Thiazole and oxazole alkaloids: isolation and synthesis

Eco-sustainable Synthesis and Potential Efficiency of Some Novel N-containing Heterocyclic Derivatives as Insecticidal and Photosensitizing Agents Against Musca domestica L

The rising application of conventional synthetic insecticides develops resistant populations of houseflies; therefore, using new chemical agents with different modes of action is essential to overcome this problem. The mechanical grinding technique was used as a green method, to synthesize the tested compounds because it is a more facile work-up and high-yield economy, simplicity and solvent-free than conventional thermal technique. Various methods were employed to synthesize new heterocycles containing anthracene (a photosensitizing agent) from chalcone 3, a building block material such as the preparation of the pyrazole derivatives 4-7, isoxazole derivative 8, pyrimidines 9-11, and oxirane derivative 12. The novel synthesized compounds were analyzed by FT-IR, 1H-NMR, 13C-NMR spectra, and elemental analysis. Herein, the toxicity of the anthracene derivatives was assessed against Musca domestica larvae and adults in different conditions to demonstrate the effect of various inserted moieties on the efficiency of tested compounds. Furthermore, the influence of sunlight on the toxicity of anthracene was studied in dark and sunlight tests against adult houseflies. Moreover, these compounds diminished the total protein and lipids contents while significantly influencing the antioxidant enzymes activities of M. domestica adults. Structure-activity relationships demonstrated the role of each moiety on the toxicity of compounds.

Natural and synthetic 5-(3'-indolyl)oxazoles: Biological activity, chemical synthesis and advanced molecules

5-(3'-Indolyl)oxazole moiety is a privileged heterocyclic scaffold, embedded in many biologically interesting natural products and potential therapeutic agents. Compounds containing this scaffold, whether from natural sources or synthesized, have demonstrated a wide array of biological activities. This has piqued the interest of synthetic chemists, leading to a large number of reported synthetic approaches to 5-(3'-indolyl)oxazole scaffold in recent years. In this review, we comprehensively overviewed the different biological activities and chemical synthetic methods for the 5-(3'-indolyl)oxazole scaffold reported in the literatures from 1963 to 2024. The focus of this study is to highlight the significance of 5-(3'-indolyl)oxazole derivatives as the lead compounds for the lead discovery of anticancer, pesticidal, antimicrobial, antiviral, antioxidant and anti-inflammatory agents, to summarize the synthetic methods for the 5-(3'-indolyl)oxazole scaffold. In addition, the reported mechanism of action of 5-(3'-indolyl)oxazoles and advanced molecules studied in animal models are also reviewed. Furthermore, this review offers perspectives on how 5-(3'-indolyl)oxazole scaffold as a privileged structure might be exploited in the future.

Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Photochemical three-component assembly of tri-substituted oxazoles through a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade

Construction of complex molecular skeletons with ubiquitous chemical feedstocks in a single transformation is highly appealing in organic synthesis. We report a novel visible-light-induced three-component reaction for the construction of complex 2,4,5-trisubstituted oxazoles, which are valuable in medicinal chemistry, from simple and readily available iodonium-phosphonium hybrid ylides, carboxylic acids, and nitriles. This reaction features a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade, in which a photo-generated α-phosphonium carbene acts as a sequence trigger. This catalyst- and additive-free transformation exhibits high efficiency and broad substrate scope for synthesizing diverse oxazoles.

Marine Streptomyces-Derived Novel Alkaloids Discovered in the Past Decade

Natural alkaloids originating from actinomycetes and synthetic derivatives have always been among the important suppliers of small-molecule drugs. Among their biological sources, Streptomyces is the highest and most extensively researched genus. Marine-derived Streptomyces strains harbor unconventional metabolic pathways and have been demonstrated to be efficient producers of biologically active alkaloids; more than 60% of these compounds exhibit valuable activity such as antibacterial, antitumor, anti-inflammatory activities. This review comprehensively summarizes novel alkaloids produced by marine Streptomyces discovered in the past decade, focusing on their structural features, biological activity, and pharmacological mechanisms. Future perspectives on the discovery and development of novel alkaloids from marine Streptomyces are also provided.

Construction of Thioamide Peptides from Chiral Amino Acids

Thioamide peptides were synthesized in a straightforward one-pot process via the linkage of diverse natural amino acids in the presence of thiolphosphonate and trichlorosilane, wherein carbonyl groups were replaced with thiono compounds with minimal racemization. Experimental and computational mechanistic studies demonstrated that the trichlorosilane enables the activation of carboxylic acids via intense interactions with the Si-O bond, followed by coupling of the carboxylic acids with thiolphosphonate to obtain the key intermediate S-acyl dithiophosphate. Silyl-activated quadrangular metathesis transition states afforded the thioamide peptides. The potential applications of these thioamide peptides were further highlighted via late-stage linkages of diverse natural products and pharmaceutical drugs and the thioamide moiety.

