Overview on the recently developed thiazolyl heterocycles as useful therapeutic agents

Thiazoles are important heterocyclic compounds which have many biological activities and different applications such as useful ligands, in optical sensors, etc. A literature survey shows that there are different routes to thiazoles. One of the most frequently used synthetic approaches consists of a reaction between α-halocarbonyl compounds with a CSNH₂ moiety. In this mini-review we have classified the contents based on the reagent or material providing the sulfur atom of the thiazole ring. Also, among many articles which have been devoted to thiazole syntheses here we presented some synthetic approaches published from 2012 to 2014.     

A progressive synthetic strategy for class B synergimycins

Described are the syntheses of four macrocyclic peptides that are the core structure of class B synergimycins, and the synthesis of a final class B derivative. Our synthetic route to these synergimycin derivatives allows the incorporation of amino acid substitutions at all points in the macrocycle, leading to structurally diverse class B analogs.     

Sanguinamide B analogs: identification of active macrocyclic structures

We report the synthesis of three new sanguinamide B (San B) analogs. We substituted in amino acids along the San B backbone with an N-Me, glycine, or an aromatic moiety (Phe or d-Phe) generating twelve derivatives in total. Testing in HCT-116 colon cancer cell lines resulted in establishing a structure–activity relationship. Our data show that the substitution of l- or d-Phe adjacent to the thiazole in the San B backbone locks the macrocycle into a single conformer, but only d-Phe analogs are cytotoxic. We demonstrate that the conformation of the macrocycle is extremely sensitive to stereochemistry and amino acid placement.     

Chiral dipeptide mimics possessing a fluoroolefin moiety: a relevant tool for conformational and medicinal studies

The replacement of the amide bond in a peptide backbone is a promising strategy in peptidomimetic drug research. Over the various amide bond surrogates, the fluoroolefin moiety has been successfully developed as an effective mimic. Today, fluorine-containing compounds account for a large proportion of new active molecules in life sciences. The synthesis of fluoroolefin peptide mimics is not a trivial task and innovative approaches often need to be addressed, in particular for the stereocontrol of the double bond configuration and the chiral centres adjacent to the fluoroalkene. These fluorinated peptidomimetics have been synthesised and evaluated as metabolically stable and/or conformationally constrained analogs of enzyme inhibitors, and as tools for probing the function, structure, and binding process of receptors.     

Synthesis,Profiling, and Bioactive Conformation of trans‐Cyclopropyl Epothilones

A series of new 3‐deoxy‐C(12),C(13)‐trans‐cyclopropyl‐epothilones have been prepared, bearing benzothiazole, quinoline, thiazol‐5‐ylvinyl, or isoxazol‐3‐ylvinyl side chains. For analogs with fused aromatic side chains, macrocyclic ring‐closure was based on ring‐closing olefin metathesis (RCM) of a precursor incorporating the fully elaborated heavy atom framework of the target structure (including the side chain moiety), while side chain attachment for the thiazole and isoxazole‐containing 16‐desmethyl analogs was performed only after establishment of the macrolactone core. Two approaches were elaborated for a macrocyclic aldehyde as the common precursor for the latter analogs that involved ring‐closure either by RCM or by macrolactonization. Benzothiazole‐ and quinoline‐based analogs were found to be highly potent antiproliferative agents; the two analogs with a thiazol‐5‐ylvinyl or an isoxazol‐3‐ylvinyl side chain likewise showed good antiproliferative activity but were significantly less potent than the parent epothilone A. Surprisingly, the desaturation of the C(10)?C(11) bond in these analogs was associated with a virtually complete loss in antiproliferative activity, which likely reflects a requirement for a ca. 60 ° C(10)?C(11) torsion angle in the tubulin‐bound conformation of 12,13‐trans‐epothilones.     

Regioselective synthesis of antiparallel loops on a macrocyclic scaffold constrained by oxazoles and thiazoles

The regioselective syntheses and structures are reported for two tris-macrocylic compounds, each possessing two antiparallel loops on a macrocyclic scaffold constrained by two oxazoles and two thiazoles. NMR solution structures show the loops projecting from the same face of the macrocycle. Such molecules are shown to be prototypes for mimicking multiple loops of proteins.[structure: see text]     

Peptidomimetics via copper-catalyzed azide-alkyne cycloadditions

This critical review concerns the impact of copper-mediated alkyne-azide cycloadditions on peptidomimetic studies. It discusses how this reaction has been used to insert triazoles into peptide chains, to link peptides to other functionalities (e.g. carbohydrates, polymers, and labels), and as a basis for evolution of less peptidic compounds as pharmaceutical leads. It will be of interest to those studying this click reaction, peptidomimetic secondary structure and function, and to medicinal chemists.     

