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Special Issue "Synthetic Heterocyclic Chemistry"

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: closed (30 April 2021).

Special Issue Editor

Prof. Dr. Fawaz Aldabbagh
E-Mail Website
Guest Editor
Department of Pharmacy, School of Life Sciences, Pharmacy & Chemistry, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK
Interests: free radical organic and polymer chemistry; heterocyclic and medicinal chemistry
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

According to "Heterocyclic Chemistry" by J. A. Joule and K. Mills (Wiley, 5th Ed), "heterocyclic chemistry comprises at least half of all organic chemistry research worldwide". A heterocycle is any organic ring-compound containing at least one non-carbon (hetero) atom within the ring. Heterocycles are mainstays of medicinal chemistry, biochemistry, natural products, specialized materials, and biomaterials. Their synthesis, functionalization, and reactivity continue to be at the forefront of organic chemistry research. Heterocycle reactions have included cycloadditions, multicomponent reactions, and ring-expansions. Some reactions are concerted, some proceed via radical intermediates, and some are mediated by metals, light or organocatalysis. This Special Issue is running in parallel with another named, “Heterocycle Reaction” in our sister journal, Molbank: https://www.mdpi.com/journal/molbank/special_issues/Heterocycle_Reaction

In the Molbank SI, single novel compounds can be published, while in this Molecules SI, we are looking for communications, full papers, and reviews on the latest developments in synthesis and reactivity of heterocycles.

Prof. Dr. Fawaz Aldabbagh
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Heterocyclic chemistry
  • Aromatic heterocycles
  • Non-aromatic heterocycles
  • Fused heterocycles
  • Nitrogen
  • Oxygen
  • Sulfur
  • Radical
  • Spectroscopic properties
  • Halogenation
  • Organic chemistry
  • Medicinal chemistry

Published Papers (10 papers)

