Chiral Copper Research Articles

Asymmetric [4+2] cycloaddition of azlactones with dipolar copper–allenylidene intermediates for chiral 3,4-dhydroquinolin-2-one derivatives

In this paper, a pybox-copper catalyzed enantioselective decarboxylative [4+2] cycloaddition reaction of ethynyl benzoxazinanones with azlactones has been developed, which provides optically active 3,4-dihydroquinolin-2-ones in high yields with good enantioselectivities and diastereoselectivities. In this transformation, the chiral dipolar copper–allenylidene intermediates are kinetically generated via decarboxylative ethynyl benzoxazinanones, followed by the attack of the enolate azlactones to form enantiomerically enriched 3,4-dihydroquinolin-2-one structures.

Tandem Chiral Cu(II) Phosphate‐Catalyzed Deoxygenation of Nitrones/Enantioselective Povarov Reaction with Enecarbamates

A new catalytic enantioselective tandem deoxygenation/aza‐Diels‐Alder reaction of nitrones with enecarbamates was serendipitously discovered in the presence of chiral copper(II) diphosphate complexes. This process affords a wide range of 4‐aminotetrahydroquinolines in respectable yields under mild conditions with good to excellent ee values.

A Stereoselective Tyrosinase Model Compound Derived from an m-Xylyl-l-histidine Ligand.

The aim of mimicking enzyme activity represents an important motivation for the development of new catalysts. A challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Herein, we report a new chiral dinuclear copper(II) complex based on a m-xylyl-bis(histidine) ligand (mXHI) as a biomimetic catalyst for tyrosinase and catechol oxidase. The new ligand improves a previous system also containing two tridentate N3 units derived from l-histidine that were connected by a short, rigid ethanediamine bridge. In mXHI the bridge is provided by the more extended m-xylyl moiety. The dicopper(II) complex [Cu2(mXHI)]4+ was studied as a catalyst for stereoselective oxidations of enantiomeric couples of chiral catechols of biological interest (L/D-dopa, L/D-dopa methyl ester, and ( R/ S)-norepinephrine), showing excellent discrimination capability, particularly for the methyl esters of dopa enantiomers. The catechol oxidation was studied in acetate buffer as slightly acidic medium, and a role of acetate as bridging ligand between the two coppers, preorganizing the dinuclear center in a more catalytic efficient structure, could be established. The oxidation of β-naphthol and 3,5-ditertbutylphenol was studied as a model monophenolase reaction. The oxidation proceeds stoichiometrically, and the partial incorporation of 18O into β-naphthol when the reaction was performed using 18O2 suggests the existence of two competitive reaction pathways, a genuine monooxygenase mechanism and a radical pathway. However, the more challenging reaction on derivatives of l-/d-tyrosine did not lead to the desired monooxygenase product but only to products of radical oxidation. Complex [Cu2(mXHI)]4+ was also used for the catalytic sulfoxidation of thioanisole in the presence of hydroxylamine as cosubstrate, in a preliminary attempt to model the reaction of external monooxygenases. The reaction proceeds with 25 turnovers, but the enantiomeric excess of sulfoxide was modest.

Optically pure chiral copper(II) complexes of rosin derivative as attractive anticancer agents with potential anti-metastatic and anti-angiogenic activities

The development of optically pure drugs is the trend of new drugs research. Searching for optically pure metallodrugs against cancer has not been taken seriously. [CuL4Cl]Cl·2CH2Cl2·H2O (1) and [CuL4Br]Br·2CH2Cl2 (2) (L = 2-amino-5-dehydroabietyl-1,3,4-thiadiazole), two rosin-derivative based optically pure chiral copper(II) complexes, are rationally synthesized as potential anticancer agents. 1 exhibits effective in vitro and in vivo anticancer activities and tolerable toxicities. 1 promotes MCF-7 cell death by combination of cell arrest at G1 phase, apoptosis (both extrinsic and intrinsic apoptotic pathways), anti-metastasis, anti-angiogenesis, damage of DNA, protein and lipid, and autophagy mediated by the oxidative stress which is confirmed by ROS generation and intracellular glutathione depletion assays. 1 can be identified as a lead anticancer molecule of therapeutic importance. This work may offer insights into the design and mechanism study of multifunctional optically pure metal-based anticancer candidates derived from natural products.

