Michael Addition Research Articles

C2-Symmetric Amino Acid Amide-Derived Organocatalysts

N-alkylated C2-symmetric amino acid amide derivatives were shown to catalyse the Michael addition of 2-hydroxy-1,4-napthoquinone to β-nitrostyrene, achieving a maximum ee of 44%. The corresponding trifluoroacetic acid salts also catalysed the aldol reaction between 4-nitrobenzaldehyde and hydroxyacetone, leading to the formation of predominantly syn-aldol products in up to 55% ee. Aspects of the solvent dependence of the aldol reaction and the H-bonding of the catalyst were investigated.

Interaction between peptide and solid lignin suggest mechanisms of abiotic covalent bond formation

The mechanisms behind the formation of soil organic matter (SOM) and associated nitrogen immobilization remain partially elusive, while ongoing research continues to shed light on carbon and nitrogen sequestration in the environment. Studies show that quinone-like structures within alkali-soluble humic extracts of SOM can bond covalently with nitrogen-containing molecules derived from proteinaceous organic matter. However, direct molecular evidence for this chemical bonding with lignin in the solid form and non-solvent-extracted SOM is lacking. Using gel-state 1H-15N heteronuclear single quantum coherence (HSQC) high-resolution magic angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that a 15N-labeled peptide (glycine-glycine-glycine-arginine, GGGR), can covalently bind to solid, untreated, brown rotted wood as well as 1,2-naphthoquinone, a quinone-model molecule typically found in degraded lignin. The new peaks in both reactions represent a change in the environment surrounding the N-functional groups. Interaction of the 15N-labeled peptide with the model quinone produced NMR cross peaks that are similar to those of the solid lignin. This suggests that the quinone-like structures are the most likely functional groups to form covalent bonds with peptides in the degraded lignin. Both Michael addition and Schiff base formation is proposed when the peptide GGGR interacts with white oak lignin and 1,2-naphthoquinone. These processes are of considerable importance to N incorporation into SOM and offer insights into how proteinaceous molecules could potentially be preserved and sequestered through covalent bond formation.

Dipeptidic Proline‐Derived Thiourea Organocatalysts for Asymmetric Michael addition Reactions between Aldehydes and Nitroolefins

AbstractThe enantiodivergent synthesis of syn‐Michael adducts can be easily achieved by switching the terminal proline in dipeptidic proline‐thiourea catalysts. In this study, a dipeptidic proline‐derived thiourea organocatalyst was rationally designed and facilely prepared from dipeptidic proline, a chiral diamine as a linker, and isothiocyanate. This asymmetric bifunctional catalyst for Michael addition between aldehydes and nitroolefins provided chiral 4‐nitro aldehyde adducts with yields of up to 99 %, 92 : 8 syn‐diastereoselectivity, and 97 % enantiomeric excess at room temperature.

Copper-Catalyzed Alkynylation of In Situ-Generated Azoalkenes: An Umpolung Approach to Pyrazoles.

A straightforward umpolung approach to regioselectively N-protected polysubstituted pyrazoles starting from aromatic α-halohydrazones and terminal alkynes has been developed. In this process, azoalkenes generated in situ from α-halohydrazones are involved in the Cu(I)-catalyzed Michael addition with alkynes to give α-alkynyl-substituted hydrazones that cyclize to give the target pyrazoles in 37-85% yield. The method employs readily available starting materials and features good functional group compatibility (nitro, sulfonyl, cyano, trimethylsilyl, and primary bromoalkyl groups, esters, and alcohols are tolerated) and scalability.

Mechanism and Origins of Weak Bonding-Controlled Selectivities in Cinchoninium-Catalyzed Umpolung Michael Addition of Imines

Mechanism and Origins of Weak Bonding-Controlled Selectivities in Cinchoninium-Catalyzed Umpolung Michael Addition of Imines

Development of a new experimental NMR strategy for covalent cysteine protease inhibitors screening: toward enhanced drug discovery.

In the development of antiviral drugs, proteases and polymerases are among the most important targets. Cysteine proteases, also known as thiol proteases, catalyze the degradation of proteins by cleaving peptide bonds using the nucleophilic thiol group of cysteine. As part of our research, we are examining how cysteine, an essential amino acid found in the active site of the main protease (Mpro) enzyme in SARS-CoV-2, interacts with electrophilic groups present in ethacrynic acid (EA) and compounds 4, 6, and 8 to form sulfur-carbon bonds. Nuclear magnetic resonance (NMR) spectroscopy was used to monitor the reaction rate between cysteine and Michael acceptors. We found that the inhibitory activity of these compounds towards Mpro is correlated to their chemical reactivity toward cysteine. This approach may serve as a valuable tool in drug development for detecting potential covalent inhibitors of Mpro and other cysteine proteases.

