Conjugate Adducts Research Articles

Nickel‐Catalyzed Asymmetric Michael Addition of Acyl‐Imidazoles to Acrolein or β‐Ester Enones

AbstractAn enantioselective Michael addition of α‐substituted acyl‐imidazoles to acrolein catalyzed by a nickel‐bisoxazoline complex has been developed. The use of strong Lewis acid TMSOTf as an additive to activate the Michael acceptors proved to be vital to the success of this process. This process tolerates a wide range of acyl‐imidazoles, and a series of conjugate adducts are obtained in high yields and good enantioselectivities. Besides acrolein, β‐ester ennones are also applicable in this process affording the corresponding conjugate adducts in high yields with good diastereoselectivities and enantioselectivites in most cases. The potential utility of the reaction system is confirmed through 2 mmol‐scale reaction and derivatization experiments.

Enantioselective enolate protonations: Evaluation of achiral templates with fluxional brønsted basic substituents in conjugate addition of malononitriles

Enoates derived from α−substituted acrylates containing 3-pyrazolidinone achiral template with Brønsted basic fluxional substituents were evaluated in chiral Lewis acid catalyzed enantioselective protonation reactions. The tandem 1,4-conjuagte addition/enantioselective protonation reaction catalyzed by chiral Lewis acids was found to depend on the fluxional group present on the achiral template. The enantioselectivity of the protonation step is highly dependent on the position of the Brønsted basic atom with respect to the pyrazolidinone nitrogen. A comparison of fluxional substituents without a Brønsted basic site and their impact on selectivity is also reported here. The results we detail here are in sharp contrast to our previous studies on enantioselective conjugate addition of malononitriles. The reaction showed reasonable substrate scope for varying α−substitution. The reactions gave conjugate adducts in good yields and good enantioselectivities up to 83 %.

Air-tolerant, ligand-free nickel-catalyzed, manganese-assisted reductive Heck reaction of alkyl/aryl halides with electron-deficient olefins

An air-tolerant reductive Heck reaction of alkyl/aryl halides with α, β-unsaturated esters, nitriles and amides catalyzed by simple nickel bromide has been developed by using manganese as reductant in refluxing acetonitrile. Water plays a double-face role in the catalytic process, hydrolysis of the enolate and/or C-tautomer intermediates to afford the conjugate adducts while suppressing the catalysis at high concentration. The scope and limitations of the nickel-catalyzed manganese-mediated process have been demonstrated by more than 40 examples, showing large steric effects and good functional group compatibility except for reduction-sensitive ones.

Photoexcited NiII-Aryl Complex-Mediated Giese Reaction of Aryl Bromides.

A Giese reaction of aryl bromides with electron-deficient alkenes was developed, enabled by a dual catalyst system containing NiII complex and IrIII photocatalyst. This protocol could accommodate a variety of aryl bromides and electron-deficient alkenes, delivering the conjugate adducts in up to 97% yield. The utilization of photoexcited (dtbbpy)NiII(aryl)Br intermediate as an aryl radical source allows this novel transformation of aryl halides, thus expanding the chemical space of excited nickel catalysis.

Chiral Silver Complex-Catalyzed Asymmetric Conjugate Addition of 1-Pyrroline-5-Carbonitrile to α-Enones.

The conjugated addition reaction of 1-pyrroline-5-carbonitrile to α-enones was effectively catalyzed by AgF/ThioClickFerrophos (TCF) in diethyl ether in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and nearly single diastereomers of adducts with good enantiomeric excesses (up to 97% ee) were obtained in high yields. The reduction of the nitrile group with H2/Raney Ni in ethanol successfully transformed the conjugate adduct to the corresponding spirocyclic piperidine (60% yield) as a single stereoisomer.

Conjugate Addition of α‐Substituted Acyl Imidazoles to Nitroalkenes Catalyzed by Nickel Bisoxazoline and B(C6F5)3

AbstractA diastereoselective and enantioselective conjugate addition of α‐substituted acyl imidazole to nitroalkene catalyzed by nickel bisoxazoline complex and B(C6F5)3 has been developed. This reaction tolerated a wide range of acyl imidazoles and nitroalkenes, affording a series of conjugate adducts in yields ranging from 38% to 99% with 4:1 to 99:1 dr and 13% to 98% ee. The use of strong Lewis acid B(C6F5)3 as co‐catalyst proved vital to the success of this process. The potential scalability and utility of the current protocol are demonstrated by a reaction on a 3 mmol scale and further transformations of the product.magnified image

Ir-Catalyzed Enantioselective Formal C-H Conjugate Addition of Pyrrole and Indoles to α,β-Unsaturated Carbonyl Compounds.

