Imidazole, oxazole and thiazole alkaloids

Oxidative Coupling of Guanidines and Isocyanides Catalyzed by Nickel(II): Access to Imidazoline Derivatives with Antibacterial Activity

A novel and concise approach to rare 2,3,5-triamino-imidazole scaffolds via Ni-catalyzed coupling of alkylisocyanides and N,N'-diarylguanidines has been developed. This reaction features include mild conditions (thermal or visible light activation), a wide substrate scope, and high efficiency. The coupling proceeds through a NiII/NiIV catalytic cycle, involving two-electron aerobic oxidation and the sequential insertion of two isocyanide units into Ni-N bonds.Testing these compounds against pathogens of the ESKAPE panel showed their high activity with a minimum inhibitory concentration down to 0.38 μg/mL.

Total Synthesis and Structural Revision of (±)-Mauritamide B

Mauritamide B (1a) is a taurine-connected cyclic guanidino-bromopyrrole alkaloid originally isolated from the marine sponge Agelas linnaei. To date, the total synthesis of taurine-connected guanidino-bromopyrrole alkaloids, including this compound, has not yet been reported. Herein, a total synthesis of (±)-mauritamide B (1b) was achieved by oxidation of 2-aminoimidazole of dihydro-sventrin (10) using activated carbon and air in the presence of taurine. The synthetic precursor of 10, 4-(3-aminopropyl)-2-aminoimidazole (22), was synthesized via our original route. The NMR data of the obtained product agreed with that reported for mauritamide B (1a). However, a detailed analysis of the NMR data of synthetic (±)-mauritamide B (1b) including 1H-15N HSQC spectrum revealed the need for a structural revision of the reported structure for mauritamide B (1b).

Multicomponent reactions (MCRs) yielding medicinally relevant rings: a recent update and chemical space analysis of the scaffolds

In this review we have compiled multicomponent reactions (MCRs) that produce cyclic structures. We have covered articles reported since 2019 to showcase the recent advances in this area. In contrast to other available reviews on this topic, we focus specifically on MCRs with strong prospects in medicinal chemistry. Consequently, the reactions operating in a single-pot and yielding novel rings or new substitution patterns under mild conditions are highlighted. Moreover, MCRs that do not require special reagents or catalysts and yield diverse products from commercially available building blocks are reviewed. The synthetic schemes, substrate scope, and other key aspects such as regio- and stereoselectivity are discussed for each MCR. Using cheminformatic tools, we have also attempted to characterize the chemical space of the scaffolds obtained from these MCRs. We show that the MCR scaffolds are novel, more complex, and globular in shape compared to the approved drugs and clinical candidates. Thus, our review represents a step towards identifying and characterizing the novel ring space that can be accessed efficiently through MCRs in a short timeframe.

Dicationic copper(I) complexes bearing ENE (E = S, Se) pincer ligands; catalytic applications in regioselective cyclization of 1,6-diynes

Two novel dicationic binuclear Cu(I) complexes of the type [{(BPPP)E2}Cu]2[BF4]2 (E = S (3a); Se (3b)) bearing (BPPP)E2 (BPPP = bis(diphenylphosphino)pyridine) pincer systems were isolated, and structurally characterized. The solid-state structures of 3a/3b display the presence of intermolecular cuprophilic (Cu⋯Cu) interactions between the two monocationic species, and consist of weak Cu⋯S bonding between the two cations. Besides, complex 3a was introduced as a molecular copper(I) catalyst in cyclization reactions, and new protocols were developed for the synthesis of a series of new oxazole and triazole derivatives bearing alkyne-phenyl propargylic ether substituents. 3a was also found to be active in achieving these two classes of heterocyclic compounds by the mechanical grinding method. One of the key intermediate copper-azide species was detected by the high-resolution mass spectrometry technique, which supports the proposed catalytic pathway. All the reported transformations were accomplished sustainably by employing a well-defined, earth-abundant, and cheap copper(I) catalytic system.

Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···H2O and 5-Methylthiazole···H2O Revealed by Microwave Spectroscopy

The pure rotational spectra of 4-methylthiazole···H2O and 5-methylthiazole···H2O were recorded by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Each complex was generated within the rotationally cold environment of a gas sample undergoing supersonic expansion in the presence of an argon buffer gas. The spectra of five isotopologues of each complex have been measured and analyzed to determine the rotational constants, A0, B0, and C0; centrifugal distortion constants, DJ, DJK, and d1; nuclear quadrupole coupling constants, χaa(N3) and [χbb(N3) - χcc(N3)]; and parameters describing the internal rotation of the CH3 group, V3 and ∠(i,b). The experimentally deduced parameters were obtained using the XIAM and the BELGI-Cs-hyperfine code. For each complex, parameters in the molecular geometry are fitted to experimentally determined moments of inertia. DFT calculations have been performed at the ωB97X-D/aug-cc-pVQZ level in support of the experiments. Each complex contains two hydrogen bonds; a comparatively strong, primary interaction between the N of thiazole and an O-H of H2O, and a weaker, secondary interaction between O and either the hydrogen atom attached to C2 (in 5-methylthiazole···H2O) or the CH3 group attached to C4 (in 4-methylthiazole···H2O). The barrier to internal rotation of the CH3 group, V3, is slightly lower for 4-methylthiazole···H2O (XIAM result is 340.05(56) cm-1) than that for the 4-methylthiazole monomer (357.6 cm-1). This is likely to be a result of internal charge redistribution within the 4-methylthiazole subunit following its coordination by H2O. At the precision of the experiments, V3 of 5-methylthiazole···H2O (XIAM result is 325.16(38) cm-1) is not significantly different from V3 of the 5-methylthiazole monomer (332.0 cm-1).

Electrochemical Trifluoromethylation and Sulfonylation of N-Allylamides: Synthesis of Oxazoline Derivatives

We report a new method for the synthesis of trifluoromethylated and sulfonylated oxazolines by electrochemical radical cascade cyclizations of N-allylamides with sodium trifluoromethanesulfinate or sulfonylhydrazines. This protocol provides a green and useful strategy to synthesize trifluoromethylated and sulfonylated oxazolines with a broad substrate scope under ambient conditions.

Triple-Consecutive Isocyanide Insertions with Aldehydes: Synthesis of 4-Cyanooxazoles

An efficient TMSOTf-promoted selective triple consecutive insertions of tert-butyl isocyanide into aldehydes has been developed, affording pharmacological interesting 4-cyanooxazoles in high yields in a one pot manner. The given method encompasses a wide range of substrates with tert-butyl isocyanide serving as sources of critical "CN" and "C-N═C" moieties. The versatile transformations of the resulting 4-cyanooxazoles were demonstrated. The key reaction intermediates for plausible mechanisms were determined.

Ir-Catalyzed Asymmetric Cascade Allylation/Spiroketalization Reaction for Stereoselective Synthesis of Oxazoline-Spiroketals

An asymmetric cascade allylation/spiroketalization reaction between 2-(1-hydroxyallyl)phenols and 5-methyleneoxazolines is accomplished by using a chiral Ir(I) catalyst derived from commercially available iridium precursor and the Carreira ligand. This protocol furnishes a class of structurally novel and unique oxazoline-spiroketals in up to 86% yield, >99% ee and >20:1 dr. Moreover, control experiments reveal that 4,4-disubstitution on 5-methyleneoxazolines is necessary to avoid the aromatization and for the spiroketalization to occur. On the basis of this, a plausible reaction mechanism is illustrated.

Lewis Acid-Promoted Three-Component Cyclization for the Construction of Functionalized Oxazoles

A simple and efficient synthetic strategy from amides, ynals, and sodium sulfinates via a Lewis acid-promoted three-component reaction has been reported. Thus, a broad range of various aryl (not alkyl)-substituted oxazoles could be synthesized via the formation of C-N, C-O, and C-S bonds in a one-pot process. In addition, this reaction possesses other unique advantages, such as transition metal-free catalysis, high step economy, good functional group tolerance, and good regioselectivity.

Cascade Nucleophilic Attack/Addition Cyclization Reactions to Synthesize Oxazolidin-2-imines via (Z)-2-Bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and Diphenyl Carbodiimides

Two concise strategies to synthesize oxazolidin-2-imines by cascade nucleophilic attack/addition cyclization reactions of (Z)-2-bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and diphenyl carbodiimides without a transition-metal catalyst have been developed. The reactions exhibited good substrate applicability tolerance, and a variety of substituted (Z)-4-((Z)-benzylidene)-N,3-diphenyloxazolidin-2-imines were synthesized in moderate to excellent yields with good stereoselectivity. The reports also provided a convenient strategy to synthesize 3-phenylprop-2-yn-1-ols by (Z)-2-bromo-3-phenylprop-2-en-1-ols. The economic and practical methods provide a great advantage for potential industrial synthesis of oxazolidin-2-imines.

