Solid Phase Synthesis of (Benzannelated) Six-Membered Heterocycles via Cyclative Cleavage of Resin-Bound Pseudo-Oxazolones

Sustainable synthesis of antibacterial 3-aryl-2H-benzo[b,1,4]oxazin-2-ones via SNAr Csp2-Csp2 coupling

Introduction

The increasing prevalence of antibiotic-resistant pathogens necessitates the urgent development of new antibacterial agents. Concurrently, synthetic chemistry is moving towards more sustainable practices that minimize environmental impact. This study aims to synthesize 3-aryl-2H-benzo[b][1,4]oxazin-2-one derivatives, including the natural product cephalandole A, using a sustainable approach that avoids metal catalysts.

Methods

We employed nucleophilic aromatic substitution (SNAr) under microwave-assisted conditions to facilitate the synthesis of the targeted compounds. This metal-free carbon-carbon coupling reaction was optimized for efficiency, yielding good results with reduced reaction times. The synthesized derivatives were then subjected to an in silico molecular docking study to predict their antibacterial potential against key bacterial targets, focusing on the binding affinity and interaction profiles.

Results

The microwave-assisted SNAr method provided good yields of 55% to 82% and significantly reduced reaction times ranging from 7 to 12 minutes, simplifying the overall workup process. Among the synthesized compounds, 3-(1H-indol-3-yl)-6-methyl-2H-benzo[b][1,4]oxazin-2-one (6b) emerged as a promising candidate, demonstrating favorable binding interactions in the molecular docking studies.

Discussion

The integration of sustainable synthetic methodologies with in silico screening offers a novel and effective strategy for drug discovery. Our findings highlight the potential of the synthesized compounds as antibacterial agents and emphasize the importance of adopting eco-friendly approaches in pharmaceutical chemistry. This research contributes to the global effort to combat antibiotic resistance by providing new compounds for further biological evaluation.

Sulfur-Promoted Access to 3-Arylquinoxalin-2-ones by Oxidative Coupling of o-Phenylenediamines with Arylacetates

We disclose the synthesis of 3-arylquinoxalin-2-ones from o-phenylenediamines and readily available arylacetates. The method harnesses the selective oxidative property of elemental sulfur in the presence of amine base catalyst and DMSO. The reactions are operationally simple and tolerate a wide range of functional groups.

Inhibition of protein misfolding and aggregation by steroidal quinoxalin-2(1H)-one and their molecular docking studies

A series of D-ring fused 16-substituted steroidal quinoxalin-2(1H)-one attached to an electron-releasing (ER) or electron-withdrawing (EW) groups via steroidal oxoacetate intermediate were synthesized to investigate their protein aggregation inhibition potential using human lysozyme (HLZ). The influence of the type of substituent at the C-6 positions of the quinoxalin-2(1H)-one ring on the protein aggregation inhibition potential was observed, showing that the EW moiety improved the protein aggregation inhibition potency. Of all the evaluated compounds, NO2-substituted quinoxalin-2(1H)-one derivative 13 was the most active compound and had a maximum protein aggregation inhibition effect. Significant stabilization effects strongly support the binding of the most biologically active steroidal quinoxalin-2(1H)-one with docking studies. The predicted physicochemical and ADME properties lie within a drug-like space which shows no violation of Lipinski's rule of five except compounds 12 and 13. Combined, our results suggest that D-ring fused 16-substituted steroidal quinoxalin-2(1H)-one has the potential to modulate the protein aggregation inhibition effect.

Photochemically Induced Hydrogen Atom Transfer and Intramolecular Radical Cyclization Reactions with Oxazolones

Photochemically induced intramolecular hydrogen atom transfer in oxazolones is reported. An acetal or thioacetal function at the side chain acts as a hydrogen donor while the photochemical exited oxazolone is the acceptor. A one-step process─the electron and the proton are simultaneously transferred─is productive, while electron transfer followed by proton transfer is inefficient. Radical combination then takes place, leading to the formation of a C-C or C-N bond. The regioselectivity of the reaction is explained by the diradical/zwitterion dichotomy of radical intermediates at the singlet state. In the present case, the zwitterion structure plays a central role, and intramolecular electron transfer favors spin-orbit coupling and thus the intersystem crossing to the singlet state. The reaction of corresponding thioacetal derivatives is less efficient. In this case, photochemical electron transfer is competitive. The photoproducts resulting from C-C bond formation easily undergo stepwise thermal decarboxylation in which zwitterionic and polar transition states are involved. A computational study of this step has also been performed.

