Asymmetric [4+2] Annulation of C1 Ammonium Enolates with Copper-Allenylidenes

Doubly Stereoconvergent Propargylic Alkylation of α-Cyanocarbonyls: Enantioselective Construction of Vicinal Stereocenters

An asymmetric propargylic alkylation of α-cyanocarbonyls is developed for the first time under cooperative Cu(I) and organocatalysis. With ethynyl benzoxazinanones as the propargylic electrophile, this decarboxylative doubly stereoconvergent reaction evades alkyne hydroamination to furnish acyclic α-propargylic cyanocarbonyls, bearing vicinal tertiary and quaternary stereocenters, with high diastereo- and enantioselectivity (up to >20:1 dr and 99.5:0.5 er).

Catalytic asymmetric intramolecular propargylation of cyclopropanols to access the cuparane core

The catalytic asymmetric propargylation of enol(ate) intermediates is a well-established method for the synthesis of α-propargyl-substituted carbonyl compounds. However, the propargylation of homo-enol(ate) or its equivalents for the synthesis of β-propargyl-substituted carbonyl compounds remains underdeveloped. A catalytic enantioselective decarboxylative intramolecular propargylation of cyclopropanols has been developed using a PyBox-complexed copper catalyst. This reaction offers an effective approach to assemble a cyclopentanone skeleton bearing an all-carbon quaternary stereogenic center and an adjacent quaternary gem-dimethyl carbon center, which is the core scaffold of the naturally occurring cuparenoids. Key to the success of this protocol is the use of a new structurally optimized PyBox ligand. This study represents the first example of catalytic asymmetric intramolecular propargylation of cyclopropanols.

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Asymmetric copper-catalyzed alkynylallylic monofluoroalkylations with fluorinated malonates

The unprecedented copper-catalyzed asymmetric alkynylallylic monofluoroalkylation reaction is described via the use of 1,3-enynes and fluorinated malonates. A series of 1,4-enynes bearing a monofluoroalkyl unit are achieved in high yields, excellent regio- and enantioselectivity and high E/Z selectivity. The asymmetric propargylic monofluoroalkylation is also developed. The reliability and synthetic value of the work are highlighted by a gram-scale test and a couple of downstream transformations. Preliminary mechanistic studies unveil a negative nonlinear effect for the catalytic process.

Rhodium and Isothiourea Dual Catalysis: Enantiodivergent Transformation of Terminal Alkynes

A dual rhodium/isothiourea catalytic system was developed for the enantiodivergent transformation of terminal alkynes. Under synergistic rhodium/isothiourea dual catalysis, terminal alkynes can be creatively utilized as precursors for C1-ammonium enolate species, which subsequently participate in [4 + 2] and [2 + 2] annulation reactions with α,β-unsaturated ketimines or ketones, respectively. A wide range of chiral lactams and lactones were obtained in excellent yields and stereoselectivities (up to >20:1 dr, 98% ee).

Elucidating the mechanism and origin of stereoselectivity in the activation/transformation of an acetic ester catalyzed by an N-heterocyclic carbene

The activation of an ester by N-heterocyclic carbene (NHC) organocatalysis is an efficient and important approach for generating an NHC-bound enolate intermediate, an important active intermediate in the transformation of carbonyl compounds. Herein, we perform a theoretical study on the NHC-catalyzed activation and transformation reaction of an acetic ester in which the NHC-bound enolate intermediate is a key intermediate. Multiple activation and transformation pathways are proposed and analyzed to identify an energetically favorable pathway. The use of different substrates for the reaction is considered. When a chalcone substrate is used, [4+2] cycloaddition between the enolate intermediate and the chalcone is identified to be both the rate- and stereoselectivity-determining step for the reaction, with the R-configured product being generated as the major isomer. Noncovalent interaction (NCI) and atoms-in-molecules (AIM) analyses are performed to identify the origin of the stereoselectivity of the reaction, and a local reactivity analysis is conducted to explore substrate and catalyst effects on the reaction.