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Research

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Article
Synthesis, Characterization and Biological Evaluation of New 3,5-Disubstituted-Pyrazoline Derivatives as Potential Anti-Mycobacterium tuberculosis H37Ra Compounds
Molecules 2021, 26(7), 2081; https://doi.org/10.3390/molecules26072081 - 05 Apr 2021
Viewed by 631
Abstract
A total of fourteen pyrazoline derivatives were synthesized through cyclo-condensation reactions by chalcone derivatives with different types of semicarbazide. These compounds were characterized by IR, 1D-NMR (1H, 13C and Distortionless Enhancement by Polarization Transfer - DEPT-135) and 2D-NMR (COSY, HSQC [...] Read more.
A total of fourteen pyrazoline derivatives were synthesized through cyclo-condensation reactions by chalcone derivatives with different types of semicarbazide. These compounds were characterized by IR, 1D-NMR (1H, 13C and Distortionless Enhancement by Polarization Transfer - DEPT-135) and 2D-NMR (COSY, HSQC and HMBC) as well as mass spectroscopy analysis (HRMS). The synthesized compounds were tested for their antituberculosis activity against Mycobacterium tuberculosis H37Ra in vitro. Based on this activity, compound 4a showed the most potent inhibitory activity, with a minimum inhibitory concentration (MIC) value of 17 μM. In addition, six other synthesized compounds, 5a and 5c–5g, exhibited moderate activity, with MIC ranges between 60 μM to 140 μM. Compound 4a showed good bactericidal activity with a minimum bactericidal concentration (MBC) value of 34 μM against Mycobacterium tuberculosis H37Ra. Molecular docking studies for compound 4a on alpha-sterol demethylase was done to understand and explore ligand–receptor interactions, and to hypothesize potential refinements for the compound. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Unexpectedly Long Lifetime of the Excited State of Benzothiadiazole Derivative and Its Adducts with Lewis Acids
Molecules 2021, 26(7), 2030; https://doi.org/10.3390/molecules26072030 - 02 Apr 2021
Viewed by 338
Abstract
We report a study of photoluminescent properties of 4-bromo-7-(3-pyridylamino)-2,1,3-benzothiadiazole (Py-btd) and its novel Lewis adducts: (PyH-btd)2(ZnCl4) and [Cu2Cl2(Py-btd)2{PPO}2]·2C7H8 [...] Read more.
We report a study of photoluminescent properties of 4-bromo-7-(3-pyridylamino)-2,1,3-benzothiadiazole (Py-btd) and its novel Lewis adducts: (PyH-btd)2(ZnCl4) and [Cu2Cl2(Py-btd)2{PPO}2]·2C7H8 (PPO = tetraphenyldiphosphine monoxide), whose crystal structure was determined by X-ray diffraction analysis. Py-btd exhibits a lifetime of 9 microseconds indicating its phosphorescent nature, which is rare for purely organic compounds. This phenomenon arises from the heavy atom effect: the presence of a bromine atom in Py-btd promotes mixing of the singlet and triplet states to allow efficient singlet-to-triplet intersystem crossing. The Lewis adducts also feature a microsecond lifetime while emitting in a higher energy range than free Py-btd, which opens up the possibility to color-tune luminescence of benzothiadiazole derivatives. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Polymer-Supported Synthesis of Various Pteridinones and Pyrimidodiazepinones
Molecules 2021, 26(6), 1603; https://doi.org/10.3390/molecules26061603 - 14 Mar 2021
Viewed by 490
Abstract
In this report, we employed the solid-phase synthetic approach to prepare variously substituted dihydropteridinones, tetrahydropyrrolopteridinones, and pyrimidodiazepinones, using a versatile building block-4,6-dichloro-5-nitropyrimidine. All these compounds are pharmacologically significant scaffolds of the great importance of medicinal chemists. The fast and efficient synthetic methodology is [...] Read more.
In this report, we employed the solid-phase synthetic approach to prepare variously substituted dihydropteridinones, tetrahydropyrrolopteridinones, and pyrimidodiazepinones, using a versatile building block-4,6-dichloro-5-nitropyrimidine. All these compounds are pharmacologically significant scaffolds of the great importance of medicinal chemists. The fast and efficient synthetic methodology is highly desirable for defining their structure-activity relationship (SAR) and optimizing pharmacokinetic properties. Our research efforts utilize simple synthetic methods to generate a library of analogues for future SAR studies. The efficiency of our approach was exemplified in various pteridinones as well as pyrimidodiazepinones. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Substituted 2-Phenacylbenzoxazole Difluoroboranes: Synthesis, Structure and Properties
Molecules 2020, 25(22), 5420; https://doi.org/10.3390/molecules25225420 - 19 Nov 2020
Viewed by 509
Abstract
Novel fluorescent dyes such as benzoxazole-boron complexes, bearing β-ketoiminate ligands, have been synthesized and characterized with a focus on the influence of a substituent on the basic photophysical properties. 1H, 11B, 13C, 15N, and 19F nuclear magnetic resonance [...] Read more.
Novel fluorescent dyes such as benzoxazole-boron complexes, bearing β-ketoiminate ligands, have been synthesized and characterized with a focus on the influence of a substituent on the basic photophysical properties. 1H, 11B, 13C, 15N, and 19F nuclear magnetic resonance (NMR) spectra of substituted 2-phenacylbenzoxazole difluoroboranes have been recorded and discussed. It is worth mentioning that a high correlation coefficient was found between 15N-NMR parameters and substituent constants. The photophysical properties of these new dyes have been investigated by fluorescence and ultraviolet-visible (UV-Vis) absorption spectroscopy. The geometry optimization, vibrational spectra, and the HOMO and LUMO energies were calculated based on density functional theory with the use of the B3LYP functional and 6-311++G(d,p) basis set. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Practical Synthesis of Quinoline-Protected Morpholino Oligomers for Light-Triggered Regulation of Gene Function
Molecules 2020, 25(9), 2078; https://doi.org/10.3390/molecules25092078 - 29 Apr 2020
Cited by 2 | Viewed by 1542
Abstract
Photoactivatable cyclic caged morpholino oligomers (ccMOs) represent a promising tool to selectively regulate gene expression with spatiotemporal control. Nevertheless, some challenges associated with the preparation of these reagents have limited their broader use in biological settings. We describe a novel ccMO design that [...] Read more.
