Recent Advances in the Development of Chiral Gold Complexes for Catalytic Asymmetric Catalysis

Asymmetric gold catalysis has been rapidly developed in the past ten years. Breakthroughs have been made by rational design and meticulous selection of chiral ligands. This review summarizes newly developed gold-catalyzed enantioselective organic transformations and recent progress in ligand design (since 2016), organized according to different types of chiral ligands, including bisphosphine ligands, monophosphine ligands, phosphite-derived ligands, and N-heterocyclic carbene ligands for asymmetric gold(I) catalysis as well as heterocyclic carbene ligands and oxazoline ligands for asymmetric gold(III) catalysis.

Enantioselective Pd0 -Catalyzed C(sp2 )-H Arylation for the Synthesis of Chiral Warped Molecules

C-H activation-based ring-forming methods are a powerful approach for the construction of complex molecular architectures, especially those containing a congested stereocenter. Therefore, this strategy seems perfectly suited to address the synthesis of chiral polycyclic aromatic hydrocarbons (PAHs) and bowl-shaped molecules, which are important target molecules in the field of organic electronic materials. Herein, we describe an enantioselective Pd⁰ -catalyzed C(sp² )-H arylation protocol for the synthesis of chiral fluoradenes and other warped molecules, which could serve to the bottom-up construction of chiral PAHs. The current approach relies on the use of chiral bifunctional phosphine-carboxylate ligands and delivers diverse polycyclic compounds in high yield and with good to excellent enantioselectivity. The chiroptical properties of the obtained products were investigated, and some of them were found to have a strong ellipticity and an emission band located in the visible region.

Uptake, Acropetal Translocation, and Enantioselectivity of Perfluorooctane Sulfonate in Maize Coexisting with Copper

Plant uptake and translocation of perfluorooctane sulfonate (PFOS) are critical for food safety and raise major concerns. However, those processes are associated with many undisclosed mechanisms, especially when PFOS coexist with heavy metals. In this study, we investigated the effect of copper (Cu) on PFOS distribution in maize tissues by assessing the PFOS concentration and enantioselectivity. The presence of <100 μmol/L Cu exerted a limited effect on PFOS bioaccumulation, while >100 μmol/L Cu damaged the root cell membrane and increased root permeability, resulting in a higher PFOS concentration in roots. The suppression of acropetal translocation might be attributed to Cu inhibition of carrier proteins. The enantiomer fraction (EF) of 1m-PFOS at <100 μmol/L Cu was higher than that in a commercial product (0.5). Racemic PFOS was detected at >100 μmol/L Cu in roots and the EF variation changed from positive to negative in shoots. These EF results evidenced the existence of a protein-mediated uptake pathway. Besides, this study indicated the challenge of chiral signature application in PFOS source identification, given the effects of heavy metals and plants on PFOS enantioselectivity. The findings provide insight into PFOS bioaccumulation in plants cocontaminated with Cu and will facilitate environmental risk assessment.

Construction of Bridged Polycyclic Skeletons via Transition-Metal Catalyzed Carbon-Carbon Bond-Forming Reactions

Transition-metal catalysis has become one of most important methods for constructing molecules with diverse architectures. Bridged polycyclic skeletons are often considered one of most challenging structures in organic synthesis. This Minireview summarizes the recent progress on synthesis of bridged polycyclic skeletons by transition-metal-catalyzed carbon-carbon bond-forming reaction. Four main ring-forming strategies including connection via olefin or carbonyl functionality, enolate intermediacy, C-H functionality, and aryl functionality are detailed and some effective methods are discussed with particular emphasis on reaction design and mechanism.

