Semi-rational screening of the inhibitors and β-lactam antibiotics against the New Delhi metallo-β-lactamase 1 (NDM-1) producing E. coli

Bacteria containing bla_NDM-1 gene are a growing threat to almost all clinically β-lactam antibiotics. Especially, the New Delhi metallo-β-lactamase (NDM-1) has become a potential public survival risk. In this study, a novel and efficient strategy for inhibitors and β-lactam antibiotics screening using recombinant New Delhi metallo-beta-lactamase (NDM-1) was developed. First, the gene of bla_NDM-1 were identified and cloned from multi-drug resistance of Acinetobacter baumannii isolate; by the means of protein expression and purification, recombinant NDM-1 activity was up to 68.5 U ml⁻¹, and high purity NDM-1 protein with activity of 347.4 U mg⁻¹ was obtained. Finally, for NDM-1, the inhibitors (aspergillomarasmine A (AMA) and EDTA) with high affinity (HI) and the β-lactam antibiotics (imipenem) with low affinity (LA) were screened out. Surprisingly, the inhibition of the NDM-1 was enhanced by the use of inhibitor combinations (AMA–EDTA (1 : 2)), where the IC₅₀ of AMA–EDTA was reduced by 88% and 95%, respectively, comparing to the AMA and EDTA alone. More interesting, AMA–EDTA could restore the activity of imipenem when tested against NDM-1 expressing strains (E. coli and Acinetobacter baumannii), with a working time of 120 min and 330 min, respectively. This method is expected to be used in high-throughput screening, drug redesign (including new inhibitors and drugs) and “old drug new use”.

Bacteria containing bla_NDM-1 gene are a growing threat to almost all clinically β-lactam antibiotics. A semi-rational screening of the inhibitors and antibiotics against the New Delhi metallo-β-lactamase 1 has been developed in this study.     

N α-arylsulfonyl histamines as selective β-glucosidase inhibitors

N ^α-benzenesulfonylhistamine, a new semi-synthetic β-glucosidase inhibitor, was obtained by bioactivity-guided isolation from a chemically engineered extract of Urtica urens L. prepared by reaction with benzenesulfonyl chloride. In order to identify better β-glucosidase inhibitors, a new series of N^α,N^τ-di-arylsulfonyl and N^α-arylsulfonyl histamine derivatives was prepared. Biological studies revealed that the β-glucosidase inhibition was in a micromolar range for several N^α-arylsulfonyl histamine compounds of the series, N^α-4-fluorobenzenesulfonyl histamine being the most powerful compound. Besides, this reversible and competitive inhibitor presented a good selectivity for β-glucosidase with respect to other target enzymes including α-glucosidase.

A selective β-glucosidase inhibitor was discovered using the chemically engineered extracts approach.     

Both the mono- and di-anions of ellagic acid are effective inhibitors of the serine β-lactamase CTX-M-15

Ellagic acid, a δ-lactone with ionisable phenolic residues, is an efficient time-dependent inhibitor of the serine β-lactamase enzyme CTX-M-15. The pH-dependence of the rate of inhibition shows that both the mono- and di-anionic species of ellagic acid are effective inhibitors, both with second order rate constants of ∼1.5 × 10⁴ M⁻¹ s⁻¹. The structurally similar δ-lactone urolithin A, which lacks the geometrically appropriate phenolic residue, shows only modest inhibitory activity against CTX-M-15. It is proposed that this inhibition by ellagic acid anions involves acylation of the active site serine and that the negative charge on the inhibitor is required for binding to the active site.

Both the mono- and di-anions of the δ-lactone containing ellagic acid are time-dependent covalent inhibitors of the active site of β-lactamase.     

Effect of C-terminus amidation of Aβ39–42 fragment derived peptides as potential inhibitors of Aβ aggregation

The C-terminus fragment (Val-Val-Ile-Ala) of amyloid-β is reported to inhibit the aggregation of the parent peptide. In an attempt to investigate the effect of sequential amino-acid scan and C-terminus amidation on the biological profile of the lead sequence, a series of tetrapeptides were synthesized using MW-SPPS. Peptide D-Phe-Val-Ile-Ala-NH₂ (12c) exhibited high protection against β-amyloid-mediated-neurotoxicity by inhibiting Aβ aggregation in the MTT cell viability and ThT-fluorescence assay. Circular dichroism studies illustrate the inability of Aβ₄₂ to form β-sheet in the presence of 12c, further confirmed by the absence of Aβ₄₂ fibrils in electron microscopy experiments. The peptide exhibits enhanced BBB permeation, no cytotoxicity along with prolonged proteolytic stability. In silico studies show that the peptide interacts with the key amino acids in Aβ, which potentiate its fibrillation, thereby arresting aggregation propensity. This structural class of designed scaffolds provides impetus towards the rational development of peptide-based-therapeutics for Alzheimer''s disease (AD).

Amidated C-terminal fragment, Aβ_39–42 derived non-cytotoxic β-sheet breaker peptides exhibit excellent potency, enhanced bioavailability and improved proteolytic stability.

