Potent and selective non-peptidic inhibitors of endothelin-converting enzyme-1 with sustained duration of action

Recent advances in the synthesis of dibenzofurans

This review reports recent accesses to the dibenzofuran nucleus described in the literature since 2008. This article starts with synthesizing dibenzofurans by creating the C-O bond of the furan ring. In the following section, we evoke the formation of dibenzofurans by cyclizing diarylether derivatives. The last part of this update concerns the construction of dibenzofurans from benzofuran or phenol derivatives. Representative examples showing the scope of these processes illustrate new approaches and biological activities of dibenzofurans. Reaction mechanisms explaining certain dibenzofuran formation are described, as suggested by their authors.

Recent advances in the synthesis of cyclic compounds using α,α-dicyanoolefins as versatile vinylogous nucleophiles

This article provides a review of the applications of α,α-dicyanoolefins as versatile vinylogous nucleophiles in the synthesis of various cyclic compounds, covering the literature from the past 13 years.

Application of the Meerwein reaction of 1,4-benzoquinone to a metal-free synthesis of benzofuropyridine analogues

Several new heterocyclic systems based on a hydroxybenzofuro[2,3-b]pyridine building block were prepared. This benzofuropyridine is easily available from the Meerwein reaction of benzoquinone and a heterocyclic diazonium salt, followed by reduction and cyclization. Electrophilic substitution and further condensations give polycyclic systems, including oxazolo- and chromeno-fused analogues.

A cascade double 1,4-addition/intramolecular annulation strategy for expeditious assembly of unsymmetrical dibenzofurans

Existing synthetic routes for accessing dibenzofuran core have intrinsic regioselectivity, limiting the substitution patterns available in heteropolycyclic arene products. Here we report a double 1,4-conjugate addition/intramolecular annulation cascade reaction between propargylamines and two equivalents of imidazolium methylides that allows efficient access of structurally versatile dibenzofurans. This transition metal-free protocol proceeds smoothly under bench-top air atmosphere and offers easy manipulation of substituents on the dibenzofuran core, and also provides good-to-excellent product yields with good functional group tolerance, particularly the -Br and -Cl groups which are often incompatible with existing metal-catalyzed C-C and/or C-O bond ring-forming processes. It is worth noting that ladder-type π-systems with all-arene quarternary carbon structure can be straightforwardly generated upon simple late-stage functionalization.

The geometry relaxation and photodeactivation from the S2 state of dibenzofuran studied by ultrafast spectroscopy

Dibenzofuran (DBF) has attracted much attention from scientists recently since it is applied as a photoluminescence material and pharmaceutically active compound. Since the polychlorinated derivatives are highly toxic and manifest photostability in the environment. Femtosecond transient absorption spectroscopy associated with quantum chemical calculations are employed to investigate the ultrafast excited state dynamics of dibenzofuran from the S2 state in 1,4-dioxane and ethanol, respectively. Following excitation at a wavelength of 266 nm, the S2 state is firstly populated in the Franck–Condon region and preserves the planar molecular structure of the ground state. The observed increase of the transient absorption spectra of the excited state within the first several picoseconds indicates a geometry relaxation occurring on the S2 potential energy surface. The subsequent kinetic traces of excited state absorption show that the S2 state in the adiabatic region decays to the S1 state through a fast internal conversion, followed by intersystem crossing to the T1 state with a decay time of tens and hundreds of picoseconds in ethanol and 1,4-dioxane, respectively. Finally, the deactivation processes from the S1 or T state are slow and take place on a time scale of about 20 ns.