Enantioselective Synthesis of 3,3'-Disubstituted 2-Amino-2'-hydroxy-1,1'-binaphthyls by Copper-Catalyzed Aerobic Oxidative Cross-Coupling

A challenging direct asymmetric catalytic aerobic oxidative cross-coupling of 2-naphthylamine and 2-naphthol, using a novel Cu^I /SPDO system, has been successfully developed for the first time. Enantioenriched 3,3'-disubstituted NOBINs were achieved and could be readily derived to divergent chiral ligands and catalysts. This reaction features high enantioselectivities (up to 96 % ee) and good yields (up to 80 %). The DFT calculations suggest that the F-H interactions between CF₃ of L17 and H-1,8 of 2-naphthol, and the π-π stacking between the two coupling partners could play vital roles in the enantiocontrol of this cross-coupling reaction.

An Enantioselective e-Nose: An Array of Nanoporous Homochiral MOF Films for Stereospecific Sensing of Chiral Odors

Chirality is essential in nature and often pivotal for biological information transfer, for example, via odor messenger molecules. While the human nose can distinguish the enantiomers of many chiral odors, the technical realization by an artificial sensor or an electronic nose, e-nose, remains a challenge. Herein, we present an array of six sensors coated with nanoporous metal-organic framework (MOF) films of different homochiral and achiral structures, working as an enantioselective e-nose. While the achiral-MOF-film sensors show identical responses for both isomers of one chiral odor molecule, the responses of the homochiral MOF films differ for different enantiomers. By machine learning algorithms, the combined array data allow the stereoselective identification of all compounds, here tested for five pairs of chiral odor molecules. We foresee the chiral-MOF-e-nose, able to enantioselectively detect and discriminate chiral odors, to be a powerful approach towards advanced odor sensing.

A novel lipase from Aspergillus oryzae WZ007 catalyzed synthesis of brivaracetam intermediate and its enzymatic characterization

Brivaracetam is a structural derivative of the chiral drug levetiracetam and has been approved for the adjuvant treatment of partial epilepsy. As a new antiepileptic drug, it is widely used in a variety of epilepsy models. In this study, a novel lipase M16 derived from Aspergillus oryzae WZ007 was cloned, expressed, and used for chiral resolution. Lipase M16 has a high enantioselectivity to the racemic substrate (R,S)-methyl 2-propylsuccinate 4-tert-butyl ester, and the intermediate (R)-2-propylsuccinic acid 4-tert-butyl ester of brivaracetam was obtained efficiently. Under optimal conditions, the enantiomeric excess of substrate was up to 99.26%, and the e.e.p was 96.23%. The conversion and apparent E value were 50.63% and 342.48, respectively. This study suggests a new biocatalytic resolution via lipase M16 for preparing the brivaracetam chiral intermediate and its potential application in the pharmaceutical industry.

Enantioselective Sulfoxidation of Thioanisole by Cascading a Choline Oxidase and a Peroxygenase in the Presence of Natural Deep Eutectic Solvents

A bienzymatic cascade for selective sulfoxidation is presented. The evolved recombinant peroxygenase from Agrocybe aegeritra catalyses the enantioselective sulfoxidation of thioanisole whereas the choline oxidase from Arthrobacter nicotianae provides the H₂ O₂ necessary via reductive activation of ambient oxygen. The reactions are performed in choline chloride-based deep eutectic solvents serving as co-solvent and stoichiometric reductant at the same time. Very promising product concentrations (up to 15 mM enantiopure sulfoxide) and catalyst performances (turnover numbers of 150,000 and 2100 for the peroxygenase and oxidase, respectively) have been achieved.

Synthetic Processes toward Nitriles without the Use of Cyanide: A Biocatalytic Concept Based on Dehydration of Aldoximes in Water

While belonging to the most fundamental functional groups, nitriles represent a class of compound that still raises challenges in terms of an efficient, cost‐effective, general and, at the same time, sustainable way for their synthesis. Complementing existing chemical routes, recently a cyanide‐free enzymatic process technology based on the use of an aldoxime dehydratase (Oxd) as a biocatalyst component has been developed and successfully applied for the synthesis of a range of nitrile products. In these biotransformations, the Oxd enzymes catalyze the dehydration of aldoximes as readily available substrates to the nitrile products. Herein, these developments with such enzymes are summarized, with a strong focus on synthetic applications. It is demonstrated that this biocatalytic technology has the potential to “cross the bridge” between the production of fine chemicals and pharmaceuticals, on one hand, and bulk and commodity chemicals, on the other.

