,2-dihydroquinoline as a precursor

Tetrahydroquinolines (THQs) with an all-carbon quaternary stereocenter were effectively obtained via the in situ formation of aza-ortho-xylylene (AOX) with easily accessible 1,2-dihydroquinolines as precursors.

Self-Assembly of a Library of Polyborate Chiral Anions for Asymmetric Catalytic Quinoline Reduction

The 'template' polyborate BOROX catalysts are shown to mediate the asymmetric transfer hydrogenation of 2-quinolines. The rapid and simple generation of a large family of BOROX catalysts with significantly altered asymmetric pockets is described. A transition state model that explains the enantioselectivity is proposed.