Catalytic enantioselective intramolecular redox reactions: ring-fused tetrahydroquinolines

Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

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.

C(sp3)-H cyclizations of 2-(2-vinyl)phenoxy-tert-anilines

1,5-hydride transfer-triggered cyclization reactions offering a robust method for C(sp3)-C(sp3) coupling and the synthesis of e.g. tetrahydroquinolines have been thoroughly investigated in the literature. Catalysts allowing milder reaction conditions or the development of enantioselective processes were important recent contributions to the field, as well as the studies on subtrates with oxygen or sulfur heteroatoms (besides the originally described nitrogen heterocycles). In a series of studies, we focused on expanded, higher order H-transfers/cyclizations by positioning the interacting substituents on distanced rings. Cyclizations of appropriately functionalized biaryl and fused bicyclic systems led to 7-9 membered rings. In the frame of this research, we set out to study the feasibility of the cyclization and the factors affecting it by in silico methods. The conclusions drawn from computational studies were complemented by cyclization screens on 2-(2-vinyl)phenoxy-tert-anilines and their CH2-expanded analogues, the results of which are presented here. Besides isolating the expected oxazonine products in several cases, we also observed a unique dimer formation, leading to an interesting 5-6-5 ring system.

Divergent synthesis of multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence

We report an effective synthetic route to multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence. The interesting feature of the present system is that it allows for the divergent synthesis of the target skeleton depending on the selected Lewis acid catalyst. When benzylidene malonates with a cyclic structure at the ortho-position were treated with BF3·OEt2, three sequential processes (internal redox reaction/elimination of the alkoxy group/ring expansion) proceeded to give phenanthrene derivatives in which the alkoxycarbonyl (CO2R) group and the alkyl (R) group were in close proximity to each other, in good chemical yields. In sharp contrast, treatment with Bi(OTf)3 exclusively led to the formation of another type of phenanthrene, whose R group was positioned distal to the CO2R group.

Synthesis of Polysubstituted Naphthalenes by a Hydride Shift Mediated C-H Bond Functionalization/Aromatization Sequence

A synthetic strategy for forming multisubstituted naphthalenes based on hydride shift mediated C(sp3)-H bond functionalization was developed. This strategy consists of three successive transformations: (1) an intramolecular hydride shift mediated C(sp3)-H bond functionalization; (2) a decarboxylative fragmentation; and (3) an oxidation reaction. When benzylidene malonates having a 2-alkoxyethyl group at the ortho position were treated with a catalytic amount of Al(OTf)3, the hydride shift/cyclization reaction proceeded smoothly to afford tetralin derivatives in good chemical yields. The resulting tetralins were easily converted into naphthalenes by exposing them to modified Krapcho decarboxylation reaction conditions (LiCl, DMSO, and heating under an O2 atmosphere). The one-pot operation of these two reactions was also realized.

Silyl-Group Boosted Internal Redox Reaction: Hydride Shift from an Aliphatic Secondary Position for the Formation of Six- and Seven-Membered Carbocycles

We report a hydride shift/cyclization reaction at the aliphatic secondary position (methylene group). The key to accomplishing this reaction was the employment of benzylidene malonate having a silyl group β to the hydride donor carbon. When the corresponding malonates were treated with a catalytic amount of Al(OTf)3, the [1,5]-hydride shift from the simple aliphatic secondary position proceeded smoothly to afford silyl-group substituted tetralin derivatives in excellent chemical yields (up to 98%). This reaction system was applied to the formation of seven-membered carbocycles via the [1,6]-hydride shift mediated process.

Chiral Crown Ethers Accessed from Catalytic Enantioselective Desymmetrization Reactions

A diversity of unprecedented chiral aza-crown ethers were synthesized straightforwardly from readily available and inexpensive aza-crown ethers. Catalyzed by a chiral phosphoric acid, desymmetrization of an array of symmetric N-arylated aza-crown ethers through tert-amino reaction proceeded efficiently under mild conditions to produce novel tetrahydroquinoline-fused aza-crown ethers in good to excellent yields with up to 96% ee. Our strategy opens a new route to functionalized chiral crown ethers.

[1,5]-Hydride Shift Triggered N-Dealkylative Cyclization into 2-Oxo-1,2,3,4-tetrahydroquinoline-3-carboxylates via Boronate Complexes

A new simple one-pot two-step protocol for the synthesis of 2-oxo-1,2,3,4-tetrahydroquinoline-3-carboxylate from 2-(2-(benzylamino)benzylidene)malonate under the action of BF3·Et2O was developed. It was shown that the reaction proceeds through the formation of a stable iminium intermediate containing a difluoroboryl bridge in the dicarbonyl fragment of the molecule.

