Studies of mild dehydrogenations in heterocyclic systems

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.

Recent advances in the synthesis and utility of thiazoline and its derivatives

Thiazolines and their derivatives hold significant importance in the field of medicinal chemistry due to their promising potential as pharmaceutical agents. These molecular entities serve as critical scaffolds within numerous natural products, including curacin A, thiangazole, and mirabazole, and play a vital role in a wide array of physiological reactions. Their pharmacological versatility encompasses anti-HIV, neurological, anti-cancer, and antibiotic activities. Over the course of recent decades, researchers have extensively explored and developed analogs of these compounds, uncovering compelling therapeutic properties such as antioxidant, anti-tumor, anti-microbial, and anti-inflammatory effects. Consequently, thiazoline-based compounds have emerged as noteworthy targets for synthetic endeavors. In this review, we provide a comprehensive summary of recent advancements in the synthesis of thiazolines and thiazoline-based derivatives, along with an exploration of their diverse potential applications across various scientific domains.

Structural Requirements for Ga3+ Coordination in Synthetic Analogues of the Siderophore Piscibactin Deduced by Chemical Synthesis and Density Functional Theory Calculations

Stereoselective total synthesis of several analogues of piscibactin (Pcb), the siderophore produced by different pathogenic Gram-negative bacteria, was performed. The acid-sensitive α-methylthiazoline moiety was replaced by a more stable thiazole ring, differing in the configuration of the OH group at the C-13 position. The ability of these Pcb analogues to form complexes with Ga³⁺ as a mimic of Fe³⁺ showed that the configuration of the hydroxyl group at C-13 as 13S is crucial for the chelation of Ga³⁺ to preserve the metal coordination, while the presence of a thiazole ring instead of the α-methylthiazoline moiety does not affect such coordination. A complete ¹H and ¹³C NMR chemical shift assignment of the diastereoisomer mixtures around C9/C10 was done for diagnostic stereochemical disposition. Additionally, density functional theory calculations were performed not only for confirming the stereochemistry of the Ga³⁺ complex among the six possible diastereoisomers but also for deducing the ability of these to form octahedral coordination spheres with gallium. Finally, the lack of antimicrobial activity of Pcb and Pcb thiazole analogue Ga³⁺ complexes against Vibrio anguillarum agrees with one of the roles of siderophores in protecting pathogens from metal ion toxicity. The efficient metal coordination shown by this scaffold suggests its possible use as a starting point for the design of new chelating agents or vectors for the development of new antibacterials that exploit the "Trojan horse" strategy using the microbial iron uptake mechanisms. The results obtained will be of great help in the development of biotechnological applications for these types of compounds.

Photooxidative Coupling of Thiols Promoted by Bromo(trichloro)methane in a Basic Aqueous Medium

A transition-metal- and organic-solvent-free oxidative coupling of thiols catalyzed by BrCCl3 and NaOH in an aqueous medium with oxygen as a green oxidant was established The facile and green method has a broad substrate scope in converting thiols into the corresponding disulfides with medium to excellent yields (up to 91%). This method could potentially be used to construct bioactive molecules containing disulfide bonds and to label bioactive molecules with disulfide bonds.

Synthesis and Stability Studies of a Simplified, Thiazole-Containing Macrocycle of the Anticancer Agent Salarin C

While salarin C (SalaC) is a potent marine cytotoxin, Kashman demonstrated that congeners which had undergone Wasserman rearrangement exhibit little to no cytotoxicity. Given that thiazoles are known to undergo Wasserman rearrangement at a significantly reduced rate, we hypothesized that a thiazole-containing SalaC would exhibit greater stability without significantly altering the macrocyclic conformation. Herein, we describe the synthesis of a simplified, thiazole-containing macrocycle which demonstrates significantly improved stability under identical aerobic conditions.