Imidazole-amino acids. Conformational switch under tautomer and pH change

Replacement of the main chain peptide bond by imidazole ring seems to be a promising tool for the peptide-based drug design, due to the specific prototropic tautomeric as well as amphoteric properties. In this study, we present that both tautomer and pH change can cause a conformational switch of the studied residues of alanine (1-4) and dehydroalanine (5-8) with the C-terminal peptide group replaced by imidazole. The DFT methods are applied and an environment of increasing polarity is simulated. The conformational maps (Ramachandram diagrams) are presented and the stability of possible conformations is discussed. The neutral forms, tautomers τ (1) and π (2), adapt the conformations αRτ (φ, ψ = - 75°, - 114°) and C7eq (φ, ψ = - 75°, 66°), respectively. Their torsion angles ψ differ by about 180°, which results in a considerable impact on the peptide chain conformation. The cation form (3) adapts both these conformations, whereas the anion analogue (4) prefers the conformations C5 (φ, ψ = - 165°, - 178°) and β2 (φ, ψ ~ - 165°, - 3°). Dehydroamino acid analogues, the tautomers τ (5) and π (6) as well as the anion form (8), have a strong tendency toward the conformations β2 (φ, ψ = - 179°, 0°) and C5 (φ, ψ = - 180°, 180°). The preferences of the protonated imidazolium form (7) depend on the environment. The imidazole ring, acting as a donor or acceptor of the hydrogen bonds created within the studied residues, has a profound effect on the type of conformation.

Synthesis, Structure and Molecular Docking of New 4,5-Dihydrothiazole Derivatives Based on 3,5-Dimethylpyrazole and Cytisine and Salsoline Alkaloids

The interaction results of 1,2-dibromo-3-isothiocyanatopropane with some pyrazoles as well as cytisine and salsoline alkaloids were presented in this paper. It was shown that the reaction resulted in one one-step and rather mild method for the preparation of the corresponding 1,3-thiazoline bromomethyl derivatives. The yield of this reaction was affected by the presence of a base and an order in which reagents were added. Molecular docking of the synthesized 1,3-thiazoline derivatives for putative antibacterial activity was carried out using the penicillin-binding target protein (PBP4) of the bacteria E. coli "Homo sapiens" and S. aureus "Homo sapiens" as an example. Molecular docking demonstrated that the compounds had insignificant binding energies at the level of selected reference drugs (Cephalotin and Chloramphenicol). The presence of natural alkaloids in the structure of thiazoline derivatives somewhat increased the affinity of these substrates for target proteins selected.

Isocyanide-Based Multicomponent Reactions in Water: Advanced Green Tools for the Synthesis of Heterocyclic Compounds

Reaction rate acceleration using green methods is an intriguing area of research for chemists. In this regard, water as a "green solvent" plays a crucial role in the acceleration of some organic transformations and reveals exclusive selectivity and reactivity in comparison with conventional organic solvents. In particular, multicomponent reactions (MCRs) as sustainable tools lead to the rapid generation of small-molecule libraries in water and aqueous media due to the prominent role of the hydrophobic effect. MCRs, as diversity-oriented synthesis (DOS) methods, have great efficiency with simple operations, atom, pot, and step economy synthesis, and mechanistic beauty. Among diverse classes of MCRs, isocyanide-based multicomponent reactions (I-MCRs), as sustainable and versatile reactions, have gained considerable attention in the synthesis of diverse heterocycle rings, especially in drug design because of the peculiar nature of isocyanide as a particular active reactant. I-MCRs that are performed in water are mild, environmentally friendly, and easily controlled, and have a reduced number of workup, purification, and extraction steps, which fit well with the advantages of "green" chemistry. Performing these powerful organic transformations in water and aqueous media is accompanied by acceleration owing to negative activation volumes, which originate from connecting several reactants together to generate a single product. It should be noted that the combination of MCR strategy and aqueous phase reaction is of growing interest for the development of sustainable synthetic techniques in organic conversions. However, an exclusive account focusing on the recent progress in eco-friendly I-MCRs for the construction of heterocycles in water and aqueous media is particularly lacking. This review highlights the progress of various kinds of I-MCRs in water and aqueous media as benign methods for the efficient construction of vital heterocyclic scaffolds, with a critical discussion of the subject in the period 2000-2021. We hope that this themed collection will be of interest and beneficial for organic and pharmaceutical chemists and will inspire more reaction development in this fascinating field.

