Exploring novel aryl/heteroaryl-isosteres of phenylthiazole against multidrug-resistant bacteria

Antimicrobial resistance has become a concern as a worldwide threat. A novel scaffold of phenylthiazoles was recently evaluated against multidrug-resistant Staphylococci to control the emergence and spread of antimicrobial resistance, showing good results. Several structural modifications are needed based on the structure-activity relationships (SARs) of this new antibiotic class. Previous studies revealed the existence of two key structural features essential for the antibacterial activity, the guanidine head and lipophilic tail. In this study, a new series of twenty-three phenylthiazole derivatives were synthesized utilizing the Suzuki coupling reaction to explore the lipophilic part. The in vitro antibacterial activity was evaluated against a range of clinical isolates. The three most promising compounds, 7d, 15d and 17d, with potent MIC values against MRSA USA300 were selected for further antimicrobial evaluation. The tested compounds exhibited potent results against the tested MSSA, MRSA, and VRSA strains (concentration: 0.5 to 4 μg mL^-1). Compound 15d inhibited MRSA USA400 at a concentration of 0.5 μg mL^-1 (one-fold more potent than vancomycin) and showed low MIC values against ten clinical isolates, including linezolid-resistant strain MRSA NRS119 and three vancomycin-resistant isolates VRSA 9/10/12. Moreover, compound 15d retained its potent antibacterial activity using the in vivo model by the burden reduction of MRSA USA300 in skin-infected mice. The tested compounds also showed good toxicity profiles and were found to be highly tolerable to Caco-2 cells at concentrations of up to 16 μg mL^-1, with 100% of the cells remaining viable.

Visible-light-driven reactions for the synthesis of sulfur dioxide-inserted compounds: generation of S-F, S-O, and S-N bonds

Sulfur dioxide-containing compounds such as sulfonyl fluorides, sulfonyl esters, and sulfonyl amides are important structural frameworks in many natural products, pharmaceuticals, and organic compounds. Thus, synthesis of these molecules is a very valuable research topic in organic chemistry. Various synthetic methods to introduce SO₂ groups into the structure of organic compounds have been developed for the synthesis of biologically and pharmaceutically useful compounds. Recently, visible-light-driven reactions were carried out to create SO₂-X (X = F, O, N) bonds, and their effective synthetic approaches were demonstrated. In this review, we summarized recent advances in visible-light-mediated synthetic strategies for generation of SO₂-X (X = F, O, N) bonds for various synthetic applications along with proposed reaction mechanisms.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Photochemical C(sp2 )-H Pyridination via Arene-Pyridinium Electron Donor-Acceptor Complexes

This report describes the development of a photochemical method for C(sp² )-H pyridination that leverages the photoexcitation of electron donor-acceptor (EDA) complexes. Experimental and DFT studies show that black light (λ_max ≈350 nm) irradiation of solutions of protonated pyridines (acceptors) and aromatic C-H substrates (donors) results in single electron transfer to form aryl radical cation intermediates that can be trapped with pyridine nucleophiles under aerobic conditions. With some modification of the reaction conditions, this EDA activation mode is also effective for promoting the oxidatively triggered S_N Ar pyridination of aryl halides. Overall, this report represents an inexpensive and atom-economical approach to photochemical pyridination reactions that eliminates the requirement of an exogenous photocatalyst.

Copper-Catalyzed Aminoarylation of Alkenes via Aminyl Radical Addition and Aryl Migration

We describe a new strategy for aminoarylation of alkenes by copper-catalyzed smiles rearrangement using O-benzoylhydroxylamines as the amine reagent. This method affords various β-amino amide derivatives possessing a quaternary carbon center with wide functional group tolerance and high regioselectivity. The mechanistic studies indicate that the transformation can involve aminyl radical intermediates under acid-free condition.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Visible Light as the Key for the Formation of Carbon–Sulfur Bonds in Sulfones, Thioethers, and Sulfonamides: An Update

This review summarizes the most relevant advancements made in the photocatalyzed synthesis of sulfones, thioethers, and sulfonamides from 2017 to the beginning of 2021. Synthetic strategies towards the construction of sulfur–carbon bonds are discussed together with the proposed reaction mechanisms. Interestingly, sulfur-based functional groups, which are of fundamental importance for the pharmaceutical field, can be assembled by photocatalysis in an easy and straightforward way under milder reaction conditions employing less toxic and expensive sulfur sources in comparison with common strategies.