Cu(II) coordination chemistry of patellamide derivatives: possible biological functions of cyclic pseudopeptides

Two synthetic derivatives of the naturally occurring cyclic pseudooctapeptides patellamide A-F and ascidiacyclamide, that is, H(4)pat(2), H(4)pat(3), as well as their Cu(II) complexes are described. These cyclic peptide derivatives differ from the naturally occurring macrocycles by the variation of the incorporated heterocyclic donor groups and the configuration of the amino acids connecting the heterocycles. The exchange of the oxazoline and thiazole groups by dimethylimidazoles or methyloxazoles leads to more rigid macrocycles, and the changes in the configuration of the side chains leads to significant differences in the folding of the cyclic peptides. These variations allow a detailed study of the various possible structural changes on the chemistry of the Cu(II) complexes formed. The coordination of Cu(II) with these macrocyclic species was monitored by high-resolution electrospray mass spectrometry (ESI-MS), spectrophotometric (UV/Vis) and circular dichroic (CD) titrations, and electron paramagnetic resonance (EPR) spectroscopy. Density functional theory (DFT) calculations and molecular mechanics (MM) simulations have been used to model the structures of the Cu(II) complexes and provide a detailed understanding of their geometric preferences and conformational flexibility. This is related to the Cu(II) coordination chemistry and the reactivity of the dinuclear Cu(II) complexes towards CO(2) fixation. The variation observed between the natural and various synthetic peptide systems enables conclusions about structure-reactivity correlations, and our results also provide information on why nature might have chosen oxazolines and thiazoles as incorporated heterocycles.     

Gas-phase thermolysis of condensed-1,2,4-triazines: interesting routes toward heterocyclic ring systems

Gas-phase thermolysis of thieno[2,3-e][1,2,4]triazines gave benzonitrile, isothiazole, pyridazine, and thieno[2,3-d]thiazole derivatives. Similar transformation of benzo[1,2,4]triazine and phenanthro[9,10-e][1,2,4]triazine derivatives into their corresponding condensed thiazoles has been achieved by heating at 350 °C with sulfur. A mechanism for these pyrolytic transformations was proposed.     

Beiträge zum Studium einiger Heterocyclen. L—Massenspektrometrische Untersuchungen von 2-Aryl-4-r-5-acetylthiazolen

The mass spectra of some 5-acetyl thiazoles were studied, and the differences emphasized between them and the corresponding 4-acetyl derivatives. This could constitute a method for their identification. The nature of the substituent in position 4 of the thiazole influences the fragmentation patterns of thiazole ring. In the case of 4-hydroxymethyl derivatives, a skeletal rearrangement occurs before fragmentation.     

Biosynthetic Strategies for Macrocyclic Peptides

Macrocyclic peptides are predominantly peptide structures bearing one or more rings and spanning multiple amino acid residues. Macrocyclization has become a common approach for improving the pharmacological properties and bioactivity of peptides. A variety of ribosomal-derived and non-ribosomal synthesized cyclization approaches have been established. The biosynthesis of backbone macrocyclic peptides using seven new emerging methodologies will be discussed with regard to the features and strengths of each platform rather than medicinal chemistry tools. The mRNA display variant, known as the random nonstandard peptide integrated discovery (RaPID) platform, utilizes flexible in vitro translation (FIT) to access macrocyclic peptides containing nonproteinogenic amino acids (NAAs). As a new discovery approach, the ribosomally synthesized and post-translationally modified peptides (RiPPs) method involves the combination of ribosomal synthesis and the phage screening platform together with macrocyclization chemistries to generate libraries of macrocyclic peptides. Meanwhile, the split-intein circular ligation of peptides and proteins (SICLOPPS) approach relies on the in vivo production of macrocyclic peptides. In vitro and in vivo peptide library screening is discussed as an advanced strategy for cyclic peptide selection. Specifically, biosynthetic bicyclic peptides are highlighted as versatile and attractive modalities. Bicyclic peptides represent another type of promising therapeutics that allow for building blocks with a heterotrimeric conjugate to address intractable challenges and enable multimer complexes via linkers. Additionally, we discuss the cell-free chemoenzymatic synthesis of macrocyclic peptides with a non-ribosomal catalase known as the non-ribosomal synthetase (NRPS) and chemo-enzymatic approach, with recombinant thioesterase (TE) domains. Novel insights into the use of peptide library tools, activity-based two-hybrid screening, structure diversification, inclusion of NAAs, combinatorial libraries, expanding the toolbox for macrocyclic peptides, bicyclic peptides, chemoenzymatic strategies, and future perspectives are presented. This review highlights the broad spectrum of strategy classes, novel platforms, structure diversity, chemical space, and functionalities of macrocyclic peptides enabled by emerging biosynthetic platforms to achieve bioactivity and for therapeutic purposes.     