First reported by Alois Alzheimer in 1906, Alzheimer''s Disease (AD) is a progressive, neurodegenerative disorder with an irreversible decline in memory and cognition.¹ It is commonly seen in elderly populations and is marked by two major histopathological hallmarks, amyloid-β (Aβ) plaques and neurofibrillary tangles (NFT).² The number of patients suffering from AD has been increasing at an alarming rate. It is estimated that around 60 million patients will be suffering from AD by the end of 2020 and half of this patient population would require the care equivalent to that of a nursing home.³ Even after a century of its discovery, there has been no treatment that targets the pathophysiology of AD.⁴ The current treatment regimen includes acetylcholine-esterase inhibitors (AChEI) comprising of donepezil, rivastigmine and galantamine as well as N-methyl-d-aspartate (NMDA)-receptor antagonist, memantine. These provide only symptomatic relief to the patient. Most of the therapeutics that are currently being tested in clinical trials couldn''t proceed beyond phase II and phase III clinical trials due to lower efficacy in elderly patients, multiple side effects and limitations in their pharmacokinetic and pharmacodynamic profiles.^5–9 This has created challenges on the societal and economic upfront.Literature analysis reveals that the soluble oligomeric Aβ species is the culprit for neurotoxicity. It interrupts normal physiological functioning of the human brain. The central hydrophobic fragment Aβ_16–22 (KLVFFAE) and the C-terminus region fragment Aβ_31–42 (IIGLMVGGVVIA) are responsible for controlling the aggregation kinetics of the monomeric species, wherein the later still remains relatively less explored.^8,9 Our group is focused on development of peptidomimetic analogues for preventing Aβ aggregation. Studies on a complete peptide scan on the C-terminus region regions has already been published previously.^10–12 In an attempt to enhance the biological efficacy of the previously designed scaffolds,¹² we rationalized the use of sequential amino acid scan by modifying/replacing individual residues, as well as amide protection of the C-terminus on the lead tetrapeptide sequence (Val-Val-Ile-Ala) to enhance the proteolytic stability of the peptides.C-terminus amidated peptides were synthesized by microwave-assisted Fmoc-solid phase peptide synthesis protocol. Scheme 1 shows the general route for the synthesis of peptides employing Rink amide resin (detailed methodology is mentioned in the ESI, Section 1^†). These peptides were characterized using by analytical HPLC, ¹H and ¹³C NMR, APCI/ESI-MS and HRMS.Open in a separate windowScheme 1Synthesis of tetrapeptide 12c using MW-assisted Fmoc-solid phase peptide synthesis protocol employing Rink amide resin. Reaction conditions: (i) 20% piperidine in DMF (7 mL), MW (40 W), 60 °C, 2 cycles – 1.5 & 3 min; (ii) 2, 4, 6, 8 (4 equiv.), TBTU (4 equiv.), HoBt (4 equiv.), DIPEA (5 equiv.), DMF (3.5 mL), MW (40 W), 60 °C, 13.5 min; (iii) TFA : TIPS : H₂O (95 : 2.5 : 2.5), rt, 2.5 h; reaction monitoring was done by: UV measurement: Fmoc deprotection-dibenzofulvene adduct, Kaiser test: 1° amines; acetaldehyde test: 2° amines.Aggregation of Aβ₄₂ results in accumulation of toxic species, which interact with neurons and hinder their functioning, leading to loss of memory and cognition. Inhibiting the process of Aβ₄₂ aggregation would alleviate the neurotoxicity imparted in PC-12 cells; this was evaluated by MTT cell viability assay.¹³ Viability of untreated cells is considered 100%. Upon treatment with 2 μM of Aβ₄₂, only 74% cells were found to be viable. Out of the total tested peptides, seven tetrapeptides 12a, 12c, 12f, 13e, 14b, 15b and 15f showed complete inhibition of Aβ₄₂-induced toxicity by restoring the cell viability to 100%, at respective concentrations. Results for cell viability assay along with Thioflavin-T fluorescence assay for all the synthesized tetrapeptides has been summarized in

Condensation of acrylonitrile and aryl acetonitrile: construction of α-amino-β-cyano cyclohexene skeletons

A representative condensation of acrylonitrile and aryl acetonitrile has been reported for the synthesis of α-amino-β-cyano cyclohexene. The reaction was carried out mildly in an open environment at room temperature. The scope and versatility of the method have been demonstrated with 20 examples, containing highly active ethynyl groups. Further applications for 4-aminopyrimidine compounds were performed. A mechanism was proposed, involving Michael additions between acrylonitrile and aryl acetonitriles as well as intramolecular condensation.

A condensation reaction between acrylonitrile and benzyl cyanide for the synthesis of α-amino-β-cyano cyclohexene was reported. The reaction could be carried out mildly with high atomic efficiency to build the cyclohexene skeleton.

