Carbene-Catalyzed Atroposelective Construction of Chiral Diaryl Ethers.

Atropoisomeric chemotypes of diaryl ethers-related scaffolds are prevalent in naturally active compounds. Nevertheless, there remains considerable research to be carried out on the catalytic asymmetric synthesis of these axially chiral molecules. In this instance, we disclose an N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. NHC desymmetrization produces axially chiral diaryl ether atropisomers with high yields and enantioselectivities in moderate circumstances. Chiral diaryl ether compounds may be precursors for highly functionalized diaryl ethers with bioactivity and chiral ligands for asymmetric catalysis.

Synthesis of silyl indenes by ruthenium-catalyzed aldehyde- and acylsilane-enabled C-H alkylation/cyclization.

A ruthenium-catalyzed C-H alkylation/cyclization sequence is presented to prepare silyl indenes with atom and step-economy. This domino reaction is triggered by acyl silane-directed C-H activation, and an aldehyde controlled the following enol cyclization/condensation other than ß-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyl indenes allows access to a diverse range of interesting indene and indanone derivatives.

Stereoselective Synthesis of Complex Polyenes through Sequential α-/ß-C-H Functionalization of trans-Styrenes.

Sequential C-H functionalization of molecules containing multiple C-H bonds can efficiently lead to structural diversity. Herein we present the first chelation-assisted sequential α-/ß-C-H functionalization of E-styrenes with simple alkenes and alkynes in excellent regio- and stereo-selectivity. The process involves α C-H functionalization by six-membered exo-cyclopalladation to result in tri- and tetrasubstituted 1,3-dienes and ß C-H functionalization through seven-membered endo-cyclopalladation to produce tetra- and pentasubstituted 1,3,5-trienes in up to 97 % yield with up to >99/1 E/Z selectivity, both enabled by the chelation assistance of pyrazinamide. The protocol is demonstrated to be widely applicable, tolerant to a wide range of functional groups and bioactive fragments, and suitable for gram-scale synthesis as well as one-pot and two step preparation of trienes. Mechanistic experiments and density functional theory (DFT) calculations were performed to elucidate the selectivity and reactivity.

Chiral NHC-Catalyzed [4+2] Cycloadditions: Highly Diastereo/Enantioselective Access to Spiro[cyclohex-4-ene-1,3'-indoles] and DFT Calculations.

A highly efficient NHC-catalyzed cycloaddition of (E)-alkenylisatins and γ-chloroenals with a broad substrate scope has been developed to provide spiro[cyclohex-4-ene-1,3'-indole] in good yields (up to 99 % yield) with excellent diastereo- and enantioselectivities (up to >20 : 1 d.r., >99 % ee) under mild conditions without the use of metal and additives. Based on computational investigations, the role of the NHC on the diastereo- and enantioselectivity is discussed.

NHC-Activations on α-, ß-, γ-, and Beyond.

Over the recent decades, due to the special electronic characteristics and diverse reactivities, N-heterocyclic carbene (NHC) has received significant interest in organocatalyzed reactions. The formation of Breslow intermediates by NHC can convert into acyl anion equivalent, enolates, homoenolate, acyl azolium, and vinyl enolate etc., and the cycloaddition reactions of these species has attracted lots of attention. In this review, we focus on the summry of the development of NHC-activation of carbonyl carbon (or imine carbon) in situ, α-, ß-, γ-, and beyond, and the cycloaddition reaction of these species.

Functionality-Directed Regio- and Enantio-Selective Olefinic C-H Functionalization of Aryl Alkenes.

Aryl alkenes represents one of the most widely occurring structural motif in countless drugs and natural products, and direct C-H functionalization of aryl alkenes provides atom- step efficient access toward valuable analogues. Among them, group-directed selective olefinic α- and ß-C-H functionalization, bearing a directing group on the aromatic ring, has attracted remarkable attentions, including alkynylation, alkenylation, amino-carbonylation, cyanation, domino cyclization and so on. These transformations proceed by endo- and exo-C-H cyclometallation and provide aryl alkene derivatives in excellent site- stereo-selectivity. Enantio-selective α- and ß- olefinic C-H functionalization were also covered to synthesis axially chiral styrenes.

Integrating network analysis and experimental validation to reveal the mitophagy-associated mechanism of Yiqi Huoxue (YQHX) prescription in the treatment of myocardial ischemia/reperfusion injury.