Cooperative Chiral Lewis Base/Palladium-Catalyzed Asymmetric Syntheses of Methylene-Containing δ-Lactams

We herein report a two-step approach for the enantioselective synthesis of novel chiral δ-lactams. By using a cooperative chiral ITU/achiral Pd-catalyst system, this protocol proceeds via an asymmetric α-allylation of activated aryl esters first, followed by an acid-mediated lactam formation. A variety of differently substituted products could be obtained with usually high levels of enantioselectivities and in reasonable yields (16 examples, up to 98 : 2 er and 73 % yield over two steps). In addition, further utilizations of the products via transformations of the exocyclic double bond were successfully carried out as well.

Photoredox Annulation of Polycyclic Aromatic Hydrocarbons

The rise of interest in using polycyclic aromatic hydrocarbons (PAHs) and molecular graphenoids in optoelectronics has recently stimulated the growth of modern synthetic methodologies giving access to intramolecular aryl-aryl couplings. Here, we show that a radical-based annulation protocol allows expansion of the planarization approaches to prepare functionalized molecular graphenoids. The enabler of this reaction is peri-xanthenoxanthene, the photocatalyst which undergoes photoinduced single electron transfer with an ortho-oligoarylenyl precursor bearing electron-withdrawing and nucleofuge groups. Dissociative electron transfer enables the formation of persistent aryl radical intermediates, the latter undergoing intramolecular C-C bond formation, allowing the planarization reaction to occur. The reaction conditions are mild and compatible with various electron-withdrawing and -donating substituents on the aryl rings as well as heterocycles and PAHs. The method could be applied to induce double annulation reactions, allowing the synthesis of π-extended scaffolds with different edge peripheries.

Asymmetric Substitution by Alkynyl Copper Driven Dearomatization and Rearomatization

Catalytic asymmetric transformations by dearomatization have developed into a widely applicable synthetic strategy, but heavily relied on the use of arenes bearing a heteroatom. In this case, the dearomatization is facilitated by the involvement of a p-orbital electron of the heteroatom. Different from the conventional substrate-dependent model, here we demonstrate that the activation by a d-orbital electron of the transition-metal center can serve as a driving force for dearomatization, and is applied to the development of a novel asymmetric alkynyl copper facilitated remote substitution reaction. A newly modified PyBox chiral ligand enables the construction of valuable diarylmethyl and triarylmethyl skeletons in high enantioselectivities. An unexpected tandem process involving sequential remote substitution/cyclization/1,5-H shift leads to the formation of the enantioenriched C-N axis. A gram-scale reaction and various downstream transformations highlight the robustness of this method and the potential transformations of the products. Preliminary mechanistic studies reveal a mononuclear Cu-catalyzed remote substitution process.

Chiral Lewis Base-Catalysed Asymmetric Syntheses of Benzo-fused ϵ-Lactones

We herein report a two-step protocol for the asymmetric synthesis of novel chiral benzofused ϵ-lactones starting from O-protected hydroxymethyl-para-quinone methides and activated aryl esters. By using chiral isothiourea Lewis base catalysts a broad variety of differently substituted products could be obtained in yields of around 50 % over both steps with high levels of enantioselectivities, albeit low diastereoselectivities only.