Multi-substituted cyclohexenes are important building blocks found in natural products, anti-influenza drugs and spices.¹ Oseltamivir, vitamin A, dynascone and beta-ionone are widely used representative molecules.² The well-known anti-flu drug oseltamivir also possesses a cyclohexene skeleton (Fig. 1a). Unlike the N-containing heterocyclic skeletons of pyridines, indoles and aminopyrimidines, little attention has been paid to the construction of cyclohexene skeletons. Shikimic acid, the raw compound for oseltamivir, is generally obtained from phytoextraction or biological fermentation.³ Further functionalization or modification is required considering the limited candidates restricted by these approaches.Open in a separate windowFig. 1Selected examples and previous works.The Diels–Alder reaction is commonly applied in the construction of the cyclohexene skeleton as a standard template.⁴ However, to the best of our knowledge, this [4 + 2] ring addition for the α-amino-β-cyano cyclohexene skeleton is unpractical. The acid-catalyzed dehydration of cyclohexanol is also used for the synthesis of cyclohexene.⁵ The use of acids and rigorous reaction conditions leads to tedious post-processing steps. The condensation between ammonium and α-cyano cyclohexanone is thought to be feasible in the construction of the cyclohexene skeleton. However, the use of expensive α-cyano cyclohexanone makes this strategy impracticable.⁶ The synthesis of α-amino-β-cyano cyclohexene via Thorpe–Ziegler condensation is commonly regarded as an intramolecular exception in the condensation of pimelic dinitrile (Fig. 1b).⁷ This reaction for building a cyclohexene skeleton is restricted by the fact that there are only a few active sites for further extension. Unlike the common synthetic approach using β-enaminonitrile, α-amino-β-cyano cyclohexene has a Z-configuration, which has high energy and high reactivity. The multifunctional unit has proved to be a key construction block in the synthesis of heterocyclic compounds such as pyrimidine, pyridine, pyrrole, pyrazole and imidazole.⁸Herein, we present a novel synthetic cyclic strategy towards 2,4-dicyano-1-amino cyclohexene skeletons. This strategy contains Michael additions between acrylonitrile and aryl acetonitriles as well as intramolecular condensation.⁹ We carried out the reaction mildly in an open environment at room temperature. Moreover, further applications of 4-aminopyrimidine compounds were performed.We began our initial study using aryl acetonitrile (1a) and acrylonitrile (2) as benchmark substrates for condensation. All the reactions were carried out at room temperature in air. We optimized the reaction conditions by changing the ingredients or conditions such as base, solvent and time to find the most suitable conditions ( En.BaseSolventTimeYield^b1 ^t BuOK THF2 h842 K ₂ CO ₃ THF2 h03 KOH THF2 hTrace4 DBU THF2 h425 LiHMDS THF2 h176^{c t} BuOKTHF2 h607^{d t} BuOKTHF2 h748 ^t BuOK Toluene 2 h569 ^t BuOK Dioxane 2 h8010 ^t BuOK Acetonitrile 2 h6711 ^t BuOK DMF 2 h5312 ^t BuOK Water 2 h013 ^t BuOKTHF 0.25 h 1614 ^t BuOKTHF 0.5 h 4315 ^t BuOKTHF 4 h 84Open in a separate window^aReaction conditions: 1a (0.4 mmol), 2 (0.84 mmol), base (1.2 mmol) and solvent (3 mL) at room temperature.^bIsolated yield.^cBase (1 equiv.).^dBase (2 equiv.).With the best conditions of this reaction in hand, we sought to investigate the generality and functional group compatibility of phenylacetonitrile (1a) under the established conditions, and the results are presented in Open in a separate window^aReaction conditions: 1a (0.4 mmol), 2 (0.84 mmol), t-BuOK (1.2 mmol) and solvent (3 mL) at room temperature for 2 h.^bIsolated yield.In addition, further studies were performed for a better understanding of the reaction, and a by-product 3n′ was isolated under the standard reaction conditions for the synthesis of 3n. 3-(2-Fluorophenyl)pentane-1,3,5-tricarbonitrile was thought to be one of the intermediates for the reaction (Scheme 3). Further utilization of compound 3n′ could afford the final product 3n in a yield of 53%. Moreover, the synthetic utility of our reaction was examined by performing gram-scale experiments. The reaction of phenylacetonitrile (1a) and acrylonitrile (2) on a 5.0 g gram scale under the standard conditions generated the compound 3a in 71% yield (Scheme 1c). For further extension, we found that the product 3a and benzonitrile (4a) could be converted to the 4-aminopyrimidines compound 5a (Scheme 2a). The same reaction was carried out using three other compounds (4b, 4c and 4d) as the starting materials (Scheme 2b–d). The crystal structures of compounds 3a and 5a were determined by X-ray crystallography (Fig. 2).Open in a separate windowScheme 1Mechanistic studies and gram-scale experiments.Open in a separate windowScheme 2Applications of the products.Open in a separate windowFig. 2Crystal structures of compounds 3a (CCDC 2158344) and 5a (CCDC 2158132).Open in a separate windowScheme 3Possible reaction mechanism.A plausible mechanism has been depicted in Scheme 3. In the presence of a base, the methylene group of aryl acetonitrile loses a proton and turns into a cyano-alkylide anion (a). Nucleophilic attack by the R–CH⁻–CN anion results in the formation of 4-phenyl-4-cyano-butyl nitrile (b) with the consumption of a molecule of acrylonitrile. Another molecule of acrylonitrile is consumed for the synthesis of the subsequent intermediate 4-phenyl-4-cyano-pimelic dinitrile (c).¹⁰ The binitrile intermediate (c) then performs a base-promoted intramolecular condensation (Scheme 1b). With the participation of the base, intermediate (c) undergoes a deprotonation process to afford the anion intermediate (d). The nucleophilic attack from the alpha-carbon anion to the cyano group leads to the formation of imine species and ultimately results in the final product 3a.     

Oxygen Equilibrium Characteristics of Abnormal Hemoglobins: Hirose (α2β237Ser), L Ferrara (α247Glyβ2), Broussais (α290Asnβ2), and Dhofar (α2β258Arg)

The oxygen equilibrium characteristics of four structural variants of hemoglobin A were correlated with their amino acid substitutions.Hemoglobin Dhofar, in which the proline at E2(58)beta is replaced by arginine, had normal oxygen equilibrium characteristics.Hemoglobin L Ferrara. in which the aspartic acid at CD5(47)alpha is replaced by glycine, and hemoglobin Broussais, in which the lysine at FG2(90)alpha is replaced by asparagine, both showed a slightly elevated oxygen affinity; nevertheless both demonstrated a normal heme-heme interaction and a normal Bohr effect.Hemoglobin Hirose, in which the tryptophan at C3 (37)beta is replaced by serine, showed abnormalities of all oxygen equilibrium characteristics; i.e., increased oxygen affinity, diminished heme-heme interaction, and reduced Bohr effect.These results suggest that aspartic acid at CD5(47)alpha and lysine at FG2(90)alpha are involved in the function of the hemoglobin molecule, despite the fact that these positions are not located directly in the heme or the alpha-beta-contact regions.Tryptophan at C3(37)beta is located at contact between alpha(1)- and beta(2)-subunits. It is suggested that the substitution by serine might disturb the quarternary structure of the mutant hemoglobin molecule during transition from oxy-form to deoxy-form resulting in an alteration of the heme function.     

Inhibition of β-Lactamases by β-Lactam Antibiotics

The inhibitory properties of a selected number of beta-lactam antibiotics were studied, with the use of three distinct types of beta-lactamases. The three enzymes were found to be distinguishable on the basis of their susceptibility to inhibition. Not one of the potential inhibitors tested was found to be a potent inhibitor of all three enzymes, but nafcillin possessed the broadest inhibitory activity. The enzyme isolated from Enterobacter cloacae was found to be the most susceptible. In some cases, the degree of inhibition varied with the time of incubation, and, depending upon the time chosen, widely different observations could be made. It is suggested that, in studies such as these, every consideration should be given to the period of incubation and to the concentration of inhibitor employed. Mixtures of inhibitor and cephaloridine did not always act synergistically against growing bacteria, and a number of reasons for failure are suggested.     