Divergent Synthesis of 3-(Indol-2-yl)quinoxalin-2-ones and 4-(Benzimidazol-2-yl)-3-methyl(aryl)cinnolines via Polyphosphoric Acid (PPA)-Mediated Intramolecular Rearrangements of 3-(Methyl/aryl(2-phenylhydrazono)methyl)quinoxalin-2-ones

Herein, we report a polyphosphoric acid (PPA)-mediated divergent metal-free operation to access a diverse collection of 3-(indol-2-yl)quinoxalin-2-ones and 4-(benzimidazol-2-yl)-3-methylcinnolines in moderate to excellent overall yields. The described process involves two distinct, and competing rearrangements of 3-(methyl(2-phenylhydrazono)methyl)quinoxalin-2-ones, namely [3,3]-sigmatropic Fischer rearrangement with the formation of an indole ring to produce 3-(indol-2-yl)-quinoxalin-2-ones, and Mamedov rearrangement with simultaneous construction of benzimidazole and cinnoline rings to form the new biheterocyclic system─4-(benzimidazol-2-yl)-3-methylcinnolines. The reaction mechanism of both rearrangement channels is explored by extensive dispersion-corrected DFT calculations. It is partcularly remarkable that when 3-(aryl(2-phenylhydrazono)methyl)quinoxalin-2-ones is used, instead of 3-(methyl(2-phenylhydrazono)methyl)quinoxalin-2-ones, reactions proceed regioselectively with the formation of only rearrangement products─4-(benzimidazol-2-yl)-3-arylcinnolines with high yields. This operationally simple protocol enables a rapid access to these scaffolds and is compatible with a wide scope of starting materials. In addition, the new rearrangement found features a promising approach for the design of unique compound libraries for drug design and discovery programs.

C-4-Modified Isotetrones Prevent Biofilm Growth and Persister Cell Resuscitation in Mycobacterium smegmatis

Hyperphosphorylated nucleotide (p)ppGpp, synthesized by Rel protein, regulates the stringent response pathway responsible for biofilm and persister cell growth in mycobacteria. The discovery of vitamin C as an inhibitor of Rel protein activities raises the prospect of tetrone lactones to prevent such pathways. The closely related isotetrone lactone derivatives are identified herein as inhibitors of the above processes in a mycobacterium. Synthesis and biochemical evaluations show that an isotetrone possessing phenyl substituent at C-4 inhibit the biofilm formation at 400 μg mL^-1, 84 h post-exposure, followed by moderate inhibition by the isotetrone possessing the p-hydroxyphenyl substituent. The latter isotetrone inhibits the growth of persister cells at 400 μg mL^-1 f.c. when monitored for 2 weeks, under PBS starvation. Isotetrones also potentiate the inhibition of antibiotic-tolerant regrowth of cells by ciprofloxacin (0.75 μg mL^-1) and thus act as bioenhancers. Molecular dynamics studies show that isotetrone derivatives bind to the Rel_Msm protein more efficiently than vitamin C at a binding site possessing serine, threonine, lysine, and arginine.

Chitosan: An Overview of Its Properties and Applications

Chitosan has garnered much interest due to its properties and possible applications. Every year the number of publications and patents based on this polymer increase. Chitosan exhibits poor solubility in neutral and basic media, limiting its use in such conditions. Another serious obstacle is directly related to its natural origin. Chitosan is not a single polymer with a defined structure but a family of molecules with differences in their composition, size, and monomer distribution. These properties have a fundamental effect on the biological and technological performance of the polymer. Moreover, some of the biological properties claimed are discrete. In this review, we discuss how chitosan chemistry can solve the problems related to its poor solubility and can boost the polymer properties. We focus on some of the main biological properties of chitosan and the relationship with the physicochemical properties of the polymer. Then, we review two polymer applications related to green processes: the use of chitosan in the green synthesis of metallic nanoparticles and its use as support for biocatalysts. Finally, we briefly describe how making use of the technological properties of chitosan makes it possible to develop a variety of systems for drug delivery.