Myocardial ischemia/reperfusion (I/R) injury is the main cause of increasing postischemic heart failure and currently there is no definite treatment for myocardial I/R injury. It has been suggested that oxidative stress-induced mitochondrial dysfunction plays an important role in the pathological development of myocardial I/R. In this study, Yiqi Huoxue (YQHX) prescription, as a kind of Chinese herbal formula, was developed and shown to alleviate I/R injury. Network analysis combined with ultrahigh-performance liquid chromatography-high resolution mass spectrometry expounded the active components of YQHX and revealed the mitophagy-regulation mechanism of YQHX treating I/R injury. In vivo experiments confirmed YQHX significantly alleviated I/R myocardial injury and relieved oxidative stress. In vitro experiments validated that YQHX could relieve hypoxia/reoxygenation injury and attenuate oxidative stress via improving the structure and function of mitochondria, which was strongly related to regulating mitophagy. In summary, this study demonstrated that YQHX, which could alleviate I/R injury via targeting mitophagy, might be a potential therapeutic strategy for myocardial I/R injury.

Chelation-assisted iridium-catalyzed hydroalkenylation and hydroarylation/cyclization with conjugated trienes.

Iridium-catalyzed hydroalkenylation of conjugated trienes by chelation-assisted alkenyl C-H activation of acrylamides has been demonstrated to produce 1,4,6-trienes atom efficiently with excellent regio- and E/Z selectivities. In contrast, the reaction of benzamides and 1,3,5-trienes proceeds by a tandem hydroarylation of the trienes and cyclization via intramolecular 1,2-addition, providing valuable trans-tetrahydroisoquinolinone derivatives. A broad range of aromatic and aliphatic 1,3,5-trienes bearing various functionalities were compatible to deliver target products with high yields and E/Z selectivity. The successful gram-scale preparation and selective hydrogenation to give the alkylation product further demonstrates the practicability of this protocol to potential applications.

A Dynamic Kinetic Resolution Approach to Axially Chiral Diaryl Ethers by Catalytic Atroposelective Transfer Hydrogenation.

Diaryl ethers are widespread in biologically active compounds, ligands and catalysts. It is known that the diaryl ether skeleton may exhibit atropisomerism when both aryl rings are unsymmetrically substituted with bulky groups. Despite recent advances, only very few catalytic asymmetric methods have been developed to construct such axially chiral compounds. We describe herein a dynamic kinetic resolution approach to axially chiral diaryl ethers via a Brønsted acid catalyzed atroposelective transfer hydrogenation (ATH) reaction of dicarbaldehydes with anilines. The desired diaryl ethers could be obtained in moderate to good chemical yields (up to 79 %) and high enantioselectivities (up to 95 % ee) under standard reaction conditions. Such structural motifs are interesting precursors for further transformations and may have potential applications in the synthesis of chiral ligands or catalysts.

Recent advances in chelation-assisted site- and stereoselective alkenyl C-H functionalization.

Olefinic C-H functionalization represents an atom- and step economic approach to valuable olefin derivatives from simpler ones, but controlling the selectivity remains a challenge. Remarkable progress has been made in the site-selective C-H functionalization of arenes and alkanes, but there are still limited examples of selective C-H functionalization of olefins presumably due to the lability and easy decomposition of the alkenyl moiety. Chelation-assisted C-H activation represents an efficient protocol for site- and stereo-selective construction of carbon-carbon and carbon-heteroatom bonds. This review highlights recent advances in vicinal- and geminal-group-directed olefinic C-H functionalization, including alkenylation, arylation, alkynylation, alkylation, halogenation, silylation, cyanation and annulation by the formation of exo-/endo-metallocycles. In particular, geminal-group-directed C-H functionalization is covered for the first time, as well as distal-selective alkenyl C-H functionalization under palladium/norbornene cooperative catalysis, which provides novel disconnections in retrosynthetic analysis and represents the future trend in green chemistry.

Ruthenium-catalyzed C-H amination of aroylsilanes.

Acylsilane represents a valuable synthon in synthetic chemistry. We report on ruthenium(ii)-catalyzed ortho-C-H amination of aroylsilanes to provide facile access to synthetically useful imidobenzoylsilanes and tosyl-amidobenzoylsilanes. The protocols, with broad substrate scope and excellent functional group tolerance, are enabled with the weak chelation-assistance of acylsilane via C-H cyclometallation.

Nitrosobenzene-Enabled Chiral Phosphoric Acid Catalyzed Enantioselective Construction of Atropisomeric N-Arylbenzimidazoles.