Ligand-Controlled Diastereodivergency in Propargylic Alkylation of Vinylogous Aza-Enamines: Construction of 1,3-Stereocenters

The first diastereodivergent propargylic alkylation reaction is developed. This Cu(I)-catalyzed formal decarboxylative [4+2] cycloaddition between ethynyl benzoxazinanone and vinylogous aza-enamine delivers each diastereomer of tetrahydroquinoline derivatives, bearing 1,3-stereocenters, using either i-Pr-Pybox or BINAP as the ligand under otherwise identical reaction conditions. This is the first application of vinylogous aza-enamines in a transition metal-catalyzed transformation and the first example of the creation of 1,3-stereocenters in a propargylic substitution reaction.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Silver/chiral pyrrolidinopyridine relay catalytic cycloisomerization/(2 + 3) cycloadditions of enynamides to asymmetrically synthesize bispirocyclopentenes as PDE1B inhibitors

Significant progress has been made in asymmetric synthesis through the use of transition metal catalysts combined with Lewis bases. However, the use of a dual catalytic system involving 4-aminopyridine and transition metal has received little attention. Here we show a metal/Lewis base relay catalytic system featuring silver acetate and a modified chiral pyrrolidinopyridine (PPY). It was successfully applied in the cycloisomerization/(2 + 3) cycloaddition reaction of enynamides. Bispirocyclopentene pyrazolone products could be efficiently synthesized in a stereoselective and economical manner (up to >19:1 dr, 99.5:0.5 er). Transformations of the product could access stereodivergent diastereoisomers and densely functionalized polycyclic derivatives. Mechanistic studies illustrated the relay catalytic model and the origin of the uncommon chemoselectivity. In subsequent bioassays, the products containing a privileged drug-like scaffold exhibited isoform-selective phosphodiesterase 1 (PDE1) inhibitory activity in vitro. The optimal lead compound displayed a good therapeutic effect for ameliorating pulmonary fibrosis via inhibiting PDE1 in vivo.

Access to Chiral Tetrahydroquinazolines/1,3-Benzoxazines via Iridium-Catalyzed Asymmetric [4 + 2] Cycloaddition

An iridium-catalyzed asymmetric [4 + 2] cycloaddition of 1,3,5-triazinanes with 2-(1-hydroxyallyl)anilines/2-(1-hydroxyallyl)phenols has been developed, providing a straightforward and efficient approach to a wide range of tetrahydroquinazolines in good yields and excellent enantioselectivities (up to >99% ee). Typically, chiral 1,3-benzoxazines, which are challenging substrates in asymmetric [4 + 2] cycloaddition, could be obtained in excellent enantioselectivities via this protocol.

Cooperative Isothiourea/Iridium-Catalyzed Asymmetric Annulation Reactions of Vinyl Aziridines with Pentafluorophenyl Esters

Chiral γ-lactam-containing skeletons are important motifs in bioactive natural products, pharmaceuticals, and bioactive molecules. Herein, we report a general and modular platform to access chiral γ-lactam compounds via an ITU/Ir cooperatively catalyzed [3 + 2] asymmetric annulation reaction of vinyl aziridines with pentafluorophenyl esters. Through the Lewis base and transition metal cooperative catalytic regime, a broad range of optically active γ-lactams were generated in good yields (up to 92%) with high asymmetric induction (up to 98% ee). Furthermore, the utility of this synthetic protocol was also demonstrated by the expedient preparation of diverse enantioenriched architectures.

Copper-Catalyzed Diastereo- and Enantioselective Decarboxylative [3 + 2] Cyclization of Alkyne-Substituted Cyclic Carbamates with Azlactones: Access to γ-Butyrolactams Bearing Two Vicinal Tetrasubstituted Carbon Stereocenters

A copper-catalyzed diastereo- and enantioselective decarboxylative [3 + 2] cyclization reaction of alkyne-substituted cyclic carbamates with azlactones has been established. A range of optically pure γ-butyrolactams bearing two vicinal tetrasubstituted carbon stereocenters were obtained in high yields with good to excellent stereoselectivities (up to 99% yield, 99:1 dr, and 99% ee). This is the first example of asymmetric synthesis γ-butyrolactams containing sterically congested vicinal tetrasubstituted stereocenters via a decarboxylative cyclization pathway.