1 Introduction

2 Sulfones

2.1 Sodium Sulfinates and Sulfinic Acids

2.2 Sulfonyl Halides

2.3 Sulfonyl Hydrazones

2.4 Sulfur Dioxide Surrogates

2.5 Miscellaneous

3 Thioethers

4 Sulfonamides

5 Conclusions

Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

Recent advancements in the development of molecular organic photocatalysts

Research in the development of molecular organic photocatalysts for applications in chemical syntheses has burgeoned in recent years. While organic photosensitizers have been known for over a century, tuning the properties of these molecules to increase photocatalytic efficiencies is now of growing importance. The properties that help improve the performance of organic photocatalysts include: a wider range of redox potentials, increased molar absorptivity (ε) in the visible spectrum, increased quantum yields (Φ), long-lived excited-state lifetimes (ns to μs), and increased chemical stability. This review examines some of the recent advancements in the development of molecular organic photocatalysts, specifically cyanoarenes, acridinium dyes, phenazines, thiazines, oxazines, and xanthenes, with respect to these properties and examines the chemical synthesis routes now achieved by organic photocatalysts.

A Mild Photocatalytic Synthesis of Guanidine from Thiourea under Visible Light

In this work, we developed the catalytic guanylation of thiourea using Ru(bpy)₃Cl₂ as a photocatalyst under irradiation by visible light. The conversion of various thioureas to the corresponding guanidines was achieved using 1-5 mol % of photocatalyst in a mixture of water and ethanol at room temperature. Key benefits of this reaction include the use of photoredox catalyst, low-toxicity solvents/base, ambient temperature, and an open-flask environment.

Recent progress in (hetero)arene cation radical-based heteroarene modification

The transformation of (hetero)arene cation radicals has become a powerful tool for the construction of highly functionalized (hetero)arenes. These (hetero)arene cation radicals could be generated under electrochemical, photochemical or chemical oxidation systems. The in situ generated (hetero)arene cation radicals can be attacked by various nucleophiles, such as (hetero)aromatics and anions, yielding structurally diverse molecules. Recently, a large number of impressive heteroarene modifications have been designed by this strategy. This review summarizes the advances in heteroarene modification via reactions of in situ formed (hetero)arene cation radicals, ranging from 2010 to 2020.

Electrochemical oxidation induced intermolecular aromatic C-H imidation

The dehydrogenative aryl C-H/N-H cross-coupling is a powerful synthetic methodology to install nitrogen functionalities into aromatic compounds. Herein, we report an electrochemical oxidation induced intermolecular cross-coupling between aromatics and sulfonimides with high regioselectivity through N-radical addition pathway under external-oxidant-free and catalyst-free conditions. A wide variety of arenes, heteroarenes, alkenes and sulfonimides are applicable scaffolds in this transformation. In addition, aryl sulfonamides or amines (aniline derivatives) can be obtained through different deprotection process. The cyclic voltammetry mechanistic study indicates that the N-centered imidyl radicals are generated via proton-coupled electron transfer event jointly mediated by tetrabutylammonium acetate and anode oxidation process.

The dehydrogenative C-H/N-H cross-coupling serves to install nitrogen functionalities into arenes with the highest atom economy. Here, the authors report an electrochemical cross-coupling between aromatics and sulfonimides through an N-radical addition pathway under oxidant- and catalyst-free conditions.