Facile synthesis of fully substituted 1H-imidazoles from oxime esters via dual photoredox/copper catalyzed multicomponent reactions

A novel and efficient photoredox/copper cocatalyzed domino cyclization of oxime esters, aldehydes, and amines has been achieved, affording a broad range of fully substituted 1H-imidazoles in good yields.

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.

TMSOTf-catalyzed synthesis of trisubstituted imidazoles using hexamethyldisilazane as a nitrogen source under neat and microwave irradiation conditions

In the process of drug discovery and development, an efficient and expedient synthetic method for imidazole-based small molecules from commercially available and cheap starting materials has great significance. Herein, we developed a TMSOTf-catalyzed synthesis of trisubstituted imidazoles through the reaction of 1,2-diketones and aldehydes using hexamethyldisilazane as a nitrogen source under microwave heating and solvent-free conditions. The chemical structures of representative trisubstituted imidazoles were confirmed using X-ray single-crystal diffraction analysis. This synthetic method has several advantages including the involvement of mild Lewis acid, being metal- and additive-free, wide substrate scope with good to excellent yields and short reaction time. Furthermore, we demonstrate the application of the methodology in the synthesis of biologically active imidazole-based drugs.

Trisubstituted imidazoles are synthesized efficiently from the readily available 1,2-diketones and aldehydes using hexamethyldisilazane as a new and stable nitrogen source under TMSOTf-catalysis system, microwave heating and solvent-free conditions.

Review on marine sponge alkaloid, aaptamine: A potential antibacterial and anticancer drug

In recent years, biological macromolecules have piqued the interest of researchers owing to their vast variety of biological uses. As a result, the marine sponge is a multicellular heterotrophic parazoan with chemicals for defence against predator assaults, biofouling and microbial diseases. These priceless molecules are known as secondary metabolites, and they are essential for survival in a highly competitive environment. So far, over 5,000 marine natural compounds have been extracted from marine sponges, making them an excellent option for drug formulation. One among them is, aaptamine, a marine alkaloid with a benzo[de][1,6]-napthyridine framework extensively distributed in marine sponges. Due to this reason, aaptamine has been intensively researched for various biological purposes, including cancer and protease inhibition, offering fresh insights into novel treatments. Keeping this in mind, we reviewed the biological significance of the marine sponge alkaloid aaptamine.

Rh(III)-Catalyzed olefination to build diverse oxazole derivatives from functional alkynes

A novel Rh(iii)-catalyzed olefination reaction of oxazoles to generate diverse oxazole skeleton derivatives has been realized by directly using oxazole as the directing group. The reaction could tolerate many functional groups, affording complex oxazole derivatives with long chain alkenyls in moderate to good yields, which might find applications in the construction of diverse compounds.

The facile and efficient synthesis of novel monocyclic cis-β-lactam conjugates with a 1-methyl-1H-imidazole-2-thiol nucleus

We developed a facile and efficient synthesis of novel monocyclic cis-β-lactam conjugates with a 1-methyl-1H-imidazole-2-thiol nucleus through a ketene-imine [2+2] cycloaddition reaction of acyl chloride and different Schiff bases.

Pd-N-heterocyclic carbene complex catalysed C–H bond activation of 2-isobutylthiazole at the C5 position with aryl bromides

An effective and efficient catalytic system has been reported for the synthesis of C5-arylated 2-isobutylthiazoles.

BF3·OEt2-TFAA Mediated Tetra-Functionalization of Amino Acids - Synthesis of Di- and Tri-Substituted 2-Trifluoromethyl Oxazoles in One Pot

A highly efficient, TFAA-BF₃·OEt₂ mediated multicomponent coupling of amino acid, TFAA, and aromatics provides a broad library of 2-trifluoromethyl equipped 2,5-disubstituted/2,4,5-trisubstituted oxazoles or N-(trifluoroacetyl)-β-aminoalkyl ketones. This amino acid tetra-functionalization approach involves amidation (C-N), anhydride (C-O), Friedel-Crafts acylation (C-C), and Robinson-Gabriel annulation (C-O) followed by dehydrative aromatization. This reaction takes place under operationally simple, mild, and metal-free conditions using readily available amino acids and aromatic compounds.