Described herein is an imidazole ring formation strategy for the synthesis of axially chiral N-arylbenzimidazoles by means of chiral phosphoric acid catalysis. Two sets of conditions were developed to transform two classes of 2-naphthylamine derivatives into structurally diverse N-arylbenzimidazole atropisomers with excellent chemo- and regioselectivity as well as high levels of enantiocontrol. It is worth reflecting on the unique roles played by the nitroso group in this domino reaction. It functions as a linchpin by first offering an electrophilic site (N) for the initial C-N bond formation while the resulting amine performs the nucleophilic addition to form the second C-N bond. Additionally, it could facilitate the final oxidative aromatization as an oxidant. The atropisomeric products could be conveniently elaborated to a series of axially chiral derivatives, enabling the exploitation of N-arylbenzimidazoles for their potential utilities in asymmetric catalysis.

Pd-Catalyzed Dearomative Asymmetric Allylic Alkylation of Naphthols with Alkoxyallenes.

A Pd-catalyzed linear selective intermolecular asymmetric dearomative allylic alkylation reaction of naphthols with alkoxyallenes under mild reaction conditions is reported. The transformation is successfully promoted by Pd2(dba)3 and the chiral Trost ligand and provides a general atom-efficient protocol to obtain various ß-naphthalenones bearing an all carbon quaternary stereogenic center in good yields and chemo- and stereoselectivities.

Stereoselective and Atom-Economic Alkenyl C-H Allylation/Alkenylation in Aqueous Media by Iridium Catalysis.

A practical and atom-economic protocol for the stereoselective preparation of various 1,4- and 1,3-diene skeletons through iridium-catalyzed directed olefinic C-H allylation and alkenylation of NH-Ts acrylamides in water was developed. This reaction tolerated a wide scope of substrates under simple reaction conditions and enabled successful gram-scale preparation. Furthermore, an asymmetric variant of this reaction giving enantioenriched 1,4-dienes was achieved employing a chiral diene-iridium complex as the catalyst.

Design and Atroposelective Construction of IAN analogues by Organocatalytic Asymmetric Heteroannulation of Alkynes.

An organocatalytic atroposelective strategy for accessing enantioenriched axially chiral IAN analogues was developed for the first time. A class of novel atropisomeric C2-arylquinoline skeletons were synthesized with high enantiocontrol via chiral phosphoric-acid-catalyzed heteroannulation of in situ generated vinylidene ortho-quinone methide (VQM) intermediates with ortho-aminophenones. The strategy tolerated a broad substrate scope, providing a facile organocatalytic approach to IAN analogues in good yields and excellent enantioselectivities under mild reaction conditions. Moreover, the synthetic utility of this methodology was illustrated through further transformations into IAN-type ligand and axially chiral thiourea.

Palladium-Catalyzed Allylic Substitution Reaction of Benzothiazolylacetamide with Allylic Alcohols in Water.

An efficient tetrakis(triphenylphosphine)palladium- and Brønsted acid catalyzed allylic substitution reaction of benzothiazolylacetamide with allylic alcohols in water has been developed, and the corresponding allylated products were afforded in good to excellent (up to 99%) yields with high regioselectivities. This straightforward protocol exhibits good functional group tolerance and scalability.

N-Heterocyclic Carbene-Catalyzed Asymmetric Benzoin Reaction in Water.

A chiral N-heterocyclic carbene-catalyzed benzoin condensation reaction in water has been developed, affording α-hydroxy ketones in good to high yields and high enantioselectivities. Water was proposed as a proton shuttle in the aqueous asymmetric condensation reaction.

Correction: Chiral NHC-catalyzed 1,3-dipolar [3 + 2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones.

Correction for 'Chiral NHC-catalyzed 1,3-dipolar [3 + 2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones' by Limin Yang, et al., Org. Biomol. Chem., 2018, 16, 4433-4438.

Chiral NHC-catalyzed 1,3-dipolar [3 + 2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones.

A chiral N-heterocyclic carbene (NHC)-catalyzed 1,3-dipolar [3 + 2] cycloaddition reaction of 3-oxopyrazolidin-1-ium-2-ides and α-chloroaldehydes was developed for the synthesis of pyrazolo[1,2-a]pyrazole-1,7-diones in moderate to good yields (up to 76%) with good enantioselectivities (up to 98% ee) and diastereoselectivities (up to 8 : 1 dr).

Copper-Catalyzed Oxidative Cyclization of 2-Amino-azaarenes with Lignin Models: Synthesis of 3-Phenoxy Imidazo Heterocycles.

A catalytic oxidative cyclization of 2-aminopyridines or 2-aminobenzothiazole with 2-phenoxyacetophenones (a kind of lignin platform compound) was developed, efficiently providing valuable 3-phenoxy imidazo[1,2-a]pyridines or 3-phenoxy benzo[d]imidazo[2,1-b]thiazoles. The reaction was realized under oxygen by simply using inexpensive CuI as the catalyst.