Discovery of a Small-Molecule Inhibitor of β-1,6-Glucan Synthesis

It is possible that antifungal drugs with novel modes of action will provide favorable options to treat fungal infections. In the course of our screening for antifungal compounds acting on the cell wall, a pyridobenzimidazole derivative with unique activities, named D75-4590, was discovered. During treatment of Saccharomyces cerevisiae with D75-4590, (i) incorporation of [¹⁴C]glucose into the β-1,6-glucan component was selectively reduced, (ii) proteins released from the cell had lost the β-1,6-glucan moiety, and (iii) cells tended to clump, resulting in impaired cell growth. Genetic analysis of a D75-4590-resistant mutant of S. cerevisiae indicated that its primary target was Kre6p, which is considered to be one of the β-1,6-glucan synthases. These results strongly suggest that D75-4590 is a specific inhibitor of β-1,6-glucan synthesis. D75-4590 showed potent activities against various Candida species. It inhibited hyphal elongation of C. albicans as well. KRE6 is conserved in various fungi, but no homologue has been found in mammalian cells. These lines of evidence indicate that D75-4590 is a promising lead compound for novel antifungal drugs. To our knowledge, this is the first report of a β-1,6-glucan inhibitor.     

Catalyst-free chemoselective α-sulfenylation/β-thiolation for α,β-unsaturated carbonyl compounds

A novel, efficient, catalyst-free and product-controllable strategy has been developed for the chemoselective α-sulfenylation/β-thiolation of α,β-unsaturated carbonyl compounds. An aromatic sulfur group could be chemoselectively introduced at α- or β-position of carbonyls with different sulfur reagents under slightly changed reaction conditions. A series of desired products were obtained in moderate to excellent yields. Mechanistic studies revealed that B₂pin₂ played the key role in activating the transformation towards the β-thiolation of α,β-unsaturated carbonyl compounds. This transition-metal-catalyst-free method provides a convenient and efficient tool for the highly chemoselective preparation of α-thiolation or β-sulfenylation products of α,β-unsaturated carbonyl compounds.

This catalyst-free method provides a useful and efficient tool for the highly chemoselective preparation of α-thiolation or β-sulfenylation products of α,β-unsaturated carbonyl compounds.     

Proteomics and metabolomics analysis reveal potential mechanism of extended-spectrum β-lactamase production in Escherichia coli

In this study, ten clinical susceptible strains and ten clinical ESBL-EC (extended-spectrum β-lactamase-producing Escherichia coli) were screened and obtained by microbial identification using ITEK® 2 Compact. TMT (Tandem Mass Tag) proteomics analysis discovered 1553 DEPs (differentially expressed proteins) between ESBL-EC and non-ESBL-EC. In addition, an untargeted metabolomics assay by using UHPLC-MS (ultra-high-performance liquid chromatography-mass spectrometry) was applied to compare the differential profiles of metabolites between β-lactam antibiotic-sensitive E. coli and multidrug-resistant ESBL-producing E. coli strains. The PCA (principal component analysis) score plots and OPLS-DA (orthogonal projections to latent structures discriminant analysis) plots clearly discriminated ESBL-EC and non-ESBL-EC, and volcano analysis presented 606 and 459 altered metabolites between ESBL-EC vs. non-ESBL-EC in positive and negative ion modes, respectively. Interestingly, the bioinformatics analysis demonstrated that the purine metabolism pathway was enriched in ESBL-EC. These results suggest that the existence of extended-spectrum β-lactamase affects the metabolite and protein profiles of E. coli. The correlation analysis of metabolomics and proteomics data established a correlation between DEPs and differential metabolites in the purine metabolism pathway. Moreover, three metabolite candidates in the purine metabolism pathway were validated by the UPLC-MRM-MS (ultra-performance liquid chromatography multiple reaction monitoring mass spectrometry) method. Our data suggest that these DEPs and differential metabolites may play important roles in the antibiotic resistance of ESBL-EC. Our study can provide scientific data for the mechanism study of antibiotic resistance of ESBL-EC at the metabolite and protein levels and targeting modulators to these pathways may be effective for treatment of ESBL-EC strains.

Proteomic and metabolomics revealed the underlying mechanism of extended-spectrum β-lactamase production in Escherichia coli.     

Electrophilic halogenations of propargyl alcohols: paths to α-haloenones, β-haloenones and mixed β,β-dihaloenones

The Meyer–Schuster rearrangement of propargyl alcohols or alkynols leading to α,β-unsaturated carbonyl compounds is well known. Yet, electrophilic halogenations of the same alkynols and their alkoxy, ester and halo derivatives are inconspicuous. This review on the halogenation reactions of propargyl alcohols and derivatives intends to give a perspective from its humble direct halogenation beginning to the present involving metal catalysis. The halogenation products of propargyl alcohols include α-fluoroenones, α-chloroenones, α-bromoenones and α-iodoenones, as well as β-haloenones and symmetrical and mixed β,β-dihaloenones. They are, in essence, tri and tetrasubstituted alkenes carrying halo-functionalization at the α- or β-carbon. This is a potential stepping stone for further construction towards challenging substituted alkenones via Pd-catalysed coupling reactions.

This review highlights the development of α-haloenone, β-haloenone and mixed β,β-dihaloenone formations from propargyl alcohols via direct electrophilic halogenations and metal catalysed-halonium interception rearrangements.     

A MoS2 based silver-doped ZnO nanocomposite and its antibacterial activity against β-lactamase expressing Escherichia coli

Multidrug-resistant (MDR) Gram-negative bacteria including Escherichia coli are increasingly resistant to current antibiotics. Among the strategies implemented to eradicate such MDR pathogens, approaches based on two-dimensional (2D) nanomaterials have received considerable attention. In particular, the excellent physicochemical properties of 2D molybdenum disulfide (MoS₂) nanosheets, including a high surface area, good conductivity, and good surface retention, are advantageous for their use as bactericidal agents. Herein, we report the fabrication of a MoS₂-based nanocomposite conjugated with silver-doped zinc oxide (AZM) as an effective antibacterial agent against E. coli species. The properties of AZM were characterized, and its antibacterial activity against MDR E. coli strains with different resistance types was evaluated. MoS₂ was found to activate the antibacterial activity of AZM and provide enhanced selectivity against MDR E. coli strains expressing β-lactamases. We proposed that membrane disruption of bacterial cell walls was the major cell death mechanism for MDR E. coli. Furthermore, surface charge perturbation could explain the differences in AZM activity against MDR E. coli strains expressing a β-lactamase and a mobilized colistin resistance (mcr-1) gene product. Thus, a MoS₂-based nanocomposite with a functional conjugation strategy could be a selective nano-antibacterial platform against infections caused by MDR E. coli with resistance against β-lactam antibiotics.