Exploring 2-Sulfonylpyrimidine Warheads as Acrylamide Surrogates for Targeted Covalent Inhibition: A BTK Story.

Targeted covalent inhibitors (TCIs) directing cysteine have historically relied on a narrow set of electrophilic "warheads". While Michael acceptors remain at the forefront of TCI design strategies, they show variable stability and selectivity under physiological conditions. Here, we show that the 2-sulfonylpyrimidine motif is an effective replacement for the acrylamide warhead of Ibrutinib, for the inhibition of Bruton's tyrosine kinase. In a few iterations, we discovered new derivatives, which inhibit BTK both in vitro and in cellulo at low nanomolar concentrations, on par with Ibrutinib. Several derivatives also displayed good plasma stability and reduced off-target binding in vitro across 135 tyrosine kinases. This proof-of-concept study on a well-studied kinase/TCI system highlights the 2-sulfonylpyrimidine group as a useful acrylamide replacement. In the future, it will be interesting to investigate its wider potential for developing TCIs with improved pharmacologies and selectivity profiles across structurally related protein families.

Preparation of a chiral hyperbranched polymer based on cinchona alkaloids and investigation of its catalytic activity in asymmetric reactions.

Cinchona alkaloid-derived sulfonamides and ester dimers containing chiral hyperbranched polymers have been successfully synthesized and applied as catalysts in asymmetric reactions. Several hyperbranched polymers derived from cinchona alkaloids, incorporating sulfonamides and esters, were synthesized through Mizoroki-Heck coupling polymerization. These polymers were subsequently applied in enantioselective Michael addition reactions. As the prepared polymers are not soluble in frequently used organic solvents, they act as efficient catalysts in the enantioselective reaction of β-ketoesters to nitroolefins, achieving up to 99% enantioselectivity with good yields. The insoluble property allows them to better satisfy "green chemistry" requirements and be used several times without losing the enantioselectivity.

Three-Component Reaction of Cyclopropanols, DABSO, and N-(Sulfonyl)acrylamides: Preparation of Sulfone-Bridged 1,7-Dicarbonyl Compounds.

A cascade reaction of cyclopropyl alcohols, DABSO (1,4-diazoniabicyclo[2.2.2]octane-1,4-disulfinate), and N-(sulfonyl)acrylamides has been developed. This tandem process went through a cyclopropanol ring opening and Michael addition sequence. The γ-keto sulfinate generated from the reaction between cyclopropanol and DABSO serves as the nucleophilic reagent, and N-(sulfonyl)acrylamide is used as the Michael addition acceptor. By utilizing this strategy, multitudinous sulfone-bridged 1,7-dicarbonyl compounds that contain both a β-sulfonyl amide unit and γ-keto sulfone skeleton were conveniently synthesized.

Structure-property relationship between different molecular precursors and the final properties of carbon dots: Preparation, characterization and applications as sensors for metal ions

Carbon dots (CDs) have attracted significant attention in recent years due to their interesting properties, such as photoluminescence, biocompatibility, excellent water dispersibility, as well as their potential applications in different fields. Despite these features, the complexity of their photoluminescent properties remains a subject of ongoing research and still presents unresolved questions. In this work, we investigated the preparation of various CDs from different low molecular mass starting materials containing carbon, nitrogen and/or sulfur, using hydrothermal carbonization as the preparation method. The CDs were characterized using various techniques, including TEM, FTIR, photoluminescence, UV–Vis, Raman and XPS. A detailed analysis of the initial reactions between the precursors was conducted, and our main results suggest that the optical properties of the CDs are related to fluorophores formed during the initial Michael condensation reactions between the precursors. This process produces molecular fluorescence, which plays a pivotal role in the photoluminescence properties of the nanoparticles. In addition, two of the obtained CDs were used as selective and sensitive probes for metal ions, achieving a detection limit of 0.58 μM and 0.55 μM. Finally, the findings described here can guide future works in fine-tuning the design of CDs with desired properties and different potential applications.