CYP2D6*10 polymorphism and the enantioselective O-desmethylation of S-(+)- and R-(-)-venlafaxine in Japanese psychiatric patients

According to previous studies, R-(-)-venlafaxine (VEN) has higher enantioselectivity than S-(+)-VEN, and the plasma concentration of R-(-)-VEN varies depending on CYP2D6 activity. Therefore, we examined the pharmacokinetic effects of CYP2D6*10 genotypes on the steady-state concentrations of the enantiomers of VEN. The individuals were 71 Japanese depressed patients treated with racemic VEN. The concentrations of the enantiomers of VEN and O-desmethylvenlafaxine (ODV) were measured. Polymerase chain reaction (PCR) was used to determine the CYP2D6*10 genotypes. The plasma concentrations of S-(+)-VEN were approximately 1.9-fold higher than those of R-(-)-VEN. The plasma concentrations of S-(+)-VEN and R-(-)-VEN seemed to be higher in individuals with two mutant alleles of CYP2D6*10, although no significant differences were found in the plasma levels of S-(+)-VEN and R-(-)-VEN between CYP2D6*10 genotypes. The number of mutant alleles of CYP2D6*10 was a significant factor associated with the R-(-)-ODV/R-(-)-VEN ratio (P = .004) in multiple regression analysis. This suggests that CYP2D6*10 mutations affect the metabolism of R-(-)-VEN and S-(+)-VEN. Further studies are needed to examine how these findings affect clinical practice.

Enantioselective fate of famoxadone during processing of apple cider and grape wine

Famoxadone enantiomers were separated on Lux Amylose-1 chiral column and determined by ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). The half-lives of R-(-)-famoxadone and S-(+)-famoxadone were 69.3 and 86.6 h in apple cider, 231.0 and 346.5 h in apple pomace, 69.3 and 77.0 h in grape wine, and 231.0 and 346.5 h in grape pomace, respectively. The enantiomeric fraction (EF) values decreased gradually from 0.498, 0.499, and 0.500 (0 h) to 0.404, 0.374, and 0.427 (144 h) and then increased gradually to 0.474, 0.427, and 0.422 (312 h) in apple cider, grape wine, and grape pomace. The EF value in apple pomace decreased gradually from 0.499 (0 h) to 0.450 (168 h) and then increased gradually to 0.482 (312 h). The processing factors (PFs) for famoxadone ranged from 0.014 to 0.024 in the overall process. The residue of famoxadone reduced 94.7-97.4% after the fermentation process.

Facile Stereoselective Reduction of Prochiral Ketones by using an F420 -dependent Alcohol Dehydrogenase

Effective procedures for the synthesis of optically pure alcohols are highly valuable. A commonly employed method involves the biocatalytic reduction of prochiral ketones. This is typically achieved by using nicotinamide cofactor-dependent reductases. In this work, we demonstrate that a rather unexplored class of enzymes can also be used for this. We used an F420 -dependent alcohol dehydrogenase (ADF) from Methanoculleus thermophilicus that was found to reduce various ketones to enantiopure alcohols. The respective (S) alcohols were obtained in excellent enantiopurity (>99 % ee). Furthermore, we discovered that the deazaflavoenzyme can be used as a self-sufficient system by merely using a sacrificial cosubstrate (isopropanol) and a catalytic amount of cofactor F420 or the unnatural cofactor FOP to achieve full conversion. This study reveals that deazaflavoenzymes complement the biocatalytic toolbox for enantioselective ketone reductions.

Enantioselective Total Synthesis of Andrographolide and 14-Hydroxy-Colladonin: Carbonyl Reductive Coupling and trans-Decalin Formation by Hydrogen Transfer

An enantioselective total synthesis of the labdane diterpene andrographolide, the bitter principle of the herb Andrographis paniculata (known as "King of Bitters"), was accomplished in 14 steps (LLS). Key transformations include iridium-catalyzed carbonyl reductive coupling to form the quaternary C4 stereocenter, diastereoselective alkene reduction to establish the trans-decalin ring, and carbonylative lactonization to install the α-alkylidene-β-hydroxy-γ-butyrolactone.