Synthesis and activity of a MoS₂ based nanoplatform.     

Thermal stability and oxidation characteristics of α-pinene, β-pinene and α-pinene/β-pinene mixture

Turpentine is a renewable resource, has good combustion performance, and is considered to be a fuel or promising additive to diesel fuel. This is very important for the investigation of thermal stability and energy oxidation characteristics, because evaluation of energy or fuel quality assurance and use safety are necessary. The main components of turpentine are α-pinene and β-pinene, which have unsaturated double bonds and high chemical activity. By investigating their thermal stability and oxidation reaction characteristics, we know the chemical thermal properties and thermal explosion hazard of turpentine. In this present study, the thermal stability and oxidation characteristics of α-pinene, β-pinene and α-pinene/β-pinene mixture were investigated using a high sensitivity accelerating rate calorimeter (ARC) and C80 calorimeter. The important parameters of oxidation reaction and thermal stability were obtained from the temperature, pressure and exothermic behavior in chemical reaction. The results show that α-pinene and β-pinene are thermally stable without chemical reaction under a nitrogen atmosphere even when the temperature reaches 473 K. The initial exothermic temperature of the two pinenes and their mixture is 333–338 K, and the heat release (−ΔH) of their oxidation is 2745–2973 J g⁻¹. The oxidation activation energy (E_a) of α-pinene, β-pinene and α-pinene/β-pinene mixture is 116.25 kJ mol⁻¹, 121.85 kJ mol⁻¹, and 115.95 kJ mol⁻¹, respectively. There are three steps in the oxidation of pinenes: the first is the induction period of the oxidation reaction; the second is the main oxidation stage, and the pressure is reduced; the third is thermal decomposition to produce gas.

Turpentine is a renewable resource, has good combustion performance, and is considered to be a fuel or promising additive to diesel fuel.     

N-Amino peptide scanning reveals inhibitors of Aβ42 aggregation

The aggregation of amyloids into toxic oligomers is believed to be a key pathogenic event in the onset of Alzheimer''s disease. Peptidomimetic modulators capable of destabilizing the propagation of an extended network of β-sheet fibrils represent a potential intervention strategy. Modifications to amyloid-beta (Aβ) peptides derived from the core domain have afforded inhibitors capable of both antagonizing aggregation and reducing amyloid toxicity. Previous work from our laboratory has shown that peptide backbone amination stabilizes β-sheet-like conformations and precludes β-strand aggregation. Here, we report the synthesis of N-aminated hexapeptides capable of inhibiting the fibrillization of full-length Aβ₄₂. A key feature of our design is N-amino substituents at alternating backbone amides within the aggregation-prone Aβ_16–21 sequence. This strategy allows for maintenance of an intact hydrogen-bonding backbone edge as well as side chain moieties important for favorable hydrophobic interactions. An N-amino scan of Aβ_16–21 resulted in the identification of peptidomimetics that block Aβ₄₂ fibrilization in several biophysical assays.

Structure-based design of backbone-aminated peptides affords novel β-strand mimics that inhibit amyloid-beta fibrillogenesis.     

Direct β-selectivity of α,β-unsaturated γ-butyrolactam for asymmetric conjugate additions in an organocatalytic manner

The β-selective asymmetric addition of γ-butyrolactam with cyclic imino esters catalyzed by a bifunctional chiral tertiary amine has been developed, which provides an efficient access to optically active β-position functionalized pyrrolidin-2-one derivatives in both high yield and enantioselectivity (up to 78% yield and 95 : 5 er). This is the first catalytic method to access chiral β-functionalized pyrrolidin-2-one via a direct organocatalytic approach.

The asymmetric addition of γ-butyrolactam with cyclic imino esters catalyzed by (DHQD)₂AQN has been developed, which provides an access to β-position functionalized pyrrolidin-2-one derivatives in high levels yield and enantioselectivity.