Photoactivatable cyclic caged morpholino oligomers (ccMOs) represent a promising tool to selectively regulate gene expression with spatiotemporal control. Nevertheless, some challenges associated with the preparation of these reagents have limited their broader use in biological settings. We describe a novel ccMO design that overcomes many of the challenges and considerably expedites the synthetic preparation. The key factor is the introduction of an ethynyl function on the photocleavable linker to facilitate the use of a Huisgen 1,3-dipolar cycloaddition for the coupling reaction with the oligonucleotide. Compared to previous strategies, this modification reduces the number of synthetic steps and significantly improves the total yield and the stability of the linker. We used the alkynyl-functionalized linker for the preparation of two different ccMOs targeting the mRNA of the glutamic acid decarboxylase genes, gad1 and gad2. HPLC analysis confirms that the caging strategy successfully inhibits the DNA binding ability, and the activity can be restored by brief illumination with 405-nm light. Overall, the straightforward preparation together with the clean and fast photochemistry make these caged antisense reagents excellent tools to modulate gene function in-vivo with spatial and temporal precision. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Synthesis and Biological Evaluation of a Novel C8-Pyrrolobenzodiazepine (PBD) Adenosine Conjugate. A Study on the Role of the PBD Ring in the Biological Activity of PBD-Conjugates
Molecules 2020, 25(5), 1243; https://doi.org/10.3390/molecules25051243 - 10 Mar 2020
Cited by 1 | Viewed by 1299
Abstract
Here we sought to evaluate the contribution of the PBD unit to the biological activity of PBD-conjugates and, to this end, an adenosine nucleoside was attached to the PBD A-ring C8 position. A convergent approach was successfully adopted for the synthesis of a [...] Read more.
Here we sought to evaluate the contribution of the PBD unit to the biological activity of PBD-conjugates and, to this end, an adenosine nucleoside was attached to the PBD A-ring C8 position. A convergent approach was successfully adopted for the synthesis of a novel C8-linked pyrrolo(2,1-c)(1,4)benzodiazepine(PBD)-adenosine(ADN) hybrid. The PBD and adenosine (ADN) moieties were synthesized separately and then linked through a pentynyl linker. To our knowledge, this is the first report of a PBD connected to a nucleoside. Surprisingly, the compound showed no cytotoxicity against murine cells and was inactive against Mycobacterium aurum and M. bovis strains and did not bind to guanine-containing DNA sequences, as shown by DNase I footprinting experiments. Molecular dynamics simulations revealed that the PBD–ADN conjugate was poorly accommodated in the DNA minor groove of two DNA sequences containing the AGA-PBD binding motif, with the adenosine moiety of the ligand preventing the covalent binding of the PBD unit to the guanine amino group of the DNA duplex. These interesting findings shed further light on the ability of the substituents attached at the C8 position of PBDs to affect and modulate the biological and biophysical properties of PBD hybrids. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Introduction of Mercaptoethyl at Sorafenib Pyridine-2-Amide Motif as a Potentially Effective Chain to Further get Sorafenib-PEG-DGL
Molecules 2020, 25(3), 573; https://doi.org/10.3390/molecules25030573 - 28 Jan 2020
Cited by 1 | Viewed by 1053
Abstract
The crystal structure of the sorafenib and B-RAF complex indicates that the binding cavity occupied by the pyridine-2-carboxamide in sorafenib has a large variable space, making it a reasonable modification site. In order to identify novel compounds with anti-cancer activity, better safety and [...] Read more.
The crystal structure of the sorafenib and B-RAF complex indicates that the binding cavity occupied by the pyridine-2-carboxamide in sorafenib has a large variable space, making it a reasonable modification site. In order to identify novel compounds with anti-cancer activity, better safety and polar groups for further application, five sorafenib analogs with new pyridine-2-amide side chains were designed and synthesized. Preliminary pharmacologic studies showed that these compounds displayed much lower toxicities than that of sorafenib. Among them, compound 10b bearing mercaptoethyl group kept relevant antiproliferation potency compared to sorafenib in Huh7 and Hela cell lines with values of IC50 58.79 and 63.67 μM, respectively. As a small molecule inhibitor targeting protein tyrosine kinases, thiol in compound 10b would be an active group to react with maleimide in a mild condition for forming nanoparticles Sorafenib-PEG-DGL, which could be developed as a delivery vehicle to improve the concentration of anti-tumor therapeutic agents in the target cancer tissue and reduce side effects in the next study. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Article
Synthesis of a New Series of Nitrogen/Sulfur Heterocycles by Linking Four Rings: Indole; 1,2,4-Triazole; Pyridazine; and Quinoxaline
Molecules 2020, 25(3), 450; https://doi.org/10.3390/molecules25030450 - 21 Jan 2020
Cited by 7 | Viewed by 1180
Abstract
A new series of nitrogen and sulfur heterocyclic systems were efficiently synthesized by linking the following four rings: indole; 1,2,4-triazole; pyridazine; and quinoxaline hybrids. The strength of the acid that catalyzes the condensation of 4-amino-5-(1H-indol-2-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione 1 with aromatic aldehydes [...] Read more.
A new series of nitrogen and sulfur heterocyclic systems were efficiently synthesized by linking the following four rings: indole; 1,2,4-triazole; pyridazine; and quinoxaline hybrids. The strength of the acid that catalyzes the condensation of 4-amino-5-(1H-indol-2-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione 1 with aromatic aldehydes controlled the final product. Reflux in glacial acetic acid yielded Schiff bases 26, whereas concentrated HCl in ethanol resulted in a cyclization product at C-3 of the indole ring to create indolo-triazolo-pyridazinethiones 716. This fascinating cyclization approach was applicable with a wide range of aromatic aldehydes to create the target cyclized compounds in excellent yield. Additionally, the coupling of the new indolo-triazolo-pyridazinethiones 713 with 2,3-bis(bromomethyl)quinoxaline, as a linker in acetone and K2CO3, yielded 2,3-bis((5,6-dihydro-14H-indolo[2,3-d]-6-aryl-[1,2,4-triazolo][4,3-b]pyridazin-3 ylsulfanyl)methyl)quinoxalines 1925 in a high yield. The formation of this new class of heterocyclic compounds in high yields warrants their use for further research. The new compounds were characterized using nuclear magnetic resonance (NMR) and mass spectral analysis. Compound 6 was further confirmed by the single crystal X-ray diffraction technique. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Review