Enantiomeric Amino Acid Schiff Base Copper(II) Complexes as a New Class of RNA-Targeted Metallo-Intercalators: Single X-ray Crystal Structural Details, Comparative in Vitro DNA/RNA Binding Profile, Cleavage, and Cytotoxicity

Chiral enantiomeric amino acid Schiff base copper(II) complexes 1 and 2 (a and b) were synthesized and characterized by various spectroscopic techniques (Fourier transform infrared, UV–vis, electron paramagnetic resonance, electrospray ionization-mass spectrometry, and circular dichroism) and single X-ray crystal diffraction analyses. To understand the selectivity and enantiomeric behavior of the complexes, binding interaction with ct-DNA and tRNA biomolecules was investigated by widely employed optical and hydrodynamic techniques. The binding experiments demonstrated that complexes 1 and 2 (a and b) interact strongly via the intercalative mode with preferential binding toward the tRNA biomolecule compared to ct-DNA. Furthermore, the order of binding propensity was 2a > 2b > 1a > 1b, implicating greater binding affinity of l-enantiomeric complexes, and complex 2a showed the highest binding propensity possessing a rigid aromatic group in the amino acid framework. Scanning electron microscopy analyses of co...

Derivative chiral copper(Ⅱ) complexes as template of an electrochemical molecular imprinting sol-gel sensor for enantiorecognition of aspartic acid

In this work, we reported a novel electrochemical sensor for enantiorecognition of electrochemically inactive aspartic acid (Asp) enantiomers. By combining sol-gel processing with molecular imprinting technology, an enantioselective interface of molecularly imprinted sol-gel (MIS) films with imprinted cavities was prepared on the surface of a glassy carbon electrode (GCE). In order to obtain the electrochemical responses of electrochemically inactive Asp, the ternary derivative of L-aspartic, (l-aspartic acid)Cu2+(N-carbobenzoxy-l-aspartic acid), was used as the template for fabrication of MIS. The stripping currents of target molecules could be detected by square-wave stripping voltammetry due to the reduction of cupric ion on the modified electrode. The resulted sensor showed good adsorbability to L-Asp derivative, and the recognition efficiency was obtained as 2.1. Meanwhile, the L-Asp was quantitatively determined by the MIS sensor. As a result, the proposed electrochemical sensor could be regarded as a potential platform for enantiorecognition of electrochemically inactive chiral compounds.

Chiral Semiconductor Nanoparticles for Protein Catalysis and Profiling

AbstractIn this study, via a simple one‐step method, chiral copper sulfide quantum dots (d/l‐QDs) were prepared using d‐/l‐penicillamine (d‐/l‐Pen). The anisotropy factor of d/l‐QDs was as high as 0.01. The d/l‐QDs can be used as photocatalysts to cleave proteins. Notably, the l‐QDs displayed the highest catalytic performance under left‐circularly polarized light irradiation. Mechanistic investigations indicate the generation of hydroxyl radicals as the reactive species that cause the cutting of proteins.

Chiral Semiconductor Nanoparticles for Protein Catalysis and Profiling.

In this study, via a simple one-step method, chiral copper sulfide quantum dots (d/l-QDs) were prepared using d-/l-penicillamine (d-/l-Pen). The anisotropy factor of d/l-QDs was as high as 0.01. The d/l-QDs can be used as photocatalysts to cleave proteins. Notably, the l-QDs displayed the highest catalytic performance under left-circularly polarized light irradiation. Mechanistic investigations indicate the generation of hydroxyl radicals as the reactive species that cause the cutting of proteins.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters.

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.