Construction of zwitterionic surface of PVDF hollow fiber membrane and the study on the mechanism of “autonomy-response” synergistic enhancement anti-fouling

In this study, polyvinylidene fluoride (PVDF)/styryl maleic anhydride (SMA) hollow fiber membrane was fabricated using the Thermally Induced Phase Separation (TIPS) technique. Subsequently, zwitterion-sulfobetaine methacrylate (SBMA) was introduced to the membrane surface through a Michael addition reaction to create a nearly electrically neutral membrane surface. This modification effectively mitigated the high adsorption of differently charged protein pollutants on the membrane surface, thereby enhancing its anti-fouling performance. Simultaneously, the membrane surfaces with positive and negative charges were separately constructed, and the impact of membrane surface charge properties on the anti-fouling performance against positively and negatively charged foulants was investigated. Furthermore, detailed studies were conducted on the chemical structure, surface morphology, performance, and anti-fouling mechanism of the modified membranes. The results indicate that the electrically neutral membrane modified by zwitterionic SBMA exhibited excellent anti-fouling properties against both bovine serum albumin (BSA) and lysozyme (LYZ), with a flux recovery rate (FRR) reaching 96.5 % and 94.6 %, respectively. The static adsorption capacities of BSA and LYZ on the membrane surface decreased to 5.89 μg cm−2 and 9.53 μg cm−2, respectively. Furthermore, the study investigated the structure of the hydration layer formed by zwitterionic SBMA on the membrane surface, as well as its hydration capacity and anti-fouling mechanism. Subsequently, through an assessment of the long-term stability of the modified membrane and its flux recovery rate (FRR), in conjunction with theories related to hydration layers and thermodynamics, it elucidated the anti-fouling mechanism involving “autonomy-response” synergistic enhancement of zwitterions. These findings will offer new strategies for researching hollow fiber membranes with superior anti-fouling properties and for applying zwitterions in membrane technology.

Dynamic monitoring of SO2 changes during ferroptosis using a light-controlled lipid droplets-targeting probe

During the process of ferroptosis, the generation of toxic lipid peroxides triggers severe inflammatory responses. Despite sulfur dioxide (SO2) being recognized as an important signal molecule and antioxidant, its specific physiological mechanism in ferroptosis remains unclear. Here, we developed a novel probe, SP-TB, tailored for detecting SO2 within lipid droplets (LDs). SP-TB features a triphenylamine moiety for LD targeting, coupled with a spiropyran serving as both the SO2 reaction site and a light-responsive switch. Under normal conditions, SP-TB exhibits yellow fluorescence at 570 nm with its SO2 reaction site closed. Upon UV light activation, the spiropyran moiety undergoes isomerization to the merocyanine form (MC-TB), resulting in red fluorescence emission at 634 nm. During this photoisomerization, the exposed CC-CN+ fragment enables specific Michael addition reactions with SO2, quenching the red fluorescence of MC-TB and enhancing yellow fluorescence. The probe exhibits stable photoreversibility and excellent LDs targeting, along with remarkable SO2 selectivity with a low detection limit of 3.5 μM. Notably, the controllability of the SO2 reaction site minimizes false positives. This probe was successfully used to monitor the changes of SO2 concentrations in LDs during ferroptosis.

Construction of central and axial chirality via Pd(II)/Bim-catalyzed asymmetric dearomative Michael reaction of polycyclic tropones.

A highly enantioselective Pd/Bim-catalyzed dearomative Michael reaction applying polycyclic tropones as non-benzenoid aromatic Michael acceptors and arylboronic acids as aryl pronucleophiles has been developed. The bridged biaryls bearing central and axial chirality, including pentacyclic cyclohepta[b]indoles and 6,7-dihydrodibenzo[a,c][7]annulen-5-ones, are generally generated in good to high yields and excellent enantioselectivities and can be readily transformed into useful derivatives.

Gene network analysis combined with preclinical studies to identify and elucidate the mechanism of action of novel irreversible Keap1 inhibitor for Parkinson's disease.

The cysteine residues of Keap1 such as C151, C273, and C288 are critical for its repressor activity on Nrf2. However, to date, no molecules have been identified to covalently modify all three cysteine residues for Nrf2 activation. Hence, in this study, our goal is to discover new Keap1 covalent inhibitors that can undergo a Michael addition with all three cysteine residues. The Keap1's intervening region was modeled using Modeller v10.4. Covalent docking and binding free energy were calculated using CovDock. Molecular dynamics (MD) was performed using Desmond. Various in-vitro assays were carried out to confirm the neuroprotective effects of the hit molecule in 6-OHDA-treated SH-SY5Y cells. Further, the best hit was evaluated in vivo for its ability to improve rotenone-induced postural instability and cognitive impairment in male rats. Finally, network pharmacology was used to summarize the complete molecular mechanism of the hit molecule. Chalcone and plumbagin were found to form the necessary covalent bonds with all three cysteine residues. However, MD analysis indicated that the binding of plumbagin is more stable than chalcone. Plumbagin displayed neuroprotective effects in 6-OHDA-treated SH-SY5Y cells at concentrations 0.01 and 0.1 μM. Plumbagin at 0.1 µM had positive effects on reactive oxygen species formation and glutathione levels. Plumbagin also improved postural instability and cognitive impairment in rotenone-treated male rats. Our network analysis indicated that plumbagin could also improve dopamine signaling. Additionally, plumbagin could exhibit anti-oxidant and anti-inflammatory activity through the activation of Nrf2. Cumulatively, our study suggests that plumbagin is a novel Keap1 covalent inhibitor for Nrf2-mediated neuroprotection in PD.