Effect of hydrogen bonding on the nonradiative properties of dibenzofuran

Femtosecond time-resolved transient absorption spectroscopy is employed to investigate the ultrafast excited-state dynamics from the S₂ state of dibenzofuran and the hydrogen bonding effect in protic and aprotic solvents. Following the excitation with 266 nm, the initial population on the S₂ state couples to the S₁ state via internal conversion on tens of picoseconds. Afterwards, vibrational energy relaxation (VER) with dozens of picoseconds is determined in all solvents and strongly depended on the polarities of aprotic solvents. And the rising species related with the hydrogen-bonded intermediate S^⁎ state are observed just in ethanol and methanol with the higher Kamlet parameter α. Subsequently, the intersystem crossing from the S₁ state to the triplet manifold is a main deactivation pathway with the decay time of ≫ns. All the relaxation processes exhibit solvent dependence, indicating that hydrogen bonding interaction between solute and solvent enhances VER and significantly influences on the excited state dynamics of dibenzofuran.

Discovery of novel (+)-Usnic acid derivatives as potential anti-leukemia agents with pan-Pim kinases inhibitory activity

Usnic acid (UA) is the main secondary metabolite isolated from lichens, with moderate anticancer activity. A small group of (+)-UA derivatives characterized with flavanone moiety was designed and synthesized, and their anticancer activities were evaluated in leukemia cells. It was demonstrated that (+)-UA derivatives 6a-6g inhibited the proliferation of leukemia cells HL-60 and K562 with low micromolar IC₅₀ values. Mechanisms of action were investigated to find that 6g induced apoptosis in HL-60 and K562 cell lines, and affected the expression of MNK/eIF4E axis-related proteins, such as Mcl-1, p-eIF4E, p-4E-BP1. Finally, kinase inhibition assay suggested 6g was a potential inhibitor of pan-Pim kinases. Meanwhile, the blocking of phosphorylation of BAD and 4E-BP1 by 6g, together with the proposed binding mode of 6g with Pim-1 further confirmed its Pim inhibition effects. Our finding provides the sight towards the potential mechanism of (+)-UA derivatives 6g as anti-leukemia agents.

Dehydroamino acids: chemical multi-tools for late-stage diversification

α,β-Dehydroamino acids (dhAAs) are noncanonical amino acids that are found in a wide array of natural products and can be easily installed into peptides and proteins. dhAAs exhibit remarkable synthetic flexibility, readily undergoing a number of reactions, such as polar and single-electron additions, transition metal catalyzed cross-couplings, and cycloadditions. Because of the relatively mild conditions required for many of these reactions, dhAAs are increasingly being used as orthogonal chemical handles for late-stage modification of biomolecules. Still, only a fraction of the chemical reactivity of dhAAs has been exploited in such biorthogonal applications. Herein, we provide an overview of the broad spectrum of chemical reactivity of dhAAs, with special emphasis on recent efforts to adapt such transformations for biomolecules such as natural products, peptides, and proteins. We also discuss examples of enzymes from natural product biosynthetic pathways that have been found to catalyze many similar reactions; these enzymes provide mild, regio- and stereoselective, biocatalytic alternatives for future development. We anticipate that the continued investigation of the innate reactivity of dhAAs will furnish a diverse portfolio dhAA-based chemistries for use in chemical biology and drug discovery.

Experimental and computational studies on the synthesis of diastereoselective natural-based Meldrum spiro dibenzofuran derivatives

Herein, a novel route to achieve chiral Meldrum spiro dibenzofuran derivatives was developed, which involved a 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid)-mediated Knoevenagel reaction of substituted aryl halides, followed by a Diels–Alder reaction with euparin as a natural compound.