Cooperative Tertiary Amine/Palladium-Catalyzed Sequential [4 + 3] Cyclization/[1,3]-Rearrangement for Stereoselective Synthesis of Spiro [Methylenecyclopentane-1,3'-oxindolines]

A cooperative tertiary amine/palladium-catalyzed sequential reaction process, proceeding via a [4 + 3] cyclization of isatin-derived Morita-Baylis-Hillman Expansion (MBH) carbonates and tert-butyl 2-(hydroxymethyl)allyl carbonates followed by a [1,3]-rearrangement, has been found and developed. A range of structurally diverse spiro[methylene cyclopentane-1,3'-oxindolines] bearing two adjacent β,γ-acyl quaternary carbon stereocenters, which are difficult to obtain by conventional strategies, were obtained in good yields. Further synthetic utility of this protocol is highlighted by its excellent regio- and stereocontrol as well as the large-scale synthesis and diverse functional transformations of the synthetic compounds. Moreover, the control experiments probably established the plausible mechanism for this sequential [4 + 3] cyclization/[1,3]-rearrangement process.

De Novo Construction of Chiral Aminoindolines by Cu-Catalyzed Asymmetric Cyclization and Subsequent Discovery of an Unexpected Sulfonyl Migration

Searching for efficient strategies to access structurally novel aminoindolines is of great significance for drug discovery. However, catalytic asymmetric de novo construction of aminoindoline scaffolds with functionality primed for diversification still remains elusive. Here, we report a Cu-catalyzed asymmetric cyclization of ethynyl benzoxazinones with amines, producing chiral 3-aminoindolines in good yield and with high enantioselectivity (up to 97% yield and 98:2 er). Moreover, a radical-mediated sulfonyl migration of these products was unexpectedly found, further affording new chiral 3-aminoindolines bearing alkenyl sulfonyl groups with retained enantiopurity (up to 84% yield and 98:2 er). Bioactivity evaluations indicate that these 3-aminoindolines show notable antitumor activities and chirality is proven to have a significant impact on their antitumor activity.

Enantioselective Construction of Vicinal Quaternary-Tetrasubstituted Carbon Stereocenters by Copper-Catalyzed Decarboxylative Propargylic Substitution

Enantioselective construction of vicinal tetrasubstituted carbon stereocenters is a formidable challenge in organic synthesis. A copper-catalyzed asymmetric decarboxylative propargylic substitution with 3-amino oxindoles as trisubstituted carbon nucleophiles and propargylic cyclic carbonates as tertiary carbon electrophiles was developed. A range of 3-amino-3,3'-disubstituted oxindoles bearing vicinal quaternary-tetrasubstituted carbon stereocenters were obtained in high yields and good to excellent stereoselectivities (up to 98% yield, >20:1 dr, and 98.5:1.5 er).

A Desilylative Approach to Alkyl Substituted C(1)-Ammonium Enolates: Application in Enantioselective [2+2] Cycloadditions

The catalytic generation of C(1)-ammonium enolates from the corresponding α-silyl-α-alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α-unsubstituted and α-alkyl substituted C(1)-ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]-cycloaddition processes with perfluoroalkylketones (31 examples, up to 96 % yield and >99 : 1 er), as well as selective [2+2]-cycloaddition with trifluoromethyl enones (4 examples, up to 75 % yield and >99 : 1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)-α-silyl-α-alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero-order kinetics.

Stereoselective synthesis of C3-tetrasubstituted oxindoles via copper catalyzed asymmetric propargylation

Herein, a copper catalyzed asymmetric propargylation of 2-oxindole-3-carboxylate esters with terminal propargylic esters is described. This strategy successfully provides a direct approach to constructing a broad range of chiral C3-tetrasubstituted oxindoles with contiguous tertiary and quaternary carbon stereocenters in high yields and excellent enantioselectivities (16 examples, up to 99% yield and 98% ee). Moreover, the diastereoisomers of the two newly formed stereocenters can be separated by silica gel chromatography, thereby providing a valuable stereoselective access to all four possible stereoisomers of C3-tetrasubstituted oxindoles.

