Green oxidations: titanium dioxide induced tandem oxidation coupling reactions

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Comparison of antifungal and cytotoxicity activities of titanium dioxide and zinc oxide nanoparticles with amphotericin B against different Candida species: In vitro evaluation

BACKGROUND

Candida species are known to cause serious fungal infections that produce cutaneous, mucosal, and systemic infections. Nowadays, mortality and morbidity candidiasis in immunocompromised patients have increased. Nanotechnology is a new world-known technology and includes particles ranging from about 1 to 100 nanometers. The purpose of this study was to evaluate the antifungal and cytotoxicity activities of titanium dioxide nanoparticles (TiO2-NPs) and zinc oxide nanoparticles (ZnO-NPs) compared to amphotericin B (AmB) on different Candida spp in in vitro conditions.

METHODS

In the present study, susceptibility of different Candida species to TiO2-NPs and ZnO-NPs compared to AmB was determined by broth microdilution (BMD) and agar well diffusion methods. Cytotoxicity of TiO2-NPs and ZnO-NPs and amphotericin B was measured by MTT (3-(4, 5-Dimethylthiazol-2-yl)-2, 5-Diphenyltetrazolium Bromide) assay.

RESULTS

The results indicated that the TiO2-NPs and ZnO-NPs showed antifungal activities against pathogenic Candida spp. The minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of TiO2-NP ranges against Candida spp. were 128-256 µg/mL and 256-512 µg/mL, respectively. The MIC and MFC values of ZnO-NPs were 64-128 µg/mL and 256-512 µg/mL, respectively. However, MICs and MFCs of AmB were 8-16 µg/mL and 16-32 µg/mL, respectively. The MTT assay results showed that the CC50% belonged to ZnO-NPs 706.2 μg/mL, for TiO2-NPs 862.1 μg/mL, and for AmB 70.19 μg/mL, respectively.

CONCLUSION

Our findings showed that TiO2-NPs and ZnO-NPs had antifungal effects against all Candida species, yet the antifungal properties of TiO2-NPs and ZnO-NPs were significantly less than those of AmB. The CC50% of AmB was significantly lower than ZnO-NPs and TiO2-NPs.

PIDA-mediated intramolecular oxidative C-N bond formation for the direct synthesis of quinoxalines from enaminones

A intramolecular oxidative C(sp2)-N bond formation mediated by hypervalent iodine(iii) to obtain quinoxalines from readily available N-(2-acetaminophenyl)enaminones was developed. A tandem process involving PIDA-mediated intramolecular condensation cyclization and a subsequent elimination was postulated, which was highly efficient and metal-free under mild conditions. Moreover, flexible structural modifications of quinoxalines bearing carbonyl groups are of interest for further transformations as building blocks in organic synthesis.

Elemental sulfur mediated synthesis of benzoxazoles, benzothiazoles and quinoxalines via decarboxylative coupling of 2-hydroxy/mercapto/amino-anilines with cinnamic acids

A practical method has been developed for the synthesis of 2-benzylbenzoxazoles and 2-benzylbenzothiazoles using sulfur mediated decarboxylative coupling of cinnamic acids with 2-hydroxyanilines and 2-mercaptoanilines respectively.

I2 catalyzed tandem protocol for synthesis of quinoxalines via sp3, sp2 and sp C–H functionalization

An efficient metal-free tandem protocol for the synthesis of quinoxalines via sp³, sp² and sp C–H functionalization has been developed.

Synthesis and optical properties of phenanthromidazole derivatives for organic electroluminescent devices

The external quantum efficiency roll-off may be due to triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA).

Recent developments in one-pot tandem oxidation process coupling reactions

The recent additions to the tandem oxidation process (TOP) research area are reviewed.