Oxidative sulfamidation as a route to N-heterocycles and unsaturated sulfonamides

Oxidative sulfamidation is a promising approach to the synthesis of numerous organic compounds, including N-heterocycles or unsaturated species having the sulfonamide group, which is a key structural motif of synthetic antimicrobial drugs. The formed products can undergo further reactions leading to a wide variety of functionalized sulfonamides. This review summarizes the current state of knowledge on the oxidative reactions of sulfonamides and their derivatives with unsaturated and CH-active compounds with an emphasis on dienes as substrates. This produces a diverse range of compounds possessing sulfonamide function and capable of further functionalization.

Applications of organocatalysed visible-light photoredox reactions for medicinal chemistry

The focus of this review is to provide an overview of the field of organocatalysed photoredox chemistry relevant to synthetic medicinal chemistry. Photoredox transformations have been shown to enable key transformations that are important to the pharmaceutical industry. This type of chemistry has also demonstrated a high degree of sustainability, especially when organic dyes can be employed in place of often toxic and environmentally damaging transition metals. The sections are arranged according to the general class of the presented reactions and the value of these methods to medicinal chemistry is considered. An overview of the general characteristics of the photocatalysts as well as some electrochemical data is presented. In addition, the general reaction mechanisms for organocatalysed photoredox transformations are discussed and some individual mechanistic considerations are highlighted in the text when appropriate.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Photoorganocatalysis, small organic molecules and light in the service of organic synthesis: the awakening of a sleeping giant

Photocatalysis, the use of light to promote organic transformations, is a field of catalysis that has received limited attention despite existing for over 100 years. With the revolution of photoredox catalysis in 2008, the rebirth or awakening of the field of photoorganocatalysis has brought new ideas and reactions to organic synthesis. This review will focus on the sudden outburst of literature regarding the use of small organic molecules as photocatalysts after 2013. In particular, it will focus on acridinium salts, benzophenones, pyrylium salts, thioxanthone derivatives, phenylglyoxylic acid, BODIPYs, flavin derivatives, and classes of organic molecules as catalysts for the photocatalytic generation of C-C and C-X bonds.

Recent advances in radical-based C–N bond formation via photo-/electrochemistry

This review highlights the recent advances in cross-dehydrogenative amination for C–N bond construction from C–H/N–H cross-coupling partners through photocatalytic and/or electronic techniques.

Synthesis of Arylamines via Aminium Radicals

Arylamines constitute the core structure of many therapeutic agents, agrochemicals, and organic materials. The development of methods for the efficient and selective construction of these structural motifs from simple building blocks is desirable but still challenging. We demonstrate that protonated electron-poor O-aryl hydroxylamines give aminium radicals in the presence of Ru(bpy)3 Cl2 . These highly electrophilic species undergo polarized radical addition to aromatic compounds in high yield and selectivity. We successfully applied this method to the late-stage modification of chiral catalyst templates, therapeutic agents, and natural products.

Direct sulfonylation of anilines mediated by visible light

Visible light photocatalysis allows the introduction of the sulfone functional group to anilines under mild reaction conditions, without the need for pre-functionalization.

Sulfones feature prominently in biologically active molecules and are key functional groups for organic synthesis. We report a mild, photoredox-catalyzed reaction for sulfonylation of aniline derivatives with sulfinate salts, and demonstrate the utility of the method by the late-stage functionalization of drugs. Key features of the method are the straightforward generation of sulfonyl radicals from bench-stable sulfinate salts and the use of simple aniline derivatives as convenient readily available coupling partners.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Visible light-induced C3-sulfonamidation of imidazopyridines with sulfamides

A visible light-induced regioselective sulfonamidation of imidazo[1,2-a]pyridines was developed using sulfamides as the nitrogen sources and aqueous NaClO solution as the oxidant under mild conditions.

Visible-light-mediated C2-amination of thiophenes by using DDQ as an organophotocatalyst

A direct C–H amination of thiophenes was presented via an oxidation pathway under visible-light irradiation, in which the thiophene radical cation serves as the key intermediate. Various thiophenes and azoles could be transformed into the corresponding amination products well, and H₂O was the only byproduct, which is environmentally benign.