Cytotoxic Activity of Piperazin-2-One-Based Structures: Cyclic Imines, Lactams, Aminophosphonates, and Their Derivatives

N-Heterocycles are considered as desirable scaffolds for the development of novel lead compounds for anticancer drug research. Among them, phosphorus-containing amino-derivatives play a crucial role. A series of imines and products of their further reactions with P-nucleophiles were obtained starting from vicinal bisamines. Reaction of ethylenediamine and α-carbonyl esters yielded in novel unexpected products, which structures were confirmed by crystallographic measurements. The cytotoxic activity evaluation was done on a variety of cell lines including HUH7, AKH12, DAOY, UW228-2, D283, D425, and U251. Human umbilical vein endothelial cells (HUVECs) were used as control. Two of the tested compounds, bearing TADDOL-derived, and trifluoromethyl substituents showed a significant effect on cell viability, though comparable to nonmalignant cells.

An approach to new chiral bicyclic imines and amines via Horner-Wadsworth-Emmons reaction

New chiral bicyclic imines, enamines and amines were prepared via Horner–Wadsworth–Emmons reaction of hexahydroquinoxalin-2(1H)-one-derived phosphonate, as the source of a phosphonate carbanion, and a wide range of structurally diverse carbonyl substrates. The simplicity of the synthetic protocol, high selectivity, and broad substrate scope are the main advantages of the presented methodology.

New chiral cyclic imines and enamines were prepared via HWE reaction, with selectivity dependent on the carbonyl substrate.

Electrochemical decarboxylative C3 alkylation of quinoxalin-2(1H)-ones with N-hydroxyphthalimide esters

Electrochemical decarboxylative C3 alkylation of a wide range of quinoxalin-2(1H)-ones under metal- and additive-free conditions was reported.

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Pd(TFA)2-catalyzed direct arylation of quinoxalinones with arenes

Pd(TFA)₂-catalyzed cross-dehydrogenative coupling reaction for the direct C-3 arylation of quinoxalin-2-ones with arenes is described.

Visible-light induced decarboxylative alkylation of quinoxalin-2(1H)-ones at the C3-position

3-Alkylquinoxalin-2(1H)-ones are synthesized via direct alkylation of quinoxalin-2(1H)-ones at the C3 position under visible light irradiation.

Polymer-Supported Syntheses of Heterocycles Bearing Oxazine and Thiazine Scaffolds

In this review, we summarize synthetic approaches to preparing single or fused oxazine and thiazine derivatives using solid-phase synthesis (SPS). The literature survey revealed that diverse compounds bearing variously functionalized 1,2-oxazine, 1,3-oxazine, or 1,4-oxazine scaffolds and the corresponding thiazines are accessible by SPS. The latest contributions involving the stereoselective polymer-supported syntheses of morpholines indicate that the field is continuing to expand.

Visible-light-induced C(sp3)-H activation for a C-C bond forming reaction of 3,4-dihydroquinoxalin-2(1H)-one with nucleophiles using oxygen with a photoredox catalyst or under catalyst-free conditions

A convenient photocatalyzed oxidative coupling reaction of 4-alkyl-3,4-dihydroquinoxalin-2(1H)-one and its derivatives with a variety of nucleophiles was developed with a ruthenium photoredox catalyst and oxygen under a household compact fluorescent light. With a slower reaction rate, the cross coupling transformation can be achieved in the absence of an external photocatalyst with a similar isolated yield. An application to the synthesis of natural product cephalandole A was also demonstrated.

Cyclic imines – preparation and application in synthesis

Cyclic imines, available from various nitrogen-containing reactants, serve as versatile synthetic intermediates for biologically active compounds.

Copper-catalyzed direct coupling of benzoxazin-2-ones with indoles for the synthesis of diverse 3-indolylbenzoxazin-2-ones: access to natural cephalandole A

Diverse and functionalized 3-indolyl benzo[b][1,4]oxazin-2-ones were synthesized via copper-catalyzed direct coupling of benzo[b][1,4]oxazin-2-one and indoles in air.

Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones

Herein is reported a practical method for the construction of 3-benzoylquinoxalinones from benzodiazepinones in the presence of DMSO which serves both as a solvent and an oxidant.