Sensitive electrochemical immunosensor for rapid detection of Salmonella in milk using polydopamine/CoFe-MOFs@Nafion modified gold electrode

Food contaminated by pathogenic bacteria poses a serious threat to human health. Consequently, we used Salmonella as a model and developed an electrochemical immunosensor based on a polydopamine/CoFe-MOFs@Nafion nanocomposite for the detection of Salmonella in milk. The CoFe-MOFs exhibit good stability, large specific surface area, and high porosity. However, after modification on the electrode surface, they were prone to detachment. This issue was effectively mitigated by incorporating Nafion into the nanocomposite. A polydopamine (PDA) film was deposited onto the surface of CoFe-MOFs@Nafion through cyclic voltammetry (CV), accompanied by an investigation into the polymerization mechanism of the PDA film. PDA contains a substantial number of quinone functional groups, which can covalently bind to amino or sulfhydryl groups via Michael addition reaction or Schiff base reaction, thereby immobilizing anti-Salmonella antibodies onto the modified electrode surface. Under the optimal experimental conditions, the Salmonella concentration exhibited a good linear relationship within the range of 1.38 × 102 to 1.38 × 108 CFU mL−1, with a detection limit of 1.38 × 102 CFU mL−1. Furthermore, the constructed immunosensor demonstrated good specificity, stability, and reproducibility, offering a novel approach for the rapid detection of foodborne pathogens.

One-Step Physical and Chemical Dual-Reinforcement with Hydrophobic Drug Delivery in Gelatin Hydrogels for Antibacterial Wound Healing.

Gelatin-based bioadhesives, especially methacrylated gelatin (GelMA), have emerged as superior alternatives to sutureless wound closure. Nowadays, their mechanical improvement and therapeutic delivery, particularly for hydrophobic antibiotics, have received ever-increasing interest. Herein, a reinforced gelatin-based hydrogel with a hydrophobic drug delivery property for skin wound treatment was reported. First, photosensitive monomers of N'-(2-nitrobenzyl)-N-acryloyl glycinamide (NBNAGA) were grafted onto GelMA via Michael addition, namely, GelMA-NBNAGA. Second, gelation of the GelMA-NBNAGA solution was accomplished in a few seconds under one step of ultraviolet (UV) light irradiation. Multiple effects were realized simultaneously, including chemical cross-linking initiated by lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), physical cross-linking of uncaged dual hydrogen bonding, and hydrophobic drug release along with o-NB group disintegration. The mechanical properties of the dual-reinforcement hydrogels were verified to be superior to those only with a chemical or physical single-cross-linked network. The hydrophobic anticancer doxorubicin (DOX) and antibiotic rifampicin (Rif) were successfully charged into the hydrogels, separately. The in vitro antimicrobial tests confirmed the antibacterial activity of the hydrogels against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. The in vivo wound-healing assessment in mice further assured their drug release and efficacy. Therefore, this NBNAGA-modified GelMA hydrogel has potential as a material in skin wound dressing with a hydrophobic antibiotic on-demand delivery.

A thiopyridine-bound mirror-image copper center in an artificial non-heme metalloenzyme