Controllable construction of pharmaceutically significant scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one via redox-neutral cascade hydride transfer/cyclization

The scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one were controllably constructed relying on cascade condensation/redox-neutral [1,6]/[1,7]-hydride transfer/cyclization from 2-aminoacetophenone and various aldehydes as well as isatins.

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.

Divergent Access to Seven/Five-Membered Rings Based on [1,6]-Hydride Shift/Cyclization Process

We have achieved a divergent access to seven/five-membered rings based on a [1,6]-hydride shift/cyclization process from benzylidenemalonate with an o-alkoxymethyl group. Whereas Yb(OTf)₃ afforded benzoxepines (with a seven-membered ring) selectively, indanes (with a five-membered ring) were the main products when Sc(OTf)₃ was employed.

1,5-Hydride-Shift-Triggered Cyclization for the Synthesis of Unsymmetric Julolidines

Directly accessible 8-substituted tetrahydroquinolines undergo 1,5-hydride-shift-triggered cyclization to provide difficult to access julolidine derivatives in yields of 21–98% under scandium(III) triflate catalysis. Additionally, the scope of the reaction, several follow-up transformations and a remarkable side process discovered during optimization of the conditions are highlighted.

Recent Advances in the Development of Catalytic Methods that Construct Medium-ring Lactams, Partially Saturated Benzazepines and their Derivatives

Recent catalytic methods to construct medium-sized lactams and partially saturated benzazepines and their derivatives are surveyed. The review is divided into the following sections: 1 Introduction 2 Non-Transition Metal Catalyzed Reactions 2.1 Beckmann Rearrangement 2.2 Non-Beckmann Rearrangement Reactions 2.3 Multi-component reactions 3 Transition Metal-Catalyzed Reactions 3.1 Au-catalyzed reactions to access medium-sized N-heterocycles 3.2 Reactions involving a metal η³-complex catalytic intermediate 3.3 Transition metal-catalyzed reactions of strained cycloalkanes 4 Conclusions.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Diastereoselective Synthesis of Dihydro-quinolin-4-ones by a Borane-Catalyzed Redox-Neutral endo-1,7-Hydride Shift

The borane-catalyzed synthesis of dihydroquinoline-4-ones is developed. The amino-substituted chalcones undergo a 1,7-hydride shift upon Lewis acid activation to form a zwitterionic iminium enolate, which collapses to the dihydroquinoline-4-one scaffold. The reaction proceeds in high yields (75-99%) with an excellent diastereoselectivity of up to >99:1 (cis:trans). The reaction mechanism is investigated by kinetic, isotope labeling, and computational experiments.

Iridium-Catalyzed Asymmetric Cascade Allylation/Pictet-Spengler Cyclization Reaction for the Enantioselective Synthesis of 1,3,4-Trisubstituted Tetrahydroisoquinolines

An iridium-catalyzed trifluoroacetic acid-promoted asymmetric cascade allylation/Pictet-Spengler cyclization reaction of azomethine ylides with aromatic allylic alcohols is reported. This protocol provides a facile and scalable method for the construction of 1,3,4-trisubstituted tetrahydroisoquinolines containing two stereogenic centers in good yields (up to 96%) with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Furthermore, a series of aromatic heterocycle-fused piperidines were also obtained with excellent enantiocontrol by this methodology.

Enantioselective Synthesis of N-Alkylamines through β-Amino C-H Functionalization Promoted by Cooperative Actions of B(C6F5)3 and a Chiral Lewis Acid Co-Catalyst

We disclose a catalytic method for β-C(sp³)-H functionalization of N-alkylamines for the synthesis of enantiomerically enriched β-substituted amines, entities prevalent in pharmaceutical compounds and used to generate different families of chiral catalysts. We demonstrate that a catalyst system comprising of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg- or Sc-based complex, promotes the highly enantioselective union of N-alkylamines and α,β-unsaturated compounds. An array of δ-amino carbonyl compounds was synthesized under redox-neutral conditions by enantioselective reaction of a N-alkylamine-derived enamine and an electrophile activated by the chiral Lewis acid co-catalyst. The utility of the approach is highlighted by late-stage β-C-H functionalization of bioactive amines. Investigations in regard to the mechanistic nuances of the catalytic processes are described.

Imidazol-5-ones as a substrate for [1,5]-hydride shift triggered cyclization

(2-Aminobenzylidene)-imidazolones were used as substrates for [1,5]-hydride shift triggered cyclization under promotion by TiCl₄ at room temperature.

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.

Rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans by hybrid use of Lewis/Brønsted acid catalysts

We report herein a rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans via a [1,4]-hydride shift/cyclizatin/intermolecular nucleophilic addition reaction sequence. In this process, a Lewis acid promoted internal redox reaction ([1,4]-hydride shift/cyclization) followed by a Brønsted acid promoted intermolecular reaction (generation of cyclic oxonium cation/intermolecular Friedel-Crafts reaction) occurred to give various 3-indolyl-1-trifluoromethyl-isobenzofurans in good chemical yields.

Expeditious Synthesis of Multisubstituted Quinolinone Derivatives Based on Ring Recombination Strategy

We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various quinolinone derivatives in good to excellent chemical yields (up to 98%).

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.

Diastereoselective Synthesis of CF3-Substituted Spiroisochromans by [1,5]-Hydride Shift/Cyclization/Intramolecular Friedel-Crafts Reaction Sequence

Developed herein is a diastereoselective synthesis of CF₃-substituted spiroisochromans via C(sp³)-H bond functionalization involving sequential transformations ([1,5]-hydride shift/cyclization/elimination of MeOH/intramolecular Friedel-Crafts reaction).

Dibenzofuran-4,6-bis(oxazoline) (DBFOX). A novel trans-chelating bis(oxazoline) ligand for asymmetric reactions

The trans-chelating bis(oxazoline) ligand (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) [(R,R)-DBFOX/Ph] coordinates metal ions to give C2-symmetric complexes which effectively catalyze a variety of asymmetric reactions. (R,R)-DBFOX/Ph·Ni(ClO4)2·3H2O, whose crystal structure is octahedral with three aqua ligands, can be stored under air for several months without loss of catalytic activity and promotes highly enantioselective reactions even in the presence of excess amounts of water, alcohols, amines, and acids. The complex shows remarkable chiral amplification in asymmetric Diels-Alder (DA) reactions. This review focuses on enantioselective reactions catalyzed by (R,R)-DBFOX/Ph·metal complexes.

Construction of seven- and eight-membered carbocycles by Lewis acid catalyzed C(sp3)–H bond functionalization

Concise construction of seven- or eight-membered carbocycles was accomplished by Lewis acid catalyzed C(sp³)–H bond functionalization.

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.

Highly diastereoselective synthesis of tricyclic fused-pyrans by sequential hydride shift mediated double C(sp3)-H bond functionalization

Described herein is the Brønsted acid-catalyzed double C(sp3)-H bond functionalization of alkyl phenethyl ether derivatives. In this process, a [1,5]-[1,5]-hydride shift occurred successively to afford tricyclic fused pyran derivatives in excellent chemical yields with excellent diastereoselectivities (up to >20 : 1). The key to achieve this reaction was the introduction of two methyl groups at the benzylic position, which was effective in both hydride shift processes: (1) the Thorpe-Ingold effect for the first hydride shift and (2) conformational control in the second hydride shift.

C-H Functionalization of Amines via Alkene-Derived Nucleophiles through Cooperative Action of Chiral and Achiral Lewis Acid Catalysts: Applications in Enantioselective Synthesis

Catalytic transformations of α-amino C-H bonds to afford valuable enantiomerically enriched α-substituted amines, entities that are prevalent in pharmaceuticals and bioactive natural products, have been developed. Typically, such processes are carried out under oxidative conditions and require precious metal-based catalysts. Here, we disclose a strategy for an enantioselective union of N-alkylamines and α,β-unsaturated compounds, performed under redox-neutral conditions, and promoted through concerted action of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg-PyBOX complex. Thus, a wide variety of β-amino carbonyl compounds may be synthesized, with complete atom economy, through stereoselective reaction of an in situ-generated enantiomerically enriched Mg-enolate and an appropriate electrophile.

Construction of 1,3-Dithio-Substituted Tetralins by [1,5]-Alkylthio Group Transfer Mediated Skeletal Rearrangement

A novel skeletal rearrangement involving a [1,5]-alkylthio group transfer/cyclization sequence is described. Treatment of benzylidene malonates having a thioketal moiety at the homobenzyl position with a catalytic amount of Sc(OTf)₃ afforded alkylthio group rearranged adducts in good chemical yields. Detailed investigation of the reaction mechanism revealed that an intramolecular conjugate addition/ring opening sequence (not through-space transfer) is the key to achieving this reaction.

Substrate-controlled divergent synthesis of polycyclic indoloazepines and indolodiazepines via 1,5-hydride shift/7-cyclization cascades

Novel and practical acid-catalyzed cyclization of 2-indolyl aryl carbinols via tandem dehydration/1,5-hydride shift/7-cyclization sequences has been developed. By appropriate variation of the substrate, diverse polycyclic-fused indoles were synthesized in good yield, thus demonstrating the high efficiency, transition-metal-free nature, and high step/atom economy of the synthesis strategy.