Exploring Free Energy Profiles of Enantioselective Organocatalytic Aldol Reactions under Full Solvent Influence

We present a computational study on the enantioselectivity of organocatalytic proline-catalyzed aldol reactions between aldehydes in dimethylformamide (DMF). To explore the free energy surface of the reaction, we apply two-dimensional metadynamics on top of ab initio molecular dynamics (AIMD) simulations with explicit solvent description on the DFT level of theory. We avoid unwanted side reactions by utilizing our newly developed hybrid AIMD (HyAIMD) simulation scheme, which adds a simple force field to the AIMD simulation to prevent unwanted bond breaking and formation. Our condensed phase simulation results are able to nicely reproduce the experimental findings, including the main stereoisomer that is formed, and give a correct qualitative prediction of the change in syn:anti product ratio with different substituents. Furthermore, we give a microscopic explanation for the selectivity. We show that both the explicit description of the solvent and the inclusion of entropic effects are vital to a good outcome-metadynamics simulations in vacuum and static nudged elastic band (NEB) calculations yield significantly worse predictions when compared to the experiment. The approach described here can be applied to a plethora of other enantioselective or organocatalytic reactions, enabling us to tune the catalyst or determine the solvent with the highest stereoselectivity.

Organocatalytic Enantioselective 1,3-Dipolar [6+4] Cycloadditions of Tropone

A highly stereoselective 1,3-dipolar [6+4] cycloaddition towards bridged azabicyclo[4.3.1]decane scaffolds has been developed, reacting aldehydes, 2-aminomalonates and tropone under mild conditions in the presence of a chiral phosphoric acid catalyst. The scope is demonstrated for a series of aldehydes and 2-aminomalonates, and the reaction proceeds in high yields, >95:5 d.r. and up to 99 % ee. A series of transformations, as well as a mechanistic proposal, are presented.

Strategies for the Catalytic Enantioselective Synthesis of α-Trifluoromethyl Amines

The exploitation of the α-trifluoromethylamino group as an amide surrogate in peptidomimetics and drug candidates has been on the rise. In a large number of these cases, this moiety bears stereochemistry with the stereochemical identity having important consequences on numerous molecular properties, such as the potency of the compound. Yet, the majority of stereoselective syntheses of α-CF3 amines rely on diastereoselective couplings with chiral reagents. Concurrent with the rapid expansion of fluorine into pharmaceuticals has been the development of catalytic enantioselective means of preparing α-trifluoromethyl amines. In this work, we outline the strategies that have been employed for accessing these enantioenriched amines, including normal polarity approaches and several recent developments in imine umpolung transformations.

Oligomeric (Salen)Mn(III) Complexes Featuring Tartrate Linkers Immobilized over Layered Double Hydroxide for Catalytically Asymmetric Epoxidation of Unfunctionalized Olefins

A series of oligomeric (salen)Mn(III) complexes featuring tartrate linkers were prepared and immobilized over layered double hydroxide, and then used as catalysts for asymmetric epoxidation of unfunctionalized olefins. Comprehensive characterizations including ¹H NMR, FT-IR, UV-Vis, elemental analysis, GPC, and ICP-AES were used to illustrate structures of oligomeric (salen)Mn(III) complexes, while powdered XRD, nitrogen physisorption, together with XPS studies provided further details to detect structures of heterogeneous catalysts. Interestingly, scanning electron microscopy found an interesting morphology change during modification of layered supporting material. Catalytic experiments indicated that configuration of major epoxide products was determined by salen chirality more than that of tartrate linker, but enantioselectivity (e.e. values) could be enhanced when tartrate and salen showed identical chiral configurations. Furthermore, the (R,R)-salen moieties linked with (R,R)-tartrate spacers usually offered higher enantioselectivity compared to other combinations. Lastly, Zn(II)/Al(III) layered double hydroxide played as a rigid supporting material in catalysis, showing positive chiral induction and high recycling potential in catalytic reactions.