Visible light-promoted intermolecular cyclization/aromatization of chalcones and 2-mercaptobenzimidazoles via an EDA complex and a mechanism study

Visible-light-promoted cyclization and aromatization of chalcones with 2-mercaptobenzimidazoles have been successfully developed to obtain diverse imidazo[2,1-b]thiazoles, and C-S and C-N bonds were constructed in one step. The reaction uses oxygen in the air as an oxidant, and the method does not need an external photocatalyst or a transition metal catalyst. The strategy features mild conditions, a simple system, readily accessible feedstocks, and a friendly environment. UV absorption spectroscopy and control experiments have shown that the reaction mechanism involves the formation of an electron-donor-acceptor (EDA) complex from thiolate anions and chalcones. In order to verify the mechanism, we studied the structure and HOMO/LUMO of the EDA complex by density functional theory (DFT) calculations. The results show that the π-π stacking between chalcones and 2-mercaptobenzimidazoles will cause a red shift of the UV absorption wavelength in the presence of Cs₂CO₃, and also provide a theoretical basis for the electron transfer of EDA complexes.

Synthesis of Trisubstituted Oxazoles via Aryne Induced [2,3] Sigmatropic Rearrangement-Annulation Cascade

A transition-metal-free, [2,3] sigmatropic rearrangement-annulation cascade of 2-substituted thio/amino acetonitriles with arynes allowing the synthesis of 2,4,5-trisubstituted oxazoles under mild conditions has been demonstrated. The key sulfur/nitrogen ylides were generated by the initial S/N arylation followed by proton transfer, which was followed by the selective [2,3] sigmatropic rearrangement involving the -CN moiety and a subsequent annulation to afford the desired products in reasonable yields.

Access to 5-fluoroalkylated trisubstituted oxazoles via copper-catalyzed cyclization of α-fluoroalkyl-α-diazoketones with amides

A novel copper-catalyzed cyclization of α-fluoroalkyl-α-diazoketones with (thio)amides has been developed. This mechanistically distinct protocol provides a robust and straightforward approach to construct 5-fluoroalkylated trisubstituted oxazoles and thiazoles with high efficiency and excellent functional group compatibility. Experimental studies suggest a mechanism involving imidate ligand migratory insertion of a copper carbenoid as the key step.

Acetylene containing 2-(2-hydrazinyl)thiazole derivatives: design, synthesis, and in vitro and in silico evaluation of antimycobacterial activity against Mycobacterium tuberculosis

Mycobacterium tuberculosis resistance to commercially available drugs is increasing day by day. To address this issue, various strategies were planned and are being implemented. However, there is a need for new drugs and rapid diagnostic methods. For this endeavour, in this paper, we present the synthesis of acetylene containing 2-(2-hydrazinyl) thiazole derivatives and in vitro evaluation against the H37Rv strain of Mycobacterium tuberculosis. Among the developed 26 acetylene containing 2-(2-hydrazinyl) thiazole derivatives, eight compounds inhibited the growth of Mycobacterium tuberculosis with MIC values ranging from 100 μg ml-1 to 50 μg ml-1. The parent acetylene containing thiosemicarbazones showed promising antimycobacterial activity by inhibiting up to 75% of the Mycobacterium at 50 μg ml-1. In addition, in silico studies were employed to understand the binding mode of all the novel acetylene-containing derivatives against the KasA protein of the Mycobacterium. Interestingly, the KasA protein interactions with the compounds were similar to the interactions of KasA protein with thiolactomycin and rifampicin. Cytotoxicity study results indicate that the compounds tested are non-toxic to human embryonic kidney cells.

Synthesis of Tetrahydrofuro[2,3-d]oxazoles and Oxazoles by Hypervalent Iodine (III)-Promoted [2 + 2 + 1] Annulation

Reaction of a hypervalent iodine reagent with bistriflimide efficiently promotes three-component regioselective cyclization of tetrahydrofuro[2,3-d]oxazoles and oxazoles from homopropargyl alcohols bearing a phenyl group, with different substituents on the aryl alkyne compounds affecting the selectivity of the resulting product. Utilizing the hydroxyethyl oxazole derivatives obtained in this research could aid in the development of various peroxisome proliferator-activated receptor agonist derivatives.

One-Pot Synthesis of α-Ketoamides from α-Keto Acids and Amines Using Ynamides as Coupling Reagents

A one-pot strategy for α-keto amide bond formation have been developed by using ynamides as coupling reagents under extremely mild reaction conditions. Diversely structural α-ketoamides were afforded in up to 98% yield for 36 examples. This reaction features advantages such as practical coupling procedure, wide functional group tolerance, and extremely mild conditions and has potential applications in synthetic and medicinal chemistry.