Metal-free organocatalytic asymmetric transformations have successfully captured considerable enthusiasm of chemists as powerful methods for the synthesis of various kinds of useful chiral compounds ranging from the preparation of biologically important molecules through to novel materials.¹ Chiral pyrrolidin-2-ones have been recognized as important structural motifs that are frequently encountered in a variety of biologically active natural and synthetic compounds.² In particular, the β-position functionalized pyrrolidin-2-one backbones, which can serve as key synthetic precursors for inhibitory neurotransmitters γ-aminobutyric acids (GABA),³ selective GABA_B receptor agonists⁴ as well as antidepressant rolipram analogues,⁵ have attracted a great deal of attention. Therefore, the development of highly efficient, environmentally friendly and convenient asymmetric synthetic methods to access these versatile frameworks is particularly appealing.As a direct precursor to pyrrolidin-2-one derivatives, recently, α,β-unsaturated γ-butyrolactam has emerged as the most attractive reactant in asymmetric organometallic or organocatalytic reactions for the synthesis of chiral γ-position functionalized pyrrolidin-2-ones (Scheme 1). These elegant developments have been achieved in the research area of catalytic asymmetric vinylogous aldol,⁶ Mannich,⁷ Michael⁸ and annulation reactions⁹ in the presence of either metal catalysts or organocatalysts (a, Scheme 1). These well-developed catalytic asymmetric methods have been related to the γ-functionalized α,β-unsaturated γ-butyrolactam to date. However, in sharp contrast, the approaches toward introducing C-3 chirality at the β-position of butyrolactam through a direct catalytic manner are underdeveloped (b, Scheme 1)¹⁰ in spite of the fact that β-selective chiral functionalization of butyrolactam can directly build up α,β-functionalized pyrrolidin-2-one frameworks.Open in a separate windowScheme 1Different reactive position of α,β-unsaturated γ-butyrolactam in catalytic asymmetric reactions.So far, only a few metal-catalytic enantioselective β-selective functionalized reactions have been reported. For examples, a rhodium/diene complex catalyzed efficient asymmetric β-selective arylation^10a and alkenylation^10b have been reported by Lin group (a, Scheme 2). Procter and co-workers reported an efficient Cu(i)–NHC-catalyzed asymmetric silylation of unsaturated lactams (b, Scheme 2).^10c Despite these creative works, considerable challenges still exist in the catalytic asymmetric β-selective functionalization of γ-butyrolactam. First, the scope of nucleophiles is limited to arylboronic acids, potassium alkenyltrifluoroborates and PhMe₂SiBpin reagents. Second, the catalytic system and activation mode is restricted to metal/chiral ligands. To our knowledge, an efficient catalytic method to access chiral β-functionalized pyrrolidin-2-one via a direct organocatalytic approach has not yet been established. Therefore, the development of organocatalytic asymmetric β-selective functionalization of γ-butyrolactam are highly desirable. In conjunction with our continuing efforts in building upon chiral precedents by using chiral tertiary amine catalytic system,¹¹ we rationalized that the activated α,β-unsaturated γ-butyrolactam might serve as a β-position electron-deficient electrophile. This γ-butyrolactam may react with a properly designed electron-rich nucleophile to conduct an expected β-selective functionalized reaction of γ-butyrolactam under a bifunctional organocatalytic fashion, while avoiding the direct γ-selective vinylogous addition reaction or β,γ-selective annulation as outlined in Scheme 2. Herein we report the β-selective asymmetric addition of γ-butyrolactam with cyclic imino esters¹² catalyzed by a bifunctional chiral tertiary amine, which provides an efficient and facile access to optically active β-position functionalized pyrrolidin-2-one derivatives with both high diastereoselectivity and enantioselectivity.Open in a separate windowScheme 2β-Selective functionalization of γ-butyrolactam via metal- (previous work) or organo- (this work) catalytic approach.To begin our initial investigation, several bifunctional organocatalysts¹³ were firstly screened to evaluate their ability to promote the β-selective asymmetric addition of γ-butyrolactam 2a with cyclic imino ester 3a in the presence of 15 mol% of catalyst loading at room temperature in CH₂Cl₂ (entries 1–6, EntryCat.SolventYield^eer^f11aCH₂Cl₂70%40 : 6021bCH₂Cl₂<5%57 : 4331cCH₂Cl₂70%65 : 3541dCH₂Cl₂68%70 : 3051eCH₂Cl₂58%63 : 4761fCH₂Cl₂71%77 : 2371fDCE72%80 : 2081fCHCl₃70%80 : 2091fMTBE68%79 : 21101fToluene63%78 : 22111fTHF45%76 : 24121fMeOH32%62 : 3813^b1fDCE : MTBE75%87 : 1314^c1fDCE : MTBE72%87 : 1315^d1fDCE : MTBE70%85 : 15Open in a separate window^aReaction conditions: unless specified, a mixture of 2a (0.2 mmol), 3a (0.3 mmol) and a catalyst (15 mmol%) in a solvent (2.0 mL) was stirred at rt. for 48 h.^bThe reaction was carried out in 2.2 mL a mixture of dichloroethane and methyl tert-butyl ether (volume ratio = 10 : 1).^cThe reaction was carried out in 2.2 mL a mixture of dichloroethane and methyl tert-butyl ether (volume ratio = 10 : 1) for 24 h.^dThe reaction was carried out in 2.2 mL a mixture of dichloroethane and methyl tert-butyl ether (volume ratio = 10 : 1) and 10 mol% of catalyst was used.^eIsolated yields.^fDetermined by chiral HPLC, the product was observed with >99 : 1 dr by ¹H NMR and HPLC. Configuration was assigned by X-ray crystal data of 4a.The results of experiments under the optimized conditions that probed the scope of the reaction are summarized in Scheme 3. The catalytic β-selective asymmetric addition of γ-butyrolactam 2a with cyclic imino esters 3a in the presence of 15 mol% (DHQD)₂AQN 1f was performed. A variety of phenyl-substituted cyclic imino esters including those bearing electron-withdrawing and electron-donating substituents on the aryl ring, heterocyclic were also examined. The electron-neutral, electron-rich, or electron-deficient groups on the para-position of phenyl ring of the cyclic imino esters afforded the products 4a–4m in 57–75% yields and 82 : 18 to 95 : 5 er values. It appears that either an electron-withdrawing or an electron-donating at the meta- or ortho-position of the aromatic ring had little influence on the yield and stereoselectivity. Similar results on the yield and enantioselectivities were obtained with 3,5-dimethoxyl substituted cyclic imino ester (71% yield and 91 : 9 er). It was notable that the system also demonstrated a good tolerance to naphthyl substituted imino ester (78% yield and 92 : 8 er value). The 2-thienyl substituted cyclic imino ester proceeded smoothly under standard conditions as well, which gave the desired product 4p in good enantioselectivity (88 : 12 er), although yield was slightly lower. However, attempts to extend this methodology to aliphatic-substituted product proved unsuccessful due to the low reactivity of the substrate 3q. It is worth noting that the replacement of Boc group with 9-fluorenylmethyl, tosyl or benzyl group as the protection, no reaction occurred. The absolute and relative configurations of the products were unambiguously determined by X-ray crystallography (4a, see the ESI^†).Open in a separate windowScheme 3Substrate scope of the asymmetric reaction of α,β-unsaturated γ-butyrolactam 2 to cyclic imino esters 3.^{a a}Reaction conditions: unless specified, a mixture of 2 (0.2 mmol), 3 (0.3 mmol) and 1f (15.0 mmol%) in 2.2 mL a mixture of dichloroethane and methyl tert-butyl ether (volume ratio = 10 : 1) was stirred at rt. ^bIsolated yields. ^cDetermined by chiral HPLC, all products were observed with >99 : 1 dr by ¹H NMR and HPLC. Configuration was assigned by comparison of HPLC data and X-ray crystal data of 4a.We then examined the substrate scope of the imide derivatives (Scheme 4). Investigations with maleimides 4r–4u gave 48–61% yield of corresponding products as lower er and dr values than most of γ-butyrolactams. As for methyl substituted maleimides, the reaction failed to give any product.Open in a separate windowScheme 4Substrate scope of the asymmetric reaction of maleimides to cyclic imino esters.^{a a}Reaction conditions: unless specified, a mixture of 2 (0.2 mmol), 3 (0.3 mmol) and 1f (15.0 mmol%) in 2.2 mL a mixture of dichloroethane and methyl tert-butyl ether (volume ratio = 10 : 1) was stirred at rt. ^bIsolated yields. ^cDetermined by ¹H NMR and chiral HPLC.The chloride product 4a ((R)-tert-butyl 4-((R)-3-((E)-(4-chlorobenzylidene)amino)-2-oxotetra hydrofuran-3-yl)-2-oxopyrrolidine-1-carboxylate) was recrystallized and the corresponding single crystal was subjected to X-ray analysis to determine the absolute structure. Based on this result and our previous work, a plausible catalytic mechanism involving multisite interactions was assumed to explain the high stereoselectivity of this process (Fig. 1). Similar to the conformation reported for the dihydroxylation and the asymmetric direct aldol reaction, the transition state structure of the substrate/catalyst complexes might be presumably in the open conformation. The acidic α-carbon atom of cyclic imino ester 3a could be activated by interaction between the tertiary amine moiety of the catalyst and the enol of 3avia a hydrogen bonding. Moreover, the enolate of 3a in the transition state might be in part stabilized through the π–π stacking between the phenyl ring of 3a and the quinoline moiety. Consequently, the Re-face of the enolate is blocked by the left half of the quinidine moiety. The steric hindrance between the Boc group of 2a and the right half of the quinidine moiety make the Re-face of 2a face to the enolate of 3a. Subsequently, the attack of the incoming nucleophiles forms the Si-face of enolate of 3a to Re-face of 2a takes place, which is consistent with the experimental results.Open in a separate windowFig. 1Proposed transition state for the reaction.In conclusion, we have disclosed the β-selective asymmetric addition of γ-butyrolactam with cyclic imino esters catalyzed by a bifunctional chiral tertiary amine, which provides an efficient and facile access to optically active β-position functionalized pyrrolidin-2-one derivatives with high diastereoselectivity and enantioselectivity. To our knowledge, this is the first catalytic method to access chiral β-functionalized pyrrolidin-2-one via a direct organocatalytic approach. Current efforts are in progress to apply this new methodology to synthesize biologically active products.     