Copper-Catalyzed Decarboxylative Cascade Cyclization of Propargylic Cyclic Carbonates/Carbamates with Pyridinium 1,4-Zwitterionic Thiolates to Fused Polyheterocyclic Structures

A copper-catalyzed decarboxylative cascade cyclization of propargylic cyclic carbonates/carbamates with pyridinium 1,4-zwitterionic thiolates is developed. A range of fused polyheterocyclic compounds are obtained in moderate to good yields with excellent diastereoselectivities. Of particular note is that four new bonds (two C-C, one C-O/N, one C-S) and four new stereocenters could be efficiently embedded into the tetracyclic fused scaffolds in a single step.

Cooperative Palladium/Isothiourea Catalyzed Enantioselective Formal (3+2) Cycloaddition of Vinylcyclopropanes and α,β-Unsaturated Esters

A protocol for the enantioselective synthesis of substituted vinylcyclopentanes has been realised using cooperative palladium and isothiourea catalysis. Treatment of vinylcyclopropanes with Pd(PPh3 )4 generates a zwitterionic π-allyl palladium intermediate that intercepts a catalytically generated α,β-unsaturated acyl ammonium species prepared from the corresponding α,β-unsaturated para-nitrophenyl ester and the isothiourea (R)-BTM. Intermolecular formal (3+2) cycloaddition between these reactive intermediates generates functionalised cyclopentanes in generally good yields and excellent diastereo- and enantiocontrol (up to >95 : 5 dr, 97 : 3 er), with the use of LiCl as an additive proving essential for optimal stereocontrol. To the best of our knowledge a dual transition metal/organocatalytic process involving α,β-unsaturated acyl ammonium intermediates has not been demonstrated previously.

Organocatalytic Enantioselective Formal (4 + 2)-Cycloadditions of Phosphine-Containing Dipoles with Isocyanates

Phosphine-catalyzed enantioselective formal (4 + 2)-cycloadditions of 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates with isocyanates have been developed for the first time. The initial S_N2' attack of the chiral phosphine organocatalyst on 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates generated the key phosphine-containing dipolar intermediates, and the subsequent formal cycloaddition with isocyanates furnished a broad scope of 3,4-dihydroquinazolin-2-ones in 60-84% yields with 61-92% ee.

Stereoselective Michael Additions of Arylacetic Acid Derivatives by Asymmetric Organocatalysis

Because of the versatility of chiral 1,5-dicarbonyl structural motifs, the development of stereoselective Michael additions of arylacetic acid derivatives to electron-deficient alkenes is an important challenge. Over recent decades, an array of enantio- and diastereoselective methods of this type have been developed through the use of chiral organocatalysts. In this article, three distinct strategies in this research area are highlighted. Catalytic generation of either a chiral iminium electrophile (iminium catalysis) or a chiral enolate nucleophile (Lewis­ base catalysis) has allowed the efficient construction of stereogenic C–C bonds. We also introduce a synergistic catalytic approach involving the merger of these two catalytic cycles that provides selective access to all four stereoisomers of products with vicinal stereocenters.

1 Introduction

2 Iminium Catalysis

3 Lewis Base Catalysis

4 Synergistic Organocatalysis

5 Summary

Solvent directed chemically divergent synthesis of β-lactams and α-amino acid derivatives with chiral isothiourea

A protocol for the chemically divergent synthesis of β-lactams and α-amino acid derivatives with isothiourea (ITU) catalysis by switching solvents was developed. The stereospecific Mannich reaction occurring between imine and C(1)-ammonium enolate generated zwitterionic intermediates, which underwent intramolecular lactamization and afforded β-lactam derivatives when DCM and CH₃CN were used as solvents. However, when EtOH was used as the solvent, the intermediates underwent an intermolecular esterification reaction, and α-amino acid derivatives were produced. Detailed mechanistic experiments were conducted to prove that these two kinds of products came from the same intermediates. Furthermore, chemically diversified transformations of β-lactam and α-amino acid derivatives were achieved.