The chiral Ir(I)-catalyzed intermolecular reaction of N-carbamoylpyrrole and indole derivatives with α,β-unsaturated carbonyl compounds such as crotonates proceeded with high enantioselectivity. The obtained chirally functionalized pyrroles and indoles are formal C-H conjugate adducts. The reaction mechanism was studied by deuterium labeling experiments.

Enantioselective Cross-Coupling of Electron-Deficient Alkenes via Ir-Catalyzed Vinylic sp2 C-H Alkylation.

A chiral Ir-catalyzed reaction of α-aryl-α,β-unsaturated amides with β-substituted acrylates proceeded to give formal conjugate adducts in high yield and ee (up to 99% yield and up to 95% ee). This is the first example of the enantioselective cross-coupling of two different electron-deficient alkenes via vinylic sp2 C-H activation, and polyfunctionalized chiral compounds were obtained.

Diurnal hepatic CYP3A11 contributes to chronotoxicity of the pyrrolizidine alkaloid retrorsine in mice

1. Retrorsine (RTS) is a pyrrolizidine alkaloid (distributed in many medicinal plants) that has significant hepatotoxicity. Here, we aimed to determine the daily variations in RTS hepatotoxicity (chronotoxicity) in mice, and to investigate the role of metabolism in generating RTS chronotoxicity. 2. Acute toxicity and pharmacokinetic studies were performed with mice after RTS administration at different times of the day. Hepatotoxicity was assessed by measuring plasma ALT (alanine aminotransferase) and AST (aspartate aminotransferase) levels. mRNA and proteins were determined by qPCR and Western blotting, respectively. Time-dependent in vitro metabolism of RTS was assessed by using mouse liver microsomes. 3. We found that RTS toxicity was more severe in the dark phase (zeitgeber time 14 or ZT14 and ZT18) than in the light phase (ZT2 and ZT6). This chronotoxicity was associated with a dosing time difference in the systemic exposures of RTS and a pyrrolic ester metabolite (a cause of hepatotoxicity, measured by the levels of pyrrole-GSH conjugate and pyrrole-protein adducts due to a high chemical reactivity). Moreover, the CYP3A11 (a major enzyme for RTS bioactivation) inhibitor ketoconazole decreased the production of pyrrole-GSH conjugate and abrogated diurnal rhythm in RTS metabolism. In addition, E4bp4 (a circadian regulator of Cyp3a11) ablation abolished the rhythm of CYP3A11 expression and abrogated the dosing time-dependency of RTS toxicity. 4. In conclusion, RTS chronotoxicity in mice was attributed to time-varying hepatic metabolism regulated by the circadian clock. Our findings have implications for reducing pyrrolizidine alkaloid-induced toxicity via a chronotherapeutic approach.

Stereoselective conjugate addition of ketones to alkylidene malonates using thiourea-sulfonamide organocatalyst.

In this study, stereoselective conjugate addition of ketones to alkylidene malonates using organocatalyst has been developed. The reaction in the presence of 20mol% of a novel thiourea-sulfonamide organocatalyst afforded conjugate adducts in moderate to high yields (up to 81%) under mild reaction conditions. Excellent diastereoselectivity (up to 98:2 dr) and enantioselectivity (up to 88% ee) were achieved.

Identification of the Histidine Residue in Vitamin D Receptor That Covalently Binds to Electrophilic Ligands.

We designed and synthesized vitamin D analogues with an electrophile as covalent modifiers for the vitamin D receptor (VDR). Novel vitamin D analogues 1-4 have an electrophilic enone group at the side chain for conjugate addition to His301 or His393 in the VDR. All compounds showed specific VDR-binding potency and agonistic activity. Covalent bond formations of 1-4 with the ligand-binding domain (LBD) of VDR were evaluated by electrospray ionization mass spectrometry. All compounds were shown to covalently bind to the VDR-LBD, and the abundance of VDR-LBD corresponding conjugate adducts of 1-4 increased with incubation time. Enone compounds 1 and 2 showed higher reactivity than the ene-ynone 3 and dienone 4 compounds. Furthermore, we successfully obtained cocrystals of VDR-LBD with analogues 1-4. X-ray crystallographic analysis showed a covalent bond with His301 in VDR-LBD. We successfully synthesized vitamin D analogues that form a covalent bond with VDR-LBD.