Artificial metalloenzymes, in which a metal complex and protein matrix are combined, have been synthesized to catalyze stereoselective reactions using the chiral environment provided by the protein cavity. Artificial metalloenzymes can be engineered by the chemical modification and mutagenesis of the protein matrix. We developed artificial non-heme metalloenzymes using a cupin superfamily protein (TM1459) with a 4-His tetrad-metal-binding motif. The Cu-bound H52A/C106D mutant with 3-His triad showed a S-enantioselective Michael addition of nitromethane to α,β-unsaturated ketone, 2-aza-chalcone 1. In this study, we demonstrated a chemical modification near the copper-binding site of this mutant to reverse its enantioselectivity. For chemical modification, the amino acid on the Si-face of the binding state of 1 to the copper center was replaced with Cys, followed by reaction with 4,4′-dithiopyridine (4-PDS) to form S-(pyridin-4-ylthio)cysteine (Cys-4py). Cu-bound I49C-4py/H52A/C106D showed reversal of the enantioselectivity from S-form to R-form (ee = 71%, (R)). The effect of steric hindrance of the amino acids at position 49 on enantioselectivity was investigated using I49X/H52A/C106D mutants (X = A, C, I, F, and W). Additionally, chemical modification with 2,2′-dithiopyridine (2-PDS) produced I49-2py/H52A/C106D, which showed lower R-enantioselectivity than I49-4py/H52A/C106D. Among the mutants, the 4py-modification on the Si-face was the most effective in reversing the enantioselectivity. By tuning the Re-face side, the H54A mutation introduced into the I49C-4py/H52A/C106D increased the R-enantioselectivity (ee = 88%, (R)). X-ray crystallography revealed a coordinated structure with ligation of thiopyridine in Cu-bound I49C-4py/H52A/H54A/C106D.

Machine Learning Algorithm Guides Catalyst Choices for Magnesium-Catalyzed Asymmetric Reactions.

Organic-chemical literature encompasses large numbers of catalysts and reactions they can effect. Many of these examples are published merely to document the catalysts' scope but do not necessarily guarantee that a given catalyst is "optimal"-in terms of yield or enantiomeric excess-for a particular reaction. This paper describes a Machine Learning model that aims to improve such catalyst-reaction assignments based on the carefully curated literature data. As we show here for the case of asymmetric magnesium catalysis, this model achieves relatively high accuracy and offers out of-the-box predictions successfully validated by experiment, e.g., in synthetically demanding asymmetric reductions or Michael additions.

Investigation of relationships between metabolic chemical reporter structures and S-glyco-modification effects

The approach of metabolic chemical reporters (MCRs) for labeling proteins has been widely used in the past several decades. Nevertheless, artificial side reaction generated with fully protected MCRs, termed S-glyco-modification, occurs with cysteine residues through base-promoted β-elimination and Michael addition, leading to false positives in the proteomic identification. Therefore, next generation of MCRs, including partially protected strategy and modifications on the backbone of monosaccharides, have emerged to improve the labeling efficiency. In this paper, we prepared fifteen kinds of unnatural monosaccharides to investigate the relationships of structures and S-glyco-modification labeling. Our results demonstrated that Ac4GlcNAz and Ac4GalNAz exhibited the most remarkable labeling effects among the detected compounds. Of note, Ac4ManNAz, Ac46AzGlucose and Ac46AzGalactose containing similar structures but did not show similar robust signals as them. Moreover, other modifications on the 1-, 2-, 3-, 4- and 6-site indicated minimal side reactions of S-glyco-modification, raising a possibility that subtle modifications of monosaccharide substrate may alter its role in the process of biosynthesis, for example, by change of electronegativity or enhancement of steric hindrance effects. In conclusion, our discoveries provide a new avenue to choose appropriate probe for selective label proteins in vitro and in vivo without undesired S-glyco-modification.

Trace benzoquinone inducing periodate for tetracyclines selective degradation: Role of the unstable intermediate

The binding behavior of tetracyclines (TCs) with natural organic matter and the subsequent induction of degradation in advanced oxidation processes has been underestimated. In this study, based on the specific oxidative effects of periodate, the selective degradation of TCs by periodate was achieved with benzoquinone (BQ) that induced poor stability intermediates. The periodate achieved the near complete degradation of TC with a trace concentration of BQ under the alkaline condition, with the kobs at 0.3090 min−1. The degradation experiments for twelve micropollutants revealed that the periodate/BQ system had a considerably efficient degradation only for TCs, and combined with the electron paramagnetic resonance (EPR) analysis, the minor involvement of reactive oxygen species during the degradation process was confirmed. The evident correlation between the observed kobs and the pre-mixing duration of TC with BQ demonstrated the dependence of the degradation process on intermediate generation. Furthermore, theoretical calculations revealed the reaction sites between the TC and BQ; the product identification and energy barrier of 18.94 kcal/mol further confirmed the interaction pathway through the Michael addition reaction. Moreover, based on the narrow energy gap (0.5350 eV) with the intermediate, only the periodate demonstrated efficient degradation of TC under alkaline conditions among the various oxidants. Little toxic iodine species were produced during the reaction, and the system exhibited resistance to natural water substances, such as anions and humic acid, highlighting its potential for application. In conclusion, the interaction mechanisms between TCs and BQ were elucidated in this research, and an efficient approach was proposed for eliminating residual TCs from aquatic environments and selective degradation from wastewater matrices.