A protocol for the solvent directed chemically divergent synthesis of β-lactam and α-amino acid derivatives with chiral isothiourea was reported.

Practical asymmetric amine nucleophilic approach for the modular construction of protected α-quaternary amino acids

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids. The key to success is the use of an alcohol solvent, which makes a rationally designed COOMe-bonded Cu-allenylidene electrophilic intermediate stable enough to couple with amine nucleophiles before its decomposition. The reaction features wide functional group tolerance with high enantioselectivity, typically >90% ee, and is amenable to the modification of commercially available bioactive molecules. The resultant protected α-amino acids could be readily converted into a number of precious enantioenriched amines featuring α-hindered tertiary carbon centers, which are otherwise synthetically quite challenging, including those of α-amino aldehyde, peptides or α-vinyl amino ester with >92% ee in excellent yields. This protocol could be utilized for the synthesis of the protected bioactive α-ethylnorvaline in 3 steps, a significant advancement in comparison to an 11-step sequence reported previously.

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids.

Mild Base-Promoted Tandem Nucleophilic Substitution/Decarboxylation/Hydroamination: Access to 3-Sulfonylindoles and 2-Methyleneindophenols

We have developed an efficient construction of 3-sulfonylindoles and 2-methyleneindophenols via decarboxylative propargylation/hydroamination of ethynyl benzoxazinanones using sodium sulfonates and phenols as the nucleophiles. The reaction featured mild conditions (K₂CO₃), simple operation, and high chemoselectivity and was transition-metal-free. Our protocol could also be extended to other nucleophiles, including malonates, alcohols, and indoles. The broad substrate scope and scalability made this protocol practical for the synthesis of indole derivatives.

Construction of poly-N-heterocyclic scaffolds via the controlled reactivity of Cu-allenylidene intermediates

Controlling the sequence of the three consecutive reactive carbon centres of Cu-allenylidene remains a challenge. One of the impressive achievements in this area is the Cu-catalyzed annulation of 4-ethynyl benzoxazinanones, which are transformed into zwitterionic Cu-stabilized allenylidenes that are trapped by interceptors to provide the annulation products. In principle, the reaction proceeds via a preferential γ-attack, while annulation reactions via an α- or β-attack are infrequent. Herein, we describe a method for controlling the annulation mode, by the manipulation of a CF₃ or CH₃ substituent, to make it proceed via either a γ-attack or an α- or β-attack. The annulation of CF₃-substituted substrates with sulfamate-imines furnished densely functionalized N-heterocycles with excellent enantioselectivity via a cascade of an internal β-attack and an external α-attack. CH₃-variants were transformed into different heterocycles that possess a spiral skeleton, via a cascade of an internal β-attack and a hydride α-migration followed by a Diels-Alder reaction.

Box-copper catalyzed cascade asymmetric amidation for chiral exo-methylene aminoindoline derivatives

Enantioselective copper-catalyzed cascade inter- and intramolecular amidation was achieved between ethynyl benzoxazinanones and α-halohydroxamates in the presence of an indapybox ligand. The one-pot cascade transformation was triggered by the attack of hydroxamates to dipolar copper-allenylidene intermediates, followed by a nucleophilic annulation reaction. Thus, a series of exo-methylene 3-aminoindoline derivatives were obtained in good yields with high enantioselectivities under mild reaction conditions.

Isothiourea-Catalyzed Enantioselective α-Alkylation of Esters via 1,6-Conjugate Addition to para-Quinone Methides

The isothiourea-catalyzed enantioselective 1,6-conjugate addition of para-nitrophenyl esters to 2,6-disubstituted para-quinone methides is reported. para-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the para-nitrophenyl ester, is proposed to facilitate catalyst turnover in this transformation. A range of para-nitrophenyl ester products can be isolated, or derivatized in situ by addition of benzylamine to give amides at up to 99% yield. Although low diastereocontrol is observed, the diastereoisomeric ester products are separable and formed with high enantiocontrol (up to 94:6 er).