Enantioselective Construction of Quaternary All-Carbon Centers via Copper-Catalyzed Arylation of Tertiary Carbon-Centered Radicals.

An enantioselective copper-catalyzed arylation of tertiary carbon-centered radicals, leading to quaternary all-carbon stereocenters, has been developed herein. The tertiary carbon-centered radicals, including both benzylic and nonbenzylic radicals, were produced by the addition of trifluoromethyl radical to α-substituted acrylamides, and subsequently captured by chiral aryl copper(II) species to give C-Ar bonds with excellent enantioselectivity. Importantly, an acylamidyl (CONHAr) group adjacent to the tertiary carbon radical is essential for the asymmetric radical coupling. The reaction itself features broad substrate scope, excellent functional group compatibility and mild conditions.

Catalytic enantioselective Hosomi-Sakurai reaction of α-ketoesters promoted by chiral copper(ii) complexes.

A catalytic enantioselective Hosomi-Sakurai reaction of α-ketoesters has been developed. A copper(ii) complex with a chiral bis(oxazoline) ligand bearing methanesulfonamide groups shows excellent catalytic activity to give α,α-disubstituted α-hydroxyesters in high yields with high enantioselectivities. This is the first successful method for the catalytic enantioselective 1,2-addition of α-ketoesters with allylic silanes.

Biological evaluation of optically pure chiral binuclear copper(ii) complexes based on a rosin derivative as highly potential anticancer agents.

Two optically pure chiral binuclear copper(ii) complexes [Cu2(μ-Cl)2L2]·CH2Cl2 (1) and Cu2L4 (2) based on the natural product rosin derivative N-(5-dehydroabietyl-1,3,4-thiadiazole)-2-substituted pyridinecarboxamide (HL) were prepared, fully characterized and their biological activities were evaluated. The circular dichroism (CD), fluorescence spectroscopy, and DNA melting studies indicate that 1 and 2 interact with calf thymus DNA (CT DNA) via intercalation. It can be concluded that 1 and 2 have a strong affinity to bovine serum albumin (BSA) based on the fluorescence and CD spectral evidence. The MTT assay illustrates that the selective cytotoxic activity of 1 is better than that of HL, 2, cisplatin and oxaliplatin. The exposure of 1 to MCF-7 cells resulted in cell cycle arrest in the G1 phase, apoptosis, mitochondrial dysfunction and an elevated ROS level. The western blot analysis results indicate that 1 might induce apoptosis through intrinsic and extrinsic pathways, autophagy and DNA damage in MCF-7 cells. Furthermore, the down-regulated VEGFR2, MMP-2 and MMP-9 expression levels indicate that 1 should have the ability to resist metastasis and angiogenesis. Thus, based on the above described results 1 has high potential value for anticancer applications.

δ C-H (hetero)arylation via Cu-catalyzed radical relay.

A Cu-catalyzed strategy has been developed that harnesses a radical relay mechanism to intercept a distal C-centered radical for C-C bond formation. This approach enables selective δ C-H (hetero)arylation of sulfonamides via intramolecular hydrogen atom transfer (HAT) by an N-centered radical. The radical relay is both initiated and terminated by a Cu catalyst, which enables incorporation of arenes and heteroarenes by cross-coupling with boronic acids. The broad scope and utility of this catalytic method for δ C-H arylation is shown, along with mechanistic probes for selectivity of the HAT mechanism. A catalytic, asymmetric variant is also presented, as well as a method for accessing 1,1-diaryl-pyrrolidines via iterative δ C-H functionalizations.