Copper-Catalyzed Carbonylative Hydroamidation of Styrenes to Branched Amides

Amides are one of the most ubiquitous functional groups in synthetic and medicinal chemistry. Novel and rapid synthesis of amides remains in high demand. In this communication, a general and efficient procedure for branch-selective hydroamidation of vinylarenes with hydroxyamine derivatives enabled by copper catalysis has been developed for the first time. The reaction proceeds under mild conditions and tolerates a broad range of functional groups. Applying a chiral phosphine ligand, an enantioselective variant of this transformation was achieved, affording a variety of chiral α-amides with excellent enantioselectivities (up to 99 % ee) and high yields.

Near-Enantiopure Trimerization of 9-Ethynylphenanthrene on a Chiral Metal Surface

Enantioselectivity in heterogeneous catalysis strongly depends on the chirality transfer between catalyst surface and all reactants, intermediates, and the product along the reaction pathway. Herein we report the first enantioselective on-surface synthesis of molecular structures from an initial racemic mixture and without the need of enantiopure modifier molecules. The reaction consists of a trimerization via an unidentified bonding motif of prochiral 9-ethynylphenanthrene (9-EP) upon annealing to 500 K on the chiral Pd₃ -terminated PdGa{111} surfaces into essentially enantiopure, homochiral 9-EP propellers. The observed behavior strongly contrasts the reaction of 9-EP on the chiral Pd₁ -terminated PdGa{111} surfaces, where 9-EP monomers that are in nearly enantiopure configuration, dimerize without enantiomeric excess. Our findings demonstrate strong chiral recognition and a significant ensemble effect in the PdGa system, hence highlighting the huge potential of chiral intermetallic compounds for enantioselective synthesis and underlining the importance to control the catalytically active sites at the atomic level.

Intramolecular C(sp3 )-H Bond Oxygenation by Transition-Metal Acylnitrenoids

This study demonstrates for the first time that easily accessible transition‐metal acylnitrenoids can be used for controlled direct C(sp³)‐H oxygenations. Specifically, a ruthenium catalyst activates N‐benzoyloxycarbamates as nitrene precursors towards regioselective intramolecular C−H oxygenations to provide cyclic carbonates, hydroxylated carbamates, or 1,2‐diols. The method can be applied to the chemoselective C−H oxygenation of benzylic, allylic, and propargylic C(sp³)−H bonds. The reaction can be performed in an enantioselective fashion and switched in a catalyst‐controlled fashion between C−H oxygenation and C−H amination. This work provides a new reaction mode for the regiocontrolled and stereocontrolled conversion of C(sp³)‐H into C(sp³)−O bonds.

Enantioselective Fluorescent Recognition of Free Amino Acids: Challenges and Opportunities

Fluorescent probes that can discriminate enantiomers of amino acids in organic media or aqueous solution are discussed. This Minireview focuses on recent progress in the studies of three classes of probes including those made of cyclodextrins, 1,1'-binaphthyl compounds, and nanomaterials, and uses them to illustrate the design strategies, applications, and limitations in this area. These probes are potentially useful for rapid analysis of asymmetric reactions for amino acid synthesis as well as the real-time imaging of amino acids in biological systems. The challenges in these applications are analyzed. Working in this field of enantioselective fluorescent recognition of amino acids offers great opportunities to make new scientific discoveries and to develop important practical applications.

Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3-Diradical Intermediate and Deracemization by a Chiral Sensitizer

The photochemical deracemization of spiro[cyclopropane‐1,3′‐indolin]‐2′‐ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2‐dichloro compound is configurationally labile upon direct irradiation at λ=350 nm and upon irradiation at λ=405 nm in the presence of achiral thioxanthen‐9‐one as the sensitizer. The triplet 1,3‐diradical intermediate generated in the latter reaction was detected by transient absorption spectroscopy and its lifetime determined (τ=22 μs). Using a chiral thioxanthone or xanthone, with a lactam hydrogen bonding site as a photosensitizer, allowed the deracemization of differently substituted chiral spirocyclopropyl oxindoles with yields of 65–98 % and in 50–85 % ee (17 examples). Three mechanistic contributions were identified to co‐act favorably for high enantioselectivity: the difference in binding constants to the chiral thioxanthone, the smaller molecular distance in the complex of the minor enantiomer, and the lifetime of the intermediate 1,3‐diradical.

A Simple Biosystem for the High-Yielding Cascade Conversion of Racemic Alcohols to Enantiopure Amines

The amination of racemic alcohols to produce enantiopure amines is an important green chemistry reaction for pharmaceutical manufacturing, requiring simple and efficient solutions. Herein, we report the development of a cascade biotransformation to aminate racemic alcohols. This cascade utilizes an ambidextrous alcohol dehydrogenase (ADH) to oxidize a racemic alcohol, an enantioselective transaminase (TA) to convert the ketone intermediate to chiral amine, and isopropylamine to recycle PMP and NAD⁺ cofactors via the reversed cascade reactions. The concept was proven by using an ambidextrous CpSADH-W286A engineered from (S)-enantioselective CpSADH as the first example of evolving ambidextrous ADHs, an enantioselective BmTA, and isopropylamine. A biosystem containing isopropylamine and E. coli (CpSADH-W286A/BmTA) expressing the two enzymes was developed for the amination of racemic alcohols to produce eight useful and high-value (S)-amines in 72-99 % yield and 98-99 % ee, providing with a simple and practical solution to this type of reaction.

Isothiourea-Catalyzed Acylative Kinetic Resolution of Tertiary α-Hydroxy Esters

A highly enantioselective isothiourea-catalyzed acylative kinetic resolution (KR) of acyclic tertiary alcohols has been developed. Selectivity factors of up to 200 were achieved for the KR of tertiary alcohols bearing an adjacent ester substituent, with both reaction conversion and enantioselectivity found to be sensitive to the steric and electronic environment at the stereogenic tertiary carbinol centre. For more sterically congested alcohols, the use of a recently-developed isoselenourea catalyst was optimal, with equivalent enantioselectivity but higher conversion achieved in comparison to the isothiourea HyperBTM. Diastereomeric acylation transition state models are proposed to rationalize the origins of enantiodiscrimination in this process. This KR procedure was also translated to a continuous-flow process using a polymer-supported variant of the catalyst.

Stereoselective uptake and metabolism of prothioconazole caused oxidative stress in zebrafish (Danio rerio)

Prothioconazole (PTA) is a novel, broad-spectrum, chiral triazole fungicide that is mainly used to prevent and control the disease of cereal crops. However, the adverse effects of PTA and its major metabolite on nontarget organisms have aroused wide concern. In the present work, the acute toxic of the metabolite prothioconazole-desthio (PTA-desthio), with an LC₅₀ of 1.31 mg L^-1, was 3.5-fold more toxic than the parent compound, indicating that the metabolism of PTA in zebrafish was toxic. The stereoselective uptake and metabolism of PTA and PTA-desthio in zebrafish was firstly investigated using LC-MS/MS. Remarkable enantioselectivity was observed: S-PTA and S-PTA-desthio were preferentially uptake with the uptake rate constants of 8.22 and 8.15 d^-1 at exposure concentration of 0.5 mg L^-1, respectively, and the R-PTA-desthio were preferentially metabolized. PTA-desthio was rapidly formed during the uptake processes. The antioxidant enzyme activities in the zebrafish changed significantly, and these effects were reversible. A metabolic pathway including 13 phase I metabolites and 2 phase II metabolites was firstly proposed. A glucuronic acid conjugate and sulfate conjugate were observed in zebrafish. The results of this work provide information that highlights and can help mitigate the potential toxicity of PTA to the ecological environment and humans health.