Development of Biligands Magnesium Catalysis in Asymmetric Conjugate Reactions of C3-Pyrrolyl-Oxindoles.

A magnesium catalyzed asymmetric conjugate reaction of C3-pyrrolyl-oxindoles with terminal alkynones is presented. The current asymmetric conjugate reaction relies on the development of novel combinational magnesium catalysis involving two chiral ligands. The current protocol proceeds smoothly and gives the corresponding enantioenriched 3,3-disubstituted oxindole skeletons with good enantioselectivities. Furthermore, the conjugate adducts could be transferred to spiro oxindole structures containing an eight-membered ring in high ee values.

Metabolism of the Tobacco Carcinogen 2-Amino-9H-pyrido[2,3-b]indole (AαC) in Primary Human Hepatocytes.

2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant carcinogenic heterocyclic aromatic amine (HAA) formed in mainstream tobacco smoke. AαC is a liver carcinogen in rodents, but its carcinogenic potential in humans is not known. To obtain a better understanding of the genotoxicity of AαC in humans, we have investigated its metabolism and its ability to form DNA adducts in human hepatocytes. Primary human hepatocytes were treated with AαC at doses ranging from 0.1-50 μM, and the metabolites were characterized by ultra-performance LC/ion trap multistage mass spectrometry (UPLC/MSn). Six major metabolites were identified: a ring-oxidized doubly conjugated metabolite, N2-acetyl-2-amino-9H-pyrido[2,3-b]indole-6-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-6-O-Gluc); two ring-oxidized glucuronide (Gluc) conjugates: 2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (AαC-3-O-Gluc) and 2-amino-9H-pyrido[2,3-b]indol-6-yl-oxo-(β-d-glucuronic acid) (AαC-6-O-Gluc); two sulfate conjugates, 2-amino-9H-pyrido[2,3-b]indol-3-yl sulfate (AαC-3-O-SO3H) and 2-amino-9H-pyrido[2,3-b]indol-6-yl sulfate (AαC-6-O-SO3H); and the Gluc conjugate, N2-(β-d-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N2-Gluc). In addition, four minor metabolites were identified: N2-acetyl-9H-pyrido[2,3-b]indol-3-yl sulfate (N2-acetyl-AαC-3-O-SO3H), N2-acetyl-9H-pyrido[2,3-b]indol-6-yl sulfate (N2-acetyl-AαC-6-O-SO3H), N2-acetyl-2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-3-O-Gluc), and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN2-O-Gluc). The latter metabolite, AαC-HN2-O-Gluc is a reactive intermediate that binds to DNA to form the covalent adduct N-(2'-deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AαC). Preincubation of hepatocytes with furafylline, a selective mechanism-based inhibitor of P450 1A2, resulted in a strong decrease in the formation of AαC-HN2-O-Gluc and a concomitant decrease in DNA adduct formation. Our findings describe the major pathways of metabolism of AαC in primary human hepatocytes and reveal the importance of N-acetylation and glucuronidation in metabolism of AαC. P450 1A2 is a major isoform involved in the bioactivation of AαC to form the reactive AαC-HN2-O-Gluc conjugate and AαC-DNA adducts.

Organocatalytic Enantioselective Conjugate Addition of Azlactones to Enolizable Linear and Cyclic Enones.

Highly diastereo- and enantioselective conjugate additions of azlactones to enolizable cyclic and linear enones were conducted by employing proline aryl sulfonamide as the organocatalyst in trifluorotoluene. The conjugate adducts bearing contiguous quaternary and tertiary stereocenters were obtained in moderate to good yields with excellent diastereoselectivities and moderate to good enantioselectivities. This developed protocol filled in the substrate gap for the organocatalytic conjugate addition of azlactone to enones.

Electrophilic Activation of α,β‐Unsaturated Amides: Catalytic Asymmetric Vinylogous Conjugate Addition of Unsaturated γ‐Butyrolactones

Although catalytic asymmetric conjugate addition reactions have remarkably advanced over the last two decades, the application of less electrophilic α,β-unsaturated carboxylic acid derivatives in this useful reaction manifold remains challenging. Herein, we report that α,β-unsaturated 7-azaindoline amides act as reactive electrophiles to participate in catalytic diastereo- and enantioselective vinylogous conjugate addition of γ-butyrolactones in the presence of a cooperative catalyst comprising of a soft Lewis acid and a Brønsted base. Reactions mostly reached completion with as little as 1 mol % of catalyst loading to give the desired conjugate adducts in a highly stereoselective manner.