A first-principles investigation of α, β, and γ-MnO2 as potential cathode materials in Al-ion batteries

An inexpensive and eco-friendly alternative energy storage solution is becoming more in demand as the world moves towards greener technology. We used first principles calculations to investigate α, β, and γ-MnO₂ and their Al-ion intercalation mechanism in potential applications for aluminum batteries. We explored these complexes through investigating properties such as volume change, binding/diffusion energy, and band gap to gauge each material. α-MnO₂ had almost no volume change. γ-MnO₂ had the lowest binding energy and diffusion barrier. Our study gives insight into the feasibility of using MnO₂ in aluminum batteries and guides investigation of the material within its different phases.

An inexpensive and eco-friendly alternative energy storage solution is becoming more in demand as the world moves towards greener technology.     

Direct access to multi-functionalized benzenes via [4 + 2] annulation of α-cyano-β-methylenones and α,β-unsaturated aldehydes

An efficient [4 + 2] benzannulation of α-cyano-β-methylenones and α,β-unsaturated aldehydes was achieved under metal-free reaction conditions selectively delivering a wide range of polyfunctional benzenes in high yields respectively (up to 94% yield).

An efficient [4 + 2] benzannulation of α-cyano-β-methylenones and α,β-unsaturated aldehydes was achieved under metal-free reaction conditions selectively delivering a wide range of polyfunctional benzenes in high yields respectively (up to 94% yield).