Ru-NHC-Catalyzed Asymmetric Hydrogenation of 2-Quinolones to Chiral 3,4-Dihydro-2-Quinolones

Direct enantioselective hydrogenation of unsaturated compounds to generate chiral three-dimensional motifs is one of the most straightforward and important approaches in synthetic chemistry. We realized the Ru(II)-NHC-catalyzed asymmetric hydrogenation of 2-quinolones under mild reaction conditions. Alkyl-, aryl- and halogen-substituted optically active dihydro-2-quinolones were obtained in high yields with moderate to excellent enantioselectivities. The reaction provides an efficient and atom-economic pathway to construct simple chiral 3,4-dihydro-2-quinolones. The desired products could be further reduced to tetrahydroquinolines and octahydroquinolones.

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Copper/Lewis base cooperatively catalyzed asymmetric allylic alkylation of Morita-Baylis-Hillman carbonates with azomethine ylides

In this paper, an asymmetric allylic alkylation of easily available azomethine ylides with Morita-Baylis-Hillman (MBH) carbonates through a copper (i)/Lewis base cooperative catalysis strategy has been realized. The co-catalyzed asymmetric allylic alkylation provided the corresponding amino acid derivatives in up to 90% yields with up to 99% ee as well as good to excellent regioselectivity.

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.

Enantioselective Synthesis of α-Aryl-β2 -Amino-Esters by Cooperative Isothiourea and Brønsted Acid Catalysis

The synthesis of α-aryl-β2 -amino esters through enantioselective aminomethylation of an arylacetic acid ester in high yields and enantioselectivity via cooperative isothiourea and Brønsted acid catalysis is demonstrated. The scope and limitations of this process are explored (25 examples, up to 94 % yield and 96:4 er), with applications to the synthesis of (S)-Venlafaxine⋅HCl and (S)-Nakinadine B. Mechanistic studies are consistent with a C(1)-ammonium enolate pathway being followed rather than an alternative dynamic kinetic resolution process. Control studies indicate that (i) a linear effect between catalyst and product er is observed; (ii) an acyl ammonium ion can be used as a precatalyst; (iii) reversible isothiourea addition to an in situ generated iminium ion leads to an off-cycle intermediate that can be used as a productive precatalyst.

CuI-Catalyzed Decarboxylative Thiolation of Propargylic Cyclic Carbonates/Carbamates to Access Allenyl Thioethers

The first CuI-catalyzed decarboxylative thiolation of terminal alkyne-substituted cyclic carbonates/carbamates to access allenes has been developed. A wide range of hydroxymethyl- and aminomethyl-containing allenyl thioethers were smoothly obtained in good to excellent yields under mild conditions. The copper-allenylidene intermediate among the process is crucial to the decarboxylative thiolation reaction. This method opens up a new channel to access allenyl thioether compounds.

Synthesis of Tetra-Substituted Trifluoromethyl-3,1-Benzoxazines by Transition-Metal-Catalyzed Decarboxylative Cyclization of N-Benzoyl Benzoxazinones

Efficient synthesis of N,O-heterocyclic tetra-substituted trifluoromethyl-3,1-benzoxazines via a transition-metal-catalyzed decarboxylative intramolecular cyclization was achieved. The decarboxylation of N-benzoyl trifluoromethyl-benzoxazinones generated the amide oxygen nucleophile, allowing a selective internal C1 -attack on Pd- or Cu-coordinated zwitterions, affording medicinally attractive tetra-substituted vinyl- or ethynyl-trifluoromethyl-3,1-benzoxazines. This protocol can be applied to the synthesis of perfluoroalkyl- and non-fluorinated 3,1-benzoxazines.