Molecular Engineering of Tetracyclic 2,3-Dihydro-1H-benzo[2,3]-benzofuro[4,5-e][1,3]oxazine Derivatives: Evaluation for Potential Anticancer Agents

Water-mediated one-pot Mannich type condensation of dibenzo[b,d]furan-2-ol with different amines resulted in a large library of novel 2,3-dihydro-1H-benzo[2,3]benzofuro[4,5-e][1,3]oxazine derivatives in moderate to excellent yields. The ortho-aminomethylation of the dibenzofuranols proceeded smoothly in the presence of various aromatic/aliphatic amines and paraformaldehyde, followed by cyclization. All the newly synthesized tetracyclic 2,3-dihydro-1H-benzo[2,3]benzofuro[4,5-e][1,3]oxazine derivatives were chemically characterized and screened for their cytotoxicity activity by cell viability assay (MTT test) against three human cancer cell lines and antibacterial activity by determining the minimum inhibitory concentration (MIC) against four bacterial strains. Among all the derivatives, MCV-24 showed promising anticancer activity by inhibiting the cell proliferation of an ovarian cancer cell line (SKOV3) with an IC₅₀ value at 7.5 µM, whereas MCV-24 to MCV-30 derivatives showed moderate activity against a lung cancer cell line (A549) with an IC₅₀ value ranging from 11 to 15.9 µM. Besides MCV-29, -30, and -31 also exhibited broad-spectrum antibacterial activity. Among all new compounds, MCV-24-30 showed promising anticancer and MCV-29-31 antibacterial activity.

Rational questing for inhibitors of endothelin converting enzyme-1 from Salvia miltiorrhiza by combining ligand- and structure-based virtual screening

In this study, a virtual screening approach based on pharmacophore and molecular docking was proposed to identify endothelin converting enzyme-1 (ECE-1) (EC 3.4.24.71) inhibitors from Salvia miltiorrhiza. First, the pharmacophore models were generated to recognize the common features of the ECE-1 inhibitors. The models were validated by a test database composed by a set of compounds known as ECE-1 inhibitors and nonactive compounds and proven to be successful in discriminating active and inactive inhibitors. Then, the best pharmacophore model was used to screen the compounds from S. miltiorrhiza. Furthermore, the Surflex-Dock procedure was used for molecular docking. All compounds from S. miltiorrhiza were docked into the active site of the target protein. An empirical scoring function was used to evaluate the affinity of the compounds and the target protein. Comparing the virtual screening results based on pharmacophore and molecular docking, respectively, 11 communal compounds with higher QFIT and docking score were hit, and the activity of some compounds was validated in the literature. The binding modes between these compounds and the ECE-1 binding site were predicted and used to identify the key interactions that contribute to the inhibitory activity of ECE-1 activity. The results show that the two methods have good consistency and can be validated and supplemented with each other.

Endothelin-converting enzymes degrade intracellular β-amyloid produced within the endosomal/lysosomal pathway and autophagosomes

Impairments in Aβ removal are increasingly being considered as a possible cause for the abnormal Aβ build-up typical of Alzheimer disease. Of particular interest is a pool of Aβ that accumulates intraneuronally and may contribute to neuronal toxicity. The mechanism for intraneuronal accumulation, however, is not well understood and is commonly attributed to impaired removal of extracellular Aβ by neurons. Based on the intracellular distribution of the well established Aβ degrading enzymes, ECE-1 and ECE-2, we tested whether impairments in their catalytic activity could lead to intracellular Aβ accumulation. Using SH-SY5Y cells overexpressing wild-type amyloid precursor protein and pharmacological inhibition of endogenous ECE activity, we found that ECEs participate in the degradation of at least two distinct pools of Aβ; one destined for secretion and the other being produced and degraded within the endosomal-autophagic-lysosomal pathways. Although ECE-1 regulates both pools of Aβ, ECE-2 regulates mainly the intracellular pool of the peptide. Consistent with this result, ECE-2 was found to co-localize with markers of the endosomal/lysosomal pathway but not with a trans-Golgi network marker. Furthermore, ECE-2 was detected in autophagic vesicles in cells treated with chloroquine. Under these conditions, ECE inhibition produced significantly higher elevations in intracellular Aβ than chloroquine treatment alone. This study highlights the existence of Aβ clearance mechanisms by ECEs at intracellular sites of production. Alterations in ECE activity may be considered as a cause for increased intraneuronal Aβ in Alzheimer disease.