Multi-substituted benzenes are privileged structural units ubiquitous in pharmaceuticals,¹ natural products² and advanced functional materials.³ Various excellent methodologies have been investigated for the construction of functionalized aromatics including nucleophilic or electrophilic substitution,⁴ transition metal-catalyzed coupling reactions⁵ and directed metalation.⁶ However, the widespread application of these strategies established thus far suffer from the limitations of functional groups introduced on the pre-existing benzene and regioselectivity issues. Among various synthetic methods, tandem benzannulation reactions arguably represent an attractive alternative to classical methods for rapid construction of polysubstituted benzenes in an atom-economical fashion.⁷ This protocol featuring an efficient transformation of acyclic building blocks into structurally valuable benzene skeletons. In this context, α-cyano-β-methylenones has been employed as substrates to format six-membered ring in tandem cyclization reactions due to the activation of the pronucleophile methyl group. In 2015, Tong and co-workers developed a phosphine-catalyzed addition/cycloaddition domino reactions of β′-acetoxy allenoate with 2-acyl-3-methyl-acrylonitriles to give 2-oxabicyclo[3.3.1]nonanes (Scheme 1a).⁸ Soon after that, the construction of benzonitrile derivatives and 1,3,5-trisubstituted benzenes via N-heterocyclic carbene catalysis has been reported by the groups of Wang and Ye independently (Scheme 1b).⁹ Then the synthesis of 1,3,5-trisubstituted benzenes by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-mediated annulation of α-cyano-β-methylenones and α,β-unsaturated carboxylic acids was also developed by Ye and co-workers (Scheme 1c).¹⁰ Shi et al. reported a base-promoted tandem cyclization reaction of α-cyano-β-methylenones and α,β-unsaturated enones, which have electron-withdrawing group (EWG), accessing to a wide range of benzonitriles in a different C–C bond formation process (Scheme 1d).¹¹ As part of our ongoing interest in harnessing enones for developing new methodologies for the construction of functionalized benzenes, we have recently demonstrated NHC-catalyzed convenient benzonitrile assembly in the presence of oxidant.^9a While the same reaction of enals and α-cyano-β-methylenones was conducted in the basic condition without NHC, a novel polyfunctionalized benzene product was obtained (Scheme 1e). The result inspired us to extend the synthetic potential of benzannulation strategy to access diverse benzonitriles, particularly from simpler, abundantly available starting materials.Open in a separate windowScheme 1α-Cyano-β-methylenones in cycloaddition domino reactions.At the outset, model reaction of 2-benzoyl-3-phenylbut-2-enenitrile 1a and cinnamaldehyde 2a was used to evaluate reaction parameters. Key results of condition optimization are summarized in 12 The configuration of products were assigned unambiguously by X-ray analysis of the product 3a. A quick solvent screening demonstrated that chloroform is the best choice to produce the benzannulation product 3a in a desirable yield (entries 10–13, †). Reducing the loading of the cinnamaldehyde or NaOH to 1.2 equivalence led to dramatical loss of the yield (entries 14 &15, EntryBaseSolventTime (h)Yield^b (%)1Cs₂CO₃Toluene24702Na₂CO₃Toluene24423K₂CO₃Toluene24384NaOHToluene12785NaOAcToluene24526KOHToluene12747K₃PO₄Toluene24588DBUToluene24339Et₃NToluene484610NaOHDCM1288 11 NaOH CHCI ₃ 12 94 12NaOHDCE128413NaOHH₂O48014^cNaOHCHCI₃128515^dNaOHCHCI₃128416^eNaOHCHCI₃1280Open in a separate window^aReaction conditions: 1a (0.1 mmol, 1.0 equiv.), 2a (0.15 mmol, 1.5 equiv.), base (0.2 mmol, 2.0 equiv.), and solvent (1 mL) for 12 h.^bIsolated yields.^c1a : 2a = 1 : 1.2.^dNaOH used 1.2 equiv.^e50 °C.Finally, the standard reaction conditions for the base-promoted synthesis of the multi-functionalized benzene derivatives identified as follows: 1.5 equivalence of NaOH and CHCl₃ as the solvent under an atmosphere of air for 12 hours at room temperature.With the optimized reaction conditions in hand, we explored the scope of the reaction. A series of enones were examined, variation of the electronic nature of the aromatic ring (R¹, including the substituted phenyl or thienyl) has little influence on the reaction efficiency (3b–f, 86–93% yields, Open in a separate window^aReaction conditions: 1a (0.1 mmol, 1.0 equiv.), 2a (0.15 mmol, 1.5 equiv.), NaOH (0.2 mmol, 2.0 equiv.), and CHCl₃ (1 mL) for 12 h.We next turned our attention to examine the scope of enals. Different substituents on the phenyl ring of cinnamaldehydes were tolerated even disregarding the position and properties, giving 4a–g in satisfying yields (82–92% yields, Open in a separate window^aReaction conditions: 1a (0.1 mmol, 1.0 equiv.), 2a (0.15 mmol, 1.5 equiv.), NaOH (0.2 mmol, 2.0 equiv.), and CHCl₃ (1 mL) for 12 h.To highlight the practicality of this mild and efficient method, the reaction of 2-benzoyl-3-phenylbut-2-enenitrile 1a at 4.0 mmol scale proceed well under the standard conditions to generate the desired product in 88% yield (Scheme 2).Open in a separate windowScheme 2Gram-Scale Synthesis of 3a.The formyl group could be easily reduced by using LiAlH₄ in THF at reflux, leading to the formation of the benzyl alcohol product 5 in 95% yield while keeping the CN group intact. Suzuki coupling of 3o with phenylboronic acid furnished derivative 6 in 90% yield¹³ (Scheme 3).Open in a separate windowScheme 3Synthetic transformation.To gain insight into the role of air in this reaction, a control experiment was designed and investigated (Scheme 4). When the reaction of 1a and 2a was carried out under an argon atmosphere, the desired product 3a was obtained in 10% yield and product 7 could be isolated in 82% yield. The results indicate that oxygen is necessary for the oxidation process and played a key role in this reaction.Open in a separate windowScheme 4Control experiment.A postulated reaction course is illustrated in Scheme 5. Briefly, α-deprotonation of enone 1a in the presence of bases, subsequent 1,4-addition of deprotonated enone I to enal 2a generates intermediate II, which undergoes an intramolecular aldol reaction to yield the adduct 7.¹⁴ Lastly, dehydration of 7 followed by spontaneous oxidative aromatization affords the polysubstituted benzonitrile 3a.Open in a separate windowScheme 5The proposed mechanism.     

Synthesis and characterization of new fluorescent boro-β-carboline dyes

The first representatives of the new fluorescent boro-β-carboline family were synthesized by the insertion of the difluoroboranyl group into the oxaza or diaza core. The resulting compounds showed good photophysical properties with fine Stokes-shifts in the range of 38–85 nm with blue and green emission. The energetics of the excitation states and molecular orbitals of two members were investigated by quantum chemical computations suggesting effects for the improved properties of diazaborinino-carbolines over oxazaborolo-carbolines. These properties nominated this chemotype as a new fluorophore for the development of fluorescent probes. As an example, diazaborinino-carbolines were used for the specific labeling of anti-Her2 antibody trastuzumab. The fluorescent conjugate showed a high fluorophore-antibody ratio and was confirmed as a useful tool for labeling and confocal microscopy imaging of tumour cells in vitro together with the ex vivo two-photon microscopy imaging of tumour slices.

The first representatives of fluorescent boro-β-carbolines were applied for labeling trastuzumab. The antibody fluorophore conjugate was confirmed as a useful tool for labeling and imaging tumour cells in confocal and two-photon microscopy.