Modulating Protein–Protein Interactions by Cyclic and Macrocyclic Peptides. Prominent Strategies and Examples

Cyclic and macrocyclic peptides constitute advanced molecules for modulating protein–protein interactions (PPIs). Although still peptide derivatives, they are metabolically more stable than linear counterparts, and should have a lower degree of flexibility, with more defined secondary structure conformations that can be adapted to imitate protein interfaces. In this review, we analyze recent progress on the main methods to access cyclic/macrocyclic peptide derivatives, with emphasis in a few selected examples designed to interfere within PPIs. These types of peptides can be from natural origin, or prepared by biochemical or synthetic methodologies, and their design could be aided by computational approaches. Some advances to facilitate the permeability of these quite big molecules by conjugation with cell penetrating peptides, and the incorporation of β-amino acid and peptoid structures to improve metabolic stability, are also commented. It is predicted that this field of research could have an important future mission, running in parallel to the discovery of new, relevant PPIs involved in pathological processes.     

Reactions of Methyl 1-(Bromoacetyl)azulene-3-carboxylate with Thioamides and Thioureas: Synthesis of 1-(Thiazol-4-yl)-3-methoxycarbonylazulenes

Thiazole derivatives have attracted a great deal of interest owing to their physiological activities. In addition, thiazoles are also useful in dye and photographic industry.[1] In the present paper we report the synthesis of azulenes containing a thiazole moiety.     

Organolithium-mediated diversification of peptide thiazoles

[Reaction: see text] We report a one-step, racemization-free method for the diversification of peptide thiazoles via direct lithiation of the thiazole ring. The method is compatible with N-Boc, N-trityl, carboxylic ester, and carboxamide protecting groups and has been used to directly functionalize the thiazole ring of cyclopeptide natural products.     

Synthesis of functionalized thiazoles via attack of heterocyclic nucleophiles on allenyl isothiocyanates

New examples of substituted thiazole derivatives carrying different heterocyclic ring systems at C-2 position were prepared via the reaction of several allenyl isothiocyanates with nucleophiles such as imidazoles, pyrazoles, benzimidazoles, indazole, 1,2,3-triazole, 1,2,4-triazole, and 1H-benzotriazole. Although these allenyl isothiocyanates are very reactive electrophiles and tend to polymerize, the yield of the thiazole products ranked between modest and very good. The regiochemistry of the reactions was proved by NMR and X-ray studies indicating that the attack of ambident nucleophiles proceeded very selectively. In some cases, however, the products were formed as mixtures of aromatic heterocycles and non-aromatic isomers. The latter could be rearranged to yield the uniform aromatic thiazoles.     

Thiocyanation, Halogenation, Dehalogenation, Transhalogenation, and Nitration of 2-Substituted 4-(2-Furyl)thiazoles

Bromination and thiocyanation of 2-amino- and 2-acetylamino-4-(2-furyl)thiazoles when 1 mol of reagent is used at 10°C are directed to the 5 position. Formation of 5'-bromo-substituted derivatives when the reaction temperature is raised is the result of a secondary, thermodynamically controlled process. Monohalogenation and mononitration of 4-(2-furyl)-2-methylthiazole are directed to the 5' position. Nitration of 2-acetylamino-4-(5-nitro-2-furyl)thiazole by a nitrating mixture is accompanied by oxidative cleavage of the 5-nitrofuran moiety and leads to formation of 5,5'- and 3',5'-dinitro derivatives.     