Gaining Insights on the Interactions of a Class of Decorated (2-([2,2'-Bipyridin]-6-yl)phenyl)platinum Compounds with c-Myc Oncogene Promoter G-Quadruplex and Other DNA Structures

Organometallic molecules offer some of the most promising scaffolds for interaction with G-quadruplex nucleic acids. We report the efficient synthesis of a family of organoplatinum(II) complexes, featuring a 2-([2,2'-bipyridin]-6-yl)phenyl tridentate (N∧ N∧ C) ligand, that incorporates peripheral side-chains aiming at enhancing and diversifying its interaction capabilities. These include a di-isopropyl carbamoyl amide, a morpholine ethylenamide, two enantiomeric proline imides and an oxazole. The binding affinities of the Pt-complexes were evaluated via UV-vis and fluorescence titrations, against 5 topologically-distinct DNA structures, including c-myc G-quadruplex, two telomeric (22AG) G-quadruplexes, a duplex (ds26) and a single-stranded (polyT) DNA. All compounds exhibited binding selectivity in favour of c-myc, with association constants (Ka ) in the range of 2-5×105  M-1 , lower affinity for both folds of 22AG and for ds26 and negligible affinity for polyT. Remarkable emission enhancements (up to 200-fold) upon addition of excess DNA were demonstrated by a subset of the compounds with c-myc, providing a basis for optical selectivity, since optical response to all other tested DNAs was low. A c-myc DNA-melting experiment showed significant stabilizing abilities for all compounds, with the most potent binder, the morpholine-Pt-complex, exhibiting a ΔTm >30 °C, at 1 : 5 DNA-to-ligand molar ratio. The same study implied contributions of the diverse side-chains to helix stabilization. To gain direct evidence of the nature of the interactions, mixtures of c-myc with the four most promising compounds were studied via UV Resonance Raman (UVRR) spectroscopy, which revealed end-stacking binding mode, combined with interactions of side-chains with loop nucleobase residues. Docking simulations were conducted to provide insights into the binding modes for the same four Pt-compounds, suggesting that the binding preference for two alternative orientations of the c-myc G-quadruplex thymine 'cap' ('open' vs. 'closed'), as well as the relative contributions to affinity from end-stacking and H-bonding, are highly dependent on the nature of the interacting Pt-complex side-chain.

Carbodiimide-based synthesis of N-heterocycles: moving from two classical reactive sites to chemical bond breaking/forming reaction

Carbodiimides are a unique class of heterocumulene compounds that display distinctive chemical properties. The rich chemistry of carbodiimides has drawn increasing attention from chemists in recent years and has made them exceedingly useful compounds in modern organic chemistry, especially in the synthesis of N-heterocycles. This review has outlined the extensive application of carbodiimides in the synthesis of N-heterocycles from the 1980s to today. A wide range of reactions for the synthesis of various types of N-heterocyclic systems (three-, four-, five-, six-, seven-, larger-membered and fused heterocycles) have been developed on the basis of carbodiimides and their derivatives.

Synthesis and Investigation of the Abiotic Formation of Pyonitrins A-D

Pyonitrins A–D are recently isolated natural products from the insect-associated Pseudomonas protegens strain, which were isolated from complex fractions that exhibited antifungal activity via an in vivo murine candidiasis assay. Genomic studies of Pseudomonas protegens suggested that pyonitrins A–D are formed via a spontaneous nonenzymatic reaction between biosynthetic intermediates of two well-known natural products pyochelin and pyrrolnitrin. Herein we have accomplished the first biomimetic total synthesis of pyonitrins A–D in three steps and studied the nonenzymatic formation of the pyonitrins using ¹⁵N NMR spectroscopy.

I2/TBHP-Mediated tandem cyclization and oxidation reaction: Facile access to 2-substituted thiazoles and benzothiazoles

The efficient synthesis of 2-substituted thiazoles and benzothiazoles has been accomplished employing readily available cysteine esters and 2-aminobenzenethiols as N and S sources.

Ruthenium(ii)-catalyzed C–O/C–S cyclization for the synthesis of 5-membered O-containing and S-containing heterocycles

An efficient and convenient method for the synthesis of oxazole derivatives from enamides has been established via a ruthenium-catalyzed C–O cyclization.

Metal-free iodine(iii)-promoted synthesis of 2,5-diaryloxazoles

A nonmetal-catalyzed oxidative cyclization to achieve 2,5-disubstituted oxazoles from inexpensive and readily available substituted chalcone, (diacetoxyiodo)benzene (PIDA) and ammonium acetate (NH4OAc) at room temperature is described. The reaction forms a variety of 2,5-diaryloxazoles in good to excellent yields with broad substrate scope under mild conditions without the requirement of ligands and additional bases.