Syntheses, spectroscopic properties and stereochemistry of bis-C-pivot macrocycles with two dialkyl phosphonate groups

Bis-C-pivot macrocycles containing dimethyl (1a, 2a) or diethyl phosphonate (1b, 2b) groups have been prepared by adding dimethyl or diethyl phosphite to two -CH=N bonds in corresponding dibenzo-bis-imino crown ethers (1 and 2). Bis-C-pivot macrocycles possess two equivalent stereogenic centres giving rise to diastereoisomers (meso and racemate). The structures were characterized by elemental analysis, FTIR, MS, TGA, DSC and NMR measurements. (1)H, (13)C and (31)P NMR assignments were made for the isolated meso form of compounds 2a and 2b and for the meso and racemic forms of compounds 1a and 1b by analysis of chemical shifts, signal intensities and splitting patterns and the DEPT and 2D HETCOR NMR techniques. Thermal analysis and (1)H NMR showed that the crystallised form of compound 1a contained an equimolar amount of water of crystallisation.

Crystal structure of Mycobacterium tuberculosis zinc-dependent metalloprotease-1 (Zmp1), a metalloprotease involved in pathogenicity

Mycobacterium tuberculosis, the causative agent of tuberculosis, parasitizes host macrophages. The resistance of the tubercle bacilli to the macrophage hostile environment relates to their ability to impair phagosome maturation and its fusion with the lysosome, thus preventing the formation of the phago-lysosome and eventually arresting the process of phagocytosis. The M. tuberculosis zinc-dependent metalloprotease Zmp1 has been proposed to play a key role in the process of phagosome maturation inhibition and emerged as an important player in pathogenesis. Here, we report the crystal structure of wild-type Zmp1 at 2.6 Å resolution in complex with the generic zinc metalloprotease inhibitor phosphoramidon, which we demonstrated to inhibit the enzyme potently. Our data represent the first structural characterization of a bacterial member of the zinc-dependent M13 endopeptidase family and revealed a significant degree of conservation with eukaryotic enzymes. However, structural comparison of the Zmp1-phosphoramidon complex with homologous human proteins neprilysin and endothelin-converting enzyme-1 revealed unique features of the Zmp1 active site to be exploited for the rational design of specific inhibitors that may prove useful as a pharmacological tool for better understanding Zmp1 biological function.

Synthesis of dibenzofurans via palladium-catalyzed phenol-directed C-H activation/C-O cyclization

A practical, Pd(0)/Pd(II)-catalyzed reaction was developed for phenol-directed C-H activation/C-O cyclization using air as an oxidant. The turnover-limiting step of the process was found to be C-O reductive elimination instead of C-H activation. This reaction can tolerate a variety of functional groups and is complementary to the previous methods for the synthesis of substituted dibenzofurans.

Study of the fragmentation pathway of α-aminophosphonates by chemical ionization and fast atom bombardment mass spectrometry

The diastereoisomers of α-aminophosphonates are key intermediates in the synthesis of enantiomerically pure α-aminophosphonic acids, which are analogs of α-amino acids. Although several methods have been reported for the diastereoselective synthesis of α-aminophosphonates, their mass spectrometry (MS) fragmentation patterns have not yet been fully investigated. The work described here involved a detailed study of the fragmentation of enriched α-aminophosphonate diastereoisomers by chemical ionization (CI-MS) and fast atom bombardment (FAB)-MS. The complete characterization of the different conventional MS fragmentation pathways is represented and this intriguing exercise required the use of tandem mass spectrometry (MS/MS) experiments and high-resolution accurate mass measurements. All α-aminophosphonates gave prominent pseudomolecular ions, protonated molecules [MH](+) , and their fragmentations mainly showed a loss of dimethyl phosphite to give the corresponding iminium ions as base peaks for α-aminophosphonates bearing methylbenzyl and 2,2-dimethylbutyl fragments. The loss of the chiral fragment from the iminium ions bearing the (S)-1-(1-naphthyl)ethyl group gave rise to a base peak due to aryl cations. The nature of all fragment ions were confirmed by high-resolution mass spectrometry (HRMS).