Designing chiral amido-oxazolines as new chelating ligands devoted to direct Cu-catalyzed oxidation of allylic C[sbnd]H bonds in cyclic olefins

A new type of amido-oxazoline ligands was conveniently synthesized from inexpensive and commercially available materials in high yields and enantiomeric excesses. The corresponding chiral copper complexes with this class of ligands [C2 symmetric S,S-bis(amido-oxazoline-Cu(II) complex] were synthesized accordingly. The ORTEP diagram of ligand 6a and complex 6a-copper were compared and characterization of the complex confirmed the involvement of both dentate parts of the ligands, the oxygen and nitrogen atoms, in complexation with copper. The utilization of this amido-oxazoline ligands in the copper-catalyzed enantioselective esterification of allylic CH bonds of cyclic olefins with tert-butyl-4-nitrobenzoperoxoate resulted in the highest activities, yields (up to 95%) and enantioselectivities (up to 96%) in the presence of HZSM-5 zeolite. These new findings highlight the protocol as one of the most attractive and useful methods for the oxidation of the asymmetric allylic CH bond of cycloalkenes compared to other methodologies reported in the literature.

Copper(II)-catalyzed enantioselective conjugate addition of nitro esters to 2-enoyl-pyridine N-oxides

A highly enantioselective Michael addition of nitro esters to 2-enoyl-pyridine N-oxides was developed by using chiral copper catalysts. The Michael addition products can be obtained in high yields and with up to 96% ee. Moreover, asymmetric Michael addition product of nitromethane to 2-enoyl-pyridine N-oxides can be obtained in one step. An analogue of nicotine, dihydro-2H-pyrrol 4b was synthesized with this developed method.

N-Heterocyclic Carbene-Cu-Catalyzed Enantioselective Allenyl Conjugate Addition.

A catalytic enantioselective conjugate addition with commercially available allenylboronic acid pinacol ester as nucleophile promoted by a chiral copper complex of N-heterocyclic carbene (NHC) is disclosed. This process constitutes an unprecedented instance of the conjugate addition that introduces an allenyl group into α,β-unsaturated carbonyl compounds, affording products that are otherwise difficult to access in up to 92% yield, >98% allenyl addition selectivity and 96:4 enantiomeric ratio. DFT calculations were performed to elucidate the origins of enantioselectivity.

Catalyst Choice for Highly Enantioselective [3 + 3]-Cycloaddition of Enoldiazocarbonyl Compounds

Chiral copper(I) catalysts are preferred over chiral dirhodium(II) catalysts for [3 + 3]-cycloaddition reactions of enoldiazocarbonyl compounds with nitrones and acyliminopyridinium ylides, forming...

Asymmetric Copper-Catalyzed C(sp)–H Bond Insertion of Carbenoids Derived from N-Tosylhydrazones

A chiral copper(I)–phosphoramidite complex efficiently catalyzed the asymmetric insertion of the carbenoids derived from N-tosyl hydrazones into alkyne C–H bonds to give the corresponding chiral alkynylated products in up to 77% yield and with up to 74% ee.

Asymmetric Synthesis of Spirocyclic β‐Lactams through Copper‐Catalyzed Kinugasa/Michael Domino Reactions

AbstractThe first copper‐catalyzed highly chemo‐, regio‐, diastereo‐, and enantioselective Kinugasa/Michael domino reaction for the desymmetrization of prochiral cyclohexadienones is described. In the presence of a chiral copper catalyst, alkyne‐tethered cyclohexadienones couple with nitrones to generate the chiral spirocyclic lactams with excellent stereoselectivity (up to 97 % ee, >20:1 dr). The new method provides direct access to versatile highly functionalized spirocyclic β‐lactams possessing four contiguous stereocenters, including one quaternary and one tetra‐substituted stereocenter.

Asymmetric Synthesis of Spirocyclic β-Lactams through Copper-Catalyzed Kinugasa/Michael Domino Reactions.

The first copper-catalyzed highly chemo-, regio-, diastereo-, and enantioselective Kinugasa/Michael domino reaction for the desymmetrization of prochiral cyclohexadienones is described. In the presence of a chiral copper catalyst, alkyne-tethered cyclohexadienones couple with nitrones to generate the chiral spirocyclic lactams with excellent stereoselectivity (up to 97 % ee, >20:1 dr). The new method provides direct access to versatile highly functionalized spirocyclic β-lactams possessing four contiguous stereocenters, including one quaternary and one tetra-substituted stereocenter.