Copper-Catalyzed Oxygenation Approach to Oxazoles from Amines, Alkynes, and Molecular Oxygen

A novel and efficient oxygenation approach to trisubstituted oxazoles via a copper-catalyzed aerobic oxidative dehydrogenative annulation of amines, alkynes, and O₂ has been developed. This transformation combines dioxygen activation and oxidative C-H bond functionalization and provides a practical protocol for the preparation of oxazole derivatives, which are privileged units found in various bioactive compounds or other natural products. ¹⁸O-labeling experiments were conducted to reveal that oxygenation was involved in this chemistry.

A Heterogeneous Gold(I)-Catalyzed [2 + 2 + 1] Annulation of Terminal Alkynes, Nitriles, and Oxygen Atoms Leading to 2,5-Disubstituted Oxazoles

The first heterogeneous gold(I)-catalyzed [2 + 2 + 1] annulation of terminal alkynes, nitriles, and oxygen atoms has been achieved by using an MCM-41-immobilized phosphine-gold(I) complex as catalyst and 8-methylquinoline N-oxide as oxidant under mild conditions, yielding a variety of 2,5-disubstituted oxazoles in good to excellent yields with broad substrate scope. The new heterogeneous gold(I) catalyst can easily be recovered by simple filtration of the reaction solution and recycled for at least eight times without significant loss of activity.

Studies of Azetidin-2-one as a Reactive Enolate Synthon of β-Alanine for Condensations with Aldehydes and Ketones

Studies describe formation of the lithium enolate of N-(4-methoxybenzyloxy)azetidin-2-one (1) and characterization of representative aldol reactions with aldehydes and ketones. Diastereoselectivity features the production of anti-aldol adducts from α,β-unsaturated ketones and α-branched aliphatic aldehydes. The stereoselectivity is rationalized via closed, six-membered transition-state arrangements leading to the formation of Felkin-Anh and anti-Felkin products. Examples illustrate the direct incorporation of monocyclic β-lactams into a variety of molecular architectures. The utility of 1 as an enolate synthon of homoglycine (β-alanine) is illustrated by the efficient synthesis of novel β-amino acid derivatives, including complex 4-hydroxy-2-pyridinones.

Stereoselective access to tubuphenylalanine and tubuvaline: improved Mn-mediated radical additions and assembly of a tubulysin tetrapeptide analog

Synthesis of tubuphenylalanine and tubuvaline (Tuv), α-substituted γ-amino acid building blocks for tubulysin family of antimitotic compounds, has been improved using a radical addition reaction in the presence of unprotected hydroxyl functionality. The key carbon-carbon bond construction entails stereoselective Mn-mediated photolytic additions of alkyl iodides to the C=N bond of chiral N-acylhydrazones, and generates the chiral amines in high yield with complete stereocontrol. Reductive N-N bond cleavage and alcohol oxidation converted these amino alcohols into the corresponding γ-amino acids. The route to Tuv proceeded via peptide coupling with serine methyl ester, followed by a high-yielding sequence to convert the serine amide to a thiazole. Finally, peptide bond construction established the tubulysin framework in the form of a C-terminal alcohol analog. Attempted oxidation to the C-terminal carboxylate was unsuccessful; control experiments with dipeptide 18 showed a cyclization interfered with the desired oxidation process.

Total synthesis of the azolemycins

The first total syntheses of newly isolated polyazole natural products azolemycins A–D, along with the synthesis of the tetra-oxazole non-natural analogue, are described.

Total Synthesis of the Posttranslationally Modified Polyazole Peptide Antibiotic Plantazolicin A

The power of rhodium-carbene methodology in chemistry is demonstrated by the synthesis of a structurally complex polyazole antibiotic. Plantazolicin A, a novel soil-bacterium metabolite, comprises a linear array of 10 five-membered rings in two pentacyclic regions that derive from ribosomal peptide synthesis followed by extensive posttranslational modification. The compound possesses potent antimicrobial activity, and is selectively active against the anthrax-causing organism. A conceptually different synthesis of plantazolicin A is reported in which the key steps are the use of rhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate up to six of the seven oxazole rings of the antibiotic. NMR spectroscopic studies and molecular modeling reveal a likely dynamic hairpin conformation with a hinge region around the two isoleucine residues. The compound has modest activity against methicillin-resistant Staphylococcus aureus (MRSA).