Endothelins and their receptors in cancer: identification of therapeutic targets

Endothelins and their receptors are important in normal physiology, but have been implicated in various pathophysiological conditions. Members of the so-called "endothelin axis" are dysregulated in a wide range of human cancers, opening the door for novel anticancer therapies. Established cancer chemotherapeutic agents and drugs that target specific components of the endothelin axis have been combined with promising results, but more work is needed in this area. The endothelin axis affects numerous signaling pathways, including Ras, mitogen activated protein kinases, β-catenin/T-cell factor/lymphoid enhancer factor, nuclear factor-κB (NFκB), SNAIL, and mammalian target of rapamycin (mTOR). There is much still to learn about optimizing drug specificity in this area, while minimizing off-target effects. Selective agonists and antagonists of endothelins, their receptors, and upstream processing enzymes, as well as knockdown strategies in vitro, are providing valuable leads for testing in the clinical setting. The endothelin axis continues to be an attractive avenue of scientific endeavor, both in the cancer arena and in other important health-related disciplines.

Phosphinic tripeptides as dual angiotensin-converting enzyme C-domain and endothelin-converting enzyme-1 inhibitors

A new series of phosphinic inhibitors able to interact with both angiotensin-converting enzyme (ACE) C-domain and endothelin-converting enzyme-1 (ECE-1), while sparing neprilysin (NEP), has been developed. The most potent and selective inhibitor in this series (compound 8(F2)) displays K(i) values of 0.65 nM, 150 nM, 14 nM and 6.7 microM toward somatic ACE C-domain, ACE N-domain, ECE-1, and NEP, respectively. Remarkably, in this series, the inhibitor's ability to discriminate between ECE-1 and NEP was observed to depend on the stereochemistry of the residue present in the inhibitor's P(1)' position. After iv administration, compound 8(F2) (10 mg/kg) lowered mean arterial blood pressure by 24 +/- 2 mmHg in spontaneously hypertensive rats, as compared with controls. Mixed ACE/ECE-1 inhibitor may lead to a new generation of vasopeptide inhibitors that should reduce the levels of angiotensin-II and endothelin-1, without interfering with bradykinin cleavage.

An Overview of Stereoselective Synthesis of α-Aminophosphonic Acids and Derivatives

An overview of all methodologies published during the last few years focused to the stereoselective (diastereoselective or enantioselective) synthesis of α-aminophosphonic acids and derivatives is reported. The procedures have been classified according a retrosynthetic strategy and taking into account the formation of each one of the bonds connected to the chiral centre.

Structure of human endothelin-converting enzyme I complexed with phosphoramidon

Endothelin-converting enzyme I (ECE-1) is a mammalian type II integral membrane zinc-containing endopeptidase. ECE-1 catalyzes the final step in the biosynthesis of endothelins in a rate-limiting fashion, through post-translational conversion of the biologically inactive big endothelins. Endothelin-1 overproduction has been implicated in a heterogeneous list of diseases including systemic and pulmonary hypertension, stroke and asthma, cardiac and renal failure. Therefore, ECE-1 is a prime therapeutic target for the regulation of endothelin-1 production in vivo and there is considerable interest in selective inhibitors of this enzyme. Here, we present the crystal structure of the extracellular domain (residues 90-770) of human ECE-1 (C428S) with the generic metalloprotease inhibitor phosphoramidon determined at 2.38 A resolution. The structure is closely related to that of human NEP, providing essential information for a detailed understanding of ligand-binding, specificity determinants as well as selectivity criteria. Selective inhibitors of ECE-1s should have beneficial effects for the treatment of diseases in which an overproduction of ETs plays a pathogenic role.