Synthesis of monofluorooxazoles with quaternary C-F centers through photoredox-catalyzed radical addition of methylene-2-oxazolines

A novel photoredox-catalyzed radical addition of methylene-2-oxazolines has been developed under visible light irradiation to synthesize monofluorooxazoles with a quaternary carbon center using 2-bromo-2-fluoro-3-oxo-3-phenylpropionates as radical source. This method with a simple protocol, scalability and high yield offers a facile path to get diverse monofluorinated oxazoles with quaternary C-F centers, which are a class of highly valuable motifs and synthons.

Copper-Catalyzed Enantioselective Synthesis of Oxazolines from Aminotriols via Asymmetric Desymmetrization

A copper-catalyzed enantioselective transformation of tris(hydroxymethyl)aminomethane-derived aminotriols was developed to provide multisubstituted oxazolines with a tetrasubstituted carbon center. The present transformation consisted of sequential reactions involving mono-sulfonylation of aminotriols, subsequent intramolecular cyclization to afford prochiral oxazoline diols, and sulfonylative asymmetric desymmetrization of resultant oxazoline diols. In addition, the kinetic resolution process would be involved in the sulfonylative asymmetric desymmetrization step, which would amplify the enantiopurities of the desired products. Various aminotriols were tolerated in the present reaction, affording the desired oxazolines in good to high yields with excellent enantioselectivities. The synthetic utility of the present reaction was demonstrated by the transformation of the optically active oxazoline into a chiral α-tertiary amine motif.

Fast and controlled ring-opening polymerization of δ-valerolactone catalyzed by benzoheterocyclic urea/MTBD catalysts

New benzoheterocyclic urea/MTBD catalysts are highly efficient and controllable in the ring-opening polymerization of δ-valerolactone under solvent-free conditions or in solution.

Synthesis of 2,4,5-Trisubstituted Oxazoles via Pd-Catalyzed C-H Addition to Nitriles/Cyclization Sequences

A practical and flexible intermolecular protocol for the diverse synthesis of trisubstituted oxazole derivatives via a Pd-catalyzed direct C-H addition of electronic-rich heteroarenes to O-acyl cyanohydrins bearing an α-hydrogen/cyclization sequence is described. A wide range of trisubstituted oxazoles can be prepared from readily available starting materials in good to high yields with high efficiency under redox neutral reaction conditions.

Direct synthesis of 2,4,5-trisubstituted imidazoles from primary alcohols by diruthenium(ii) catalysts under aerobic conditions

A straightforward synthetic approach to 2,4,5-trisubstituted imidazoles from readily available primary alcohols using arene diruthenium(ii) catalysts has been described.

Contemplating 1,2,4-Thiadiazole-Inspired Cyclic Peptide Mimics: A Computational Investigation

Marine derived cyclic peptides have inspired chemists for decades as the cavitand architecture can be compared with macrocyclic ligands, and hence easily conceived as mediators of metal-ion transport. Lissoclinamide 5 and ascidiacyclamide are two such cyclic peptides that have received much attention both for their metal ion complexation properties and biological activity; the metal ion binding properties of mimics of these two systems have been reported. Reported herein is a computational study aimed at evaluating the stability, and potential for copper(ii) ion binding by lissoclinamide 5 mimics that substitute the naturally occurring 4-carboxy-1,3-thiazole units for novel valine- and phenylalanine-derived 1,2,4-thiadiazole units. Our results suggest that one lissoclinamide 5 mimic, 1,2,4-thiadiazole (TDA)-lissoclinamide 9, may be capable of forming a complex with one CuII ion, [Cu(9-H)(H2O)]+. A complex with two CuII ions, [Cu2(9-H)(μ-OH)]2+, was also considered. These results set the stage for synthetic and experimental metal binding studies.

Preparation of oxazolines and oxazoles via a PhI(OAc)2-promoted cyclization of N-propargylamides

A metal-free cyclization of N-propargylamides for the synthesis of various oxazolines and oxazoles via a 5-exo-dig process is presented. Using (diacetoxyiodo)benzene (PIDA) as a reaction promoter and lithium iodide (LiI) as an iodine source, intramolecular iodooxygenation of N-propargylamides proceeded readily, leading to the corresponding (E)-5-iodomethylene-2-oxazolines in good to excellent isolated yields. In addition, using the PhI(OAc)2/LiI system, N-propargylamides can be converted to the corresponding oxazole-5-carbaldehydes in the presence of oxygen under visible light irradiation. The resulting products can be further converted into various oxazoline and oxazole derivatives after simple derivatizations, and this method ultimately offers an efficient route to a variety of biologically active structures.