Concise synthesis of 2,4-disubstituted thiazoles from β-azido disulfides and carboxylic acids or anhydrides: asymmetric synthesis of cystothiazole C

A novel and efficient method for the one-pot synthesis of 2,4-disubstituted thiazoles from carboxylic acids or anhydrides is presented. Based on this new method, the total synthesis of the bis-2,4-disubstituted bis(thiazoles) natural product cystothiazole C is also presented.

Electrolytic macrocyclizations: scalable synthesis of a diazonamide-based drug development candidate

An electrochemical method to synthesize the core macrolactam of diazonamides is described. Large ring-forming dehydrogenation is initiated by anodic oxidation at a graphite surface. The reaction requires no tailoring of the substrate and occurs at ambient temperature in aqueous DMF in an undivided cell open to air. This unique chemistry has enabled a concise, scalable preparation of DZ-2384; a refined analog of diazonamide A slated for clinical development as a cancer therapeutic.

A facile synthesis of 2,5-disubstituted oxazoles via a copper-catalyzed cascade reaction of alkenes with azides

A novel approach to the synthesis of 2,5-disubstituted oxazoles is developed via a 1,3-dipolar cycloaddition/ring cleavage/1,2-H migration/denitrogenation/copper-catalyzed aerobic oxidative dehydrogenative cyclization cascade.

Synthesis and structural reconfirmation of bacillamide B

Synthesis of bacillamide B was accomplished, the absolute configuration was reconfirmed as S, and the specific optical rotation was revised to (−).

Inhibition of group IVA cytosolic phospholipase A2 by thiazolyl ketones in vitro, ex vivo, and in vivo

Group IVA cytosolic phospholipase A2 (GIVA cPLA2) is the rate-limiting provider of pro-inflammatory mediators in many tissues and is thus an attractive target for the development of novel anti-inflammatory agents. In this work, we present the synthesis of new thiazolyl ketones and the study of their activities in vitro, in cells, and in vivo. Within this series of compounds, methyl 2-(2-(4-octylphenoxy)acetyl)thiazole-4-carboxylate (GK470) was found to be the most potent inhibitor of GIVA cPLA2, exhibiting an XI(50) value of 0.011 mole fraction in a mixed micelle assay and an IC50 of 300 nM in a vesicle assay. In a cellular assay using SW982 fibroblast-like synoviocytes, it suppressed the release of arachidonic acid with an IC50 value of 0.6 μM. In a prophylactic collagen-induced arthritis model, it exhibited an anti-inflammatory effect comparable to the reference drug methotrexate, whereas in a therapeutic model, it showed results comparable to those of the reference drug Enbrel. In both models, it significantly reduced plasma PGE2 levels.

Regioselective synthesis of oxazole derivatives via palladium-catalyzed and copper-mediated cascade oxidative cyclization

A novel Pd-catalyzed/Cu-mediated oxidative cyclization to synthesize oxazoles via cascade formation of C–N and C–O bonds has been developed.

Electrochemically promoted synthesis of polysubstituted oxazoles from β-diketone derivatives and benzylamines under mild conditions

An efficient electrochemical route has been developed to synthesize polysubstituted oxazoles, without the use of hazardous oxidants and transition metal catalysts.

Synthesis and biological properties of thiazole-analogues of pyochelin, a siderophore of Pseudomonas aeruginosa

Pyochelin is a siderophore common to all strains of Pseudomonas aeruginosa utilized by this Gram-negative bacterium to acquire iron(III). FptA is the outer membrane transporter responsible of ferric-pyochelin uptake in P. aeruginosa. We describe in this Letter the synthesis and the biological properties ((55)Fe uptake, binding to FptA) of several thiazole analogues of pyochelin. Among them we report in this Letter the two first pyochelin analogues able to bind FptA without promoting any iron uptake in P. aeruginosa.