Protein ligand design: from phage display to synthetic protein epitope mimetics in human antibody Fc-binding peptidomimetics

Phage display is a powerful method for selecting peptides with novel binding functions. Synthetic peptidomimetic chemistry is a powerful tool for creating structural diversity in ligands as a means to establish structure-activity relationships. Here we illustrate a method of bridging these two methodologies, by starting with a disulfide bridged phage display peptide which binds a human antibody Fc fragment (Delano et al. Science 2000, 287, 1279) and creating a backbone cyclic beta-hairpin peptidomimetic with 80-fold higher affinity for the Fc domain. The peptidomimetic is shown to adopt a well-defined beta-hairpin conformation in aqueous solution, with a bulge in one beta-strand, as seen in the crystal structure of the phage peptide bound to the Fc domain. The higher binding affinity of the peptidomimetic presumably reflects the effect of constraining the free ligand into the conformation required for binding, thus highlighting in this example the influence that ligand flexibility has on the binding energy. Since phage display peptides against a wide variety of different proteins are now accessible, this approach to synthetic ligand design might be applied to many other medicinally and biotechnologically interesting target proteins.     

Synthesis of novel scaffolds based on thiazole or triazolothiadiazine linked to benzofuran or benzo[d]thiazole moieties as new hybrid molecules

Abstract

A synthesis of novel hybrid molecules containing thiazole or bis(thiazoles) each bearing benzofuran and/or benzo[d]thiazole moieties by the reaction of the appropriate thioamide derivatives with the corresponding bis-bromoacetyl derivatives is reported. Mono- and bis(triazolothiadiazine) derivatives based on benzofuran or benzo[d]thiazole moieties were also synthesized in good yields by the reaction of the appropriate bis(bromoacetyl) derivatives with each of 4-amino-5-mercapto-1,2,4-triazoles and their corresponding bis-derivatives.     

Synthesis and derivatization of daptomycin: a chemoenzymatic route to acidic lipopeptide antibiotics

Daptomycin is a branched cyclic nonribosomally assembled acidic lipopeptide, which is the first clinically approved antibiotic of this class. Here we show that the recombinant cyclization domain of the Streptomyces coelicolor calcium-dependent antibiotic (CDA) nonribosomal peptide synthetase (NRPS) is a versatile tool for the chemoenzymatic generation of daptomycin derivatives. Linear CDA undecapeptide thioesters with single exchanges at six daptomycin-specific residues were successfully cyclized by CDA cyclase. Simultaneous incorporation of all six of these residues into the peptide backbone and elongation of the N-terminus of CDA by two residues yielded a daptomycin derivative that lacked only the beta-methyl group of l-3-methylglutamate. Bioactivity studies with several substrate analogues revealed a significant role of nonproteinogenic constituents for antibacterial potency. In accordance with acidic lipopeptides, the bioactivity of the chemoenzymatic assembled daptomycin analogue is dependent on the concentration of calcium ions. Single deletions of the four acidic residues in the peptide backbone suggest that only two aspartic acid residues are essential for antimicrobial potency. These two residues are strictly conserved among other nonribosomal acidic lipopeptides and the EF-motif of ribosomally assembled calmodulin. Based on these findings CDA cyclase is a versatile catalyst that can be used to generate novel daptomycin derivatives that are otherwise difficult to obtain by chemical modification of the parental tridecapeptide to improve further its therapeutic activity.     

Nonribosomal peptides: from genes to products

The ability to synthesize nonribosomally small bioactive peptides that find application in modern medicine is widely spread among microorganisms. As broad as the spectrum of biological activities is the structural diversity of these peptides, which are mostly cyclic or branched cyclic compounds containing non-proteinogenic amino acids, small heterocyclic rings and other unusual modifications in the peptide backbone. They are synthesized by multimodular enzymes, the so-called nonribosomal peptide synthetases (NRPSs), from simple building blocks. Biochemical and genetic studies have unveiled the key principles of nonribosomal peptide syntheses, as well as the realization of many structural features of these peptides. This review focuses on recent results in NRPS research and highlights how this knowledge can be exploited for biotechnological purposes. In addition, possibilities and limitations for prediction of structural features of uncharacterized NRPSs and approaches for their engineering are discussed.