Efficient synthesis of ethyl 2-(oxazolin-2-yl)alkanoates via ethoxycarbonylketene-induced electrophilic ring expansion of aziridines

Alkyl 2-diazo-3-oxoalkanoates generate alkoxycarbonylketenes, which undergo an electrophilic ring expansion with aziridines to afford alkyl 2-(oxazolin-2-yl)alkanoates in good to excellent yields under microwave heating. The method is a convenient and clean reaction without any activators and catalysts and can be also applied in the synthesis of 2-(oxazolin-2-yl)alkanamides and 1-(oxazolin-2-yl)alkylphosphonates.

Neat synthesis of isothiazole compounds, and studies on their synthetic applications and photophysical properties

Ammonium thiocyanate-promoted simple, rapid and eco-friendly neat synthesis of isothiazoles is developed for the first time.

A new anti-proliferative compound from an endophytic fungus, Phoma sp

A new oxazole-type compound (1), named macrooxazole E, and three known macrooxazoles A-C (2-4), were isolated from ethyl acetate (EtOAc) extracts of Phoma sp. JS0228 cultures, an endophytic fungus of Morus alba (M. alba). Structures of the isolated compounds were determined using spectroscopic methods, such as 1 D- and 2 D-nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS). Macrooxazole E (1) differed from macrooxazole C only in the presence of methyl carboxylate instead of free carboxylic acid. Macrooxazole C showed moderate anti-proliferative activities against human breast cancer (MCF-7) and prostate cancer (LNCaP) cell lines with IC₅₀ values of 0.29 mM and 0.36 mM, respectively. This study presents the possibility of the endophytic fungus Phoma sp. JS0228 to produce new bioactive natural compounds.

Asymmetric Total Synthesis and Revision of Absolute Stereochemistry for (+)-Taumycin A: An Approach that Exploits Orthogonally Protected Quasienantiomers

The first asymmetric total synthesis of C(9)-S-(+)-taumycin A is now reported using an approach that targeted both C(9) diastereomers concurrently. To facilitate this work, we called upon the symmetrical nature of a C(5)-C(13) side-chain intermediate and exploited orthogonal protecting groups as a tactic to access both stereoisomers from a single chiral, nonracemic intermediate. In addition to our successful approach, several minor detours that helped refine our strategy and a detailed analysis of ¹H NMR data will be discussed. Select compounds included in this work were screened against the NCI60 cell line panel and displayed modest growth inhibition activity.

Antimelanoma activities of chimeric thiazole-androstenone derivatives

The discovery of chimeric anti-melanoma agents is reported. These molecules are potent growth suppressors of melanoma cells in vitro with growth inhibition of 50% (GI₅₀) values as low as 1.32 µM. Compounds were more toxic to melanoma cells in vitro than commonly used anti-melanoma agent dacarbazine as measured by TUNEL assay. They induced both caspase-independent apoptosis evident by colocalization of TUNEL with endonuclease G (EndoG) and caspase-mediated apoptosis measured by colocalization of TUNEL with caspase-activated DNase (CAD). In addition, compounds 3 and 5 strongly induced oxidative injury to melanoma cells as measured by TUNEL colocalization with heme oxygenase-1 (HO1). Dacarbazine induced only caspase-independent apoptosis, which may explain why it is less cytotoxic to melanoma cells than compounds 3, 4 and 5.

NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

Thiazole-amino acids: influence of thiazole ring on conformational properties of amino acid residues

Post-translational modified thiazole-amino acid (Xaa-Tzl) residues have been found in macrocyclic peptides (e.g., thiopeptides and cyanobactins), which mostly inhibit protein synthesis in Gram + bacteria. Conformational study of the series of model compounds containing this structural motif with alanine, dehydroalanine, dehydrobutyrine and dehydrophenylalanine were performed using DFT method in various environments. The solid-state crystal structure conformations of thiazole-amino acid residues retrieved from the Cambridge Structural Database were also analysed. The studied structural units tend to adopt the unique semi-extended β2 conformation; which is stabilised mainly by N-H⋯NTzl hydrogen bond, and for dehydroamino acids also by π-electron conjugation. The conformational preferences of amino acids with a thiazole ring were compared with oxazole analogues and the role of the sulfur atom in stabilising the conformations of studied peptides was discussed.