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Review
1,2,3-Triazoles as Biomimetics in Peptide Science
Molecules 2021, 26(10), 2937; https://doi.org/10.3390/molecules26102937 - 14 May 2021
Viewed by 482
Abstract
Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules [...] Read more.
Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules mimicking peptides is of great importance for the development of new biologically active molecules. Therefore, replacing the amide bond in a peptide with a heterocyclic bioisostere, such as the 1,2,3-triazole ring, can be considered an effective solution for the synthesis of biologically relevant peptidomimetics. These 1,2,3-triazoles may have an interesting biological activity, because they behave as rigid link units, which can mimic the electronic properties of amide bonds and show bioisosteric effects. Additionally, triazole can be used as a linker moiety to link peptides to other functional groups. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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Review
Advances in the Synthesis of Ring-Fused Benzimidazoles and Imidazobenzimidazoles
Molecules 2021, 26(9), 2684; https://doi.org/10.3390/molecules26092684 - 04 May 2021
Viewed by 518
Abstract
This review article provides a perspective on the synthesis of alicyclic and heterocyclic ring-fused benzimidazoles, imidazo[4,5-f]benzimidazoles, and imidazo[5,4-f]benzimidazoles. These heterocycles have a plethora of biological activities with the iminoquinone and quinone derivatives displaying potent bioreductive antitumor activity. Synthesis is [...] Read more.
This review article provides a perspective on the synthesis of alicyclic and heterocyclic ring-fused benzimidazoles, imidazo[4,5-f]benzimidazoles, and imidazo[5,4-f]benzimidazoles. These heterocycles have a plethora of biological activities with the iminoquinone and quinone derivatives displaying potent bioreductive antitumor activity. Synthesis is categorized according to the cyclization reaction and mechanisms are detailed. Nitrobenzene reduction, cyclization of aryl amidines, lactams and isothiocyanates are described. Protocols include condensation, cross-dehydrogenative coupling with transition metal catalysis, annulation onto benzimidazole, often using CuI-catalysis, and radical cyclization with homolytic aromatic substitution. Many oxidative transformations are under metal-free conditions, including using thermal, photochemical, and electrochemical methods. Syntheses of diazole analogues of mitomycin C derivatives are described. Traditional oxidations of o-(cycloamino)anilines using peroxides in acid via the t-amino effect remain popular. Full article
(This article belongs to the Special Issue Synthetic Heterocyclic Chemistry)
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