Copper- and Silver-Catalyzed Diastereo- and Enantioselective Conjugate Addition Reaction of 1-Pyrroline Esters to Nitroalkenes: Diastereoselectivity Switch by Chiral Metal Complexes.

syn-Diastereoselective conjugate addition of 1-pyrroline esters to nitroalkenes in good yields with an excellent enantioselectivity by using CuOAc/Me-FcPHOX catalyst in the presence of pyridine. In contrast, AgOAc/tBu-ThioClickFerrophos catalyzed the anti diastereoselective conjugate addition with a high enantioselectivity without additional base. Thus, the preparation of chiral 1-pyrroline derivatives bearing diverse stereochemistry could be achieved. The diastereoselective reduction of the imine group in the conjugate adduct could afford the 2,5-cis-proline ester derivative.

Development and Application of α-Heteroatom Ketones in Asymmetric Michael Reaction with β-trans-Nitroalkenes.

The successful design and application of a new type of N-phenyl-imidazole-modified α-heteroatom ketones in asymmetric anti-selective Michael reactions with β-trans-nitroalkenes is reported. High yields and enantioselectivities could be obtained, and the corresponding conjugate adducts could be further transformed into related chiral esters and cyclopropane derivatives with excellent enantioselectivities.

Experimental and computational evidence for hydrogen bonding interaction between 2′-deoxyadenosine conjugate adduct and amino-terminated organic film on Si(001)

A simple method for immobilization of malonaldehyde-acetaldehyde conjugate adduct with DNA base onto an amino-terminated surface of silicon from water solution is proposed. The Si(001) surface which contains OH groups was modified with 3-aminopropyltrimethoxysilane (APTMS) to serve as a linker between the silica surface and the organic adduct. The 2′-deoxyadenosine adduct was adsorbed on the APTMS/Si surface from acetonitrile/water solution. This nucleoside derivative is stable under laboratory conditions and emits a natural fluorescence, allowing for its adsorption on the APTMS/Si surface to be easily verified by fluorescence microscopy, non-contact atomic force microscopy and attenuated total reflectance Fourier transform infrared spectroscopy. The degree of surface coverage by the adduct was evaluated by X-ray photoelectron spectroscopy (XPS). Analysis of the XPS spectra revealed bands at 400.2eV and 533.1eV which are characteristic of a hydrogen bonded –NH2 and –OH group. This observation implies that the free electron donating –NH2 groups from the APTMS layer makes hydrogen bonds with the fluorescent adduct and immobilize it on the surface. The wetting angle of the APTMS/Si surface before and after adsorption of the nucleoside derivative does not differ significantly, which points to the involvement of an – OH group from 2′-deoxyadenosine to be involved in hydrogen bonding. These experimental results were further supported using quantum chemical calculations to demonstrate that the 2′deoxyadenosine adduct makes hydrogen bonds with the APTMS molecule. Furthermore, this hydrogen bond involves the –NH2 group from APTMS and –OH group at carbon atoms C3 and C6 from the deoxyribose ring of 2′deoxyadenosine.

Regioselectivity switch in chiral amine-catalysed asymmetric addition of aldehydes to reactive enals.

In this communication, we present a regioselectivity switch for the chiral amine-catalysed asymmetric addition of aldehydes to reactive enals to afford either aldol adducts or conjugate adducts in a stereoselective fashion. The unprecedented asymmetric aldol reaction of aldehydes with enals was realized by the use of a diarylprolinol catalyst, giving synthetically useful and important chiral allylic alcohols.

Asymmetric anti-selective Michael reaction of imidazole-modified ketones with trans-β-nitroalkenes.

The successful application of imidazole-modified ketones in asymmetric anti-selective Michael reactions with trans-β-nitroalkenes is presented by employing a newly developed 3-bromothiophene-modified chiral diamine ligand. The corresponding conjugate adduct was submitted to further transformations with Grignard reagents to solve the problem of α-site selectivity of simple linear ketones. Additionally, the syn-selective product was obtained by treating the anti-selective adduct with a simple base. In this way, the site-specific products for both diastereomers in the asymmetric conjugate addition of simple ketones to nitroalkenes can be obtained.