Selective C-H chalcogenation of thiazoles via thiazol-2-yl-phosphonium salts

Thiazoles and benzothiazoles undergo regioselective C2-H chalcogenation via the sequence of thiazole C2-functionalization with phosphines to produce phosphonium salts which in turn react with S- and Se-centered nucleophiles to give products of C2-H chalcogenation and allow for recovery of the starting phosphine. The atom economical sequence proceeds under mild conditions and features broad scope for both the nucleophiles (electron-rich, electron-poor, sterically hindered thiols) and the various substituted benzothiazoles. The access to the substituted medicinally relevant C2-thio benzothiazoles also enables stereoselectivity improvements in the modified Julia olefinations.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Macrooxazoles A-D, New 2,5-Disubstituted Oxazole-4-Carboxylic Acid Derivatives from the Plant Pathogenic Fungus Phoma macrostoma

In our ongoing search for new bioactive fungal metabolites, four previously undescribed oxazole carboxylic acid derivatives (1–4) for which we proposed the trivial names macrooxazoles A–D together with two known tetramic acids (5–6) were isolated from the plant pathogenic fungus Phoma macrostoma. Their structures were elucidated based on high-resolution mass spectrometry (HR-MS) and nuclear magnetic resonance (NMR) spectroscopy. The hitherto unclear structure of macrocidin Z (6) was also confirmed by its first total synthesis. The isolated compounds were evaluated for their antimicrobial activities against a panel of bacteria and fungi. Cytotoxic and anti-biofilm activities of the isolates are also reported herein. The new compound 3 exhibited weak-to-moderate antimicrobial activity as well as the known macrocidins 5 and 6. Only the mixture of compounds 2 and 4 (ratio 1:2) showed weak cytotoxic activity against the tested cancer cell lines with an IC₅₀ of 23 µg/mL. Moreover, the new compounds 2 and 3, as well as the known compounds 5 and 6, interfered with the biofilm formation of Staphylococcus aureus, inhibiting 65%, 75%, 79%, and 76% of biofilm at 250 µg/mL, respectively. Compounds 5 and 6 also exhibited moderate activity against S. aureus preformed biofilm with the highest inhibition percentage of 75% and 73% at 250 µg/mL, respectively.

Clickable Transformation of Nitriles (RCN) to Oxazolyl Sulfonyl Fluoride Warheads

The protocol for simple, efficient, and mild synthesis of oxazolyl sulfonyl fluorides was developed through Rh₂(OAc)₄-catalyzed annulation of methyl-2-diazo-2-(fluorosulfonyl)acetate (MDF) or its ethyl ester derivative with nitriles. This practical method provides a general and direct route to a unique class of highly functionalized oxazolyl-decorated sulfonyl fluoride warheads with great potential in medicinal chemistry, chemical biology, and drug discovery.

Synthesis of biphenyl oxazole derivatives via Suzuki coupling and biological evaluations as nucleotide pyrophosphatase/phosphodiesterase-1 and -3 inhibitors

Oxazole derivatives are important medicinal compounds which are inhibitors of various enzymes such as NPP1, NPP2, NPP3, tyrosine kinase, dipeptidyl-peptidase IV, cyclooxygenase-2, and protein tyrosine phosphatase. In this study, an extensive range of new biologically active biphenyl oxazole derivatives was synthesized in high to excellent yields (57-93%) through Suzuki-Miyaura cross-coupling of bromophenyloxazole with different boronic acids. The reaction was carried out in wet toluene under mild conditions. Overexpression of nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) and NPP3 has been associated with various health disorders including chondrocalcinosis, cancer, osteoarthritis, and type 2 diabetes. We evaluated the inhibitory potential and selectivity of the synthesized compounds (3a-3q) towards NPP1 and NPP3 at 100 μM concentrations. We found two compounds that were selective and potent inhibitors of these two enzymes on the artificial substrate thymidine 5'-monophosphate para-nitrophenyl ester: compound 3n inhibited NPP1 with an IC₅₀ of 0.15 μM, and compound 3f inhibited NPP3 with an IC₅₀ value of 0.17 μM. The compounds with promising inhibitory potential were docked inside the proteins of NPP1 and NPP3 isozymes to get insight into the plausible binding interactions with active site residues.

Diverse N-Substituted Azole-Containing Amino Acids as Building Blocks for Cyclopeptides

The preparation of 16 oxazole- or thiazole-containing amino esters bearing a wide array of N-substitution is reported. These were accessed in 40-92% yield via an AgClO4-promoted substitution reaction between a primary amine and a chloromethyl-functionalized thiazole or oxazole. These new synthetic building blocks will be useful for the preparation of new cyclopeptide analogues bearing heterocyclic backbone modifications. Four macrocyclic N-substituted oligoamides that include thiazole or oxazole heterocycles were obtained, following cyclooligomerization reactions of azole-modified N-substituted amino acids.