Cu-Catalyzed Aerobic Oxidative Sulfuration/Annulation Approach to Thiazoles via Multiple Csp3-H Bond Cleavage

Electrochemical Enaminone-Thioamide Annulation and Thioamide Dimeric Annulation for the Tunable Synthesis of Thiazoles and 1,2,4-Thiadiazole

A green and sustainable electrochemical oxidative cyclization of enaminones with thioamides under metal- and oxidant-free conditions has been developed, providing an efficient approach for thiazole synthesis. Furthermore, 1,2,4-thiadiazoles can be selectively accessed via the electrochemical dimerization of thioamides in the absence of enaminones.

HFIP-Mediated Multicomponent Reactions: Synthesis of Pyrazole-Linked Thiazole Derivatives

The development of one-pot, atom, and step-economic new methods avoiding metal, harsh reaction conditions, and toxic reagents for the synthesis of medicinally important hybrid molecules bearing more than one bioactive moieties is currently one of the hot topics in organic synthesis. Herein, we report a green and efficient room temperature multicomponent reaction for the synthesis of novel pyrazole-linked thiazoles involving a one-pot C-C, C-N, and C-S bond-forming process from the reaction of aryl glyoxal, aryl thioamide, and pyrazolones in 1,1,1,3,3,3-hexafluoroisopropanol, a hydrogen bond donating reaction medium. A set of diverse hybrid molecules bearing thiazole and pyrazole moieties were prepared in good to excellent yields by using this method. This methodology can also be extended to prepare thiazole-linked barbiturates as well as imidazole-linked pyrazoles. All the products were fully characterized by spectroscopic techniques. The notable features of this protocol are room temperature, metal as well as additive-free reaction conditions, use of recyclable solvent, water as the byproduct, wide substrate scope, operational simplicity, easy purification, applicability for gram-scale synthesis, high atom economy, and the presence of two bioactive pyrazole and thiazole moieties in the products.

I2-catalyzed one-pot oxidative condensation of thiourea, methyl ketones, and aryl thiols into 5-sulfenylated 2-amino-1,3-thiazoles by DMSO

A one-pot, efficient oxidative-condensation process for constructing both 4-alkyl and 4-aryl-5-(arylthio) thiazol-2-amines using DMSO/I2 is introduced. In this procedure, methyl ketones, thiourea, DMSO, and thiols are reacted together in the presence of molecular I2 at 80 °C simply to produce 4-alkyl or aryl-5-(arylthio)thiazol-2-amines due to formation of a C-S bond between thiourea and methyl carbon linked to carbonyl group and the another C-S bond formation between thiol and thiazol ring. Under reaction conditions, both aryl and alkyl methyl ketones including acetophenone and substituted acetophenones also, 2-alkanones such as acetone, 2-butanone, 2-pentanone, and 2-heptanone yield those products successfully.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

Synthesis and Characterization of Novel 2-(1,2,3-Triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazoles and 2-(4,5-Dihydro-1H-pyrazol-1-yl)-4-(1H-1,2,3-triazol-4-yl)thiazoles

Reactions of 1-(5-methyl)-1H-1,2,3-triazol-4-yl)ethan-1-ones and benzaldehydes in ethanol under basic conditions gave the corresponding chalcones. Reactions of the chalcones combined with thiosemicarbazide in dry ethanol containing sodium hydroxide afforded the corresponding pyrazolin-N-thioamides. Reactions of the synthesized pyrazolin-N-thioamides and several ketones (namely, ethyl 2-chloro-3-oxobutanoate, 2-bromoacetylbenzofuran, and hydrazonoyl chloride) gave the corresponding novel 2-(1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazoles in high yields (77-90%). Additionally, 2-(4,5-dihydro-1H-pyrazol-1-yl)-4-(1H-1,2,3-triazol-4-yl)thiazoles were obtained in high yields (84-87%) from reactions with N-pyrazoline-thioamides and 4-bromoacetyl-1,2,3-triazoles under basic conditions. The structures of six of the newly synthesized heterocycles were confirmed by X-ray crystallography.

Tris(4-bromophenyl)aminium Hexachloroantimonate as a "Waste-Utilized"-Type Initiator-Promoted C-H Chlorination via C-H Activation Relay: Synthesis of Chlorinated Pyrroles

Using tris(4-bromophenyl)aminium hexachloroantimonate as a "waste-utilized"-type initiator, the aerobic oxidation of the sp³ C-H bond of proline esters was realized via C-H activation relay, giving a series of halogenated pyrroles in high yields. The mechanistic study revealed that the counterion, SbCl₆^-, was involved in the radical chlorination process, which provides a new way to understand the role of the counterions.

Synthesis of a Thiazole Library via an Iridium-Catalyzed Sulfur Ylide Insertion Reaction

A library of thiazoles and selenothiazoles were synthesized via Ir-catalyzed ylide insertion chemistry. This process is a functional group, particularly heterocycle-substituent tolerant. This was applied to the synthesis of fanetizole, an anti-inflammatory drug, and a thiazole-containing drug fragment that binds to the peptidyl-tRNA hydrolase (Pth) in Neisseria gonorrheae bacteria.

Multicomponent Synthesis of Fluorescent Thiazole-Indole Hybrids and Thiazole-Based Novel Polymers

Herein, we report an efficient multicomponent reaction for the synthesis of trisubstituted thiazoles involving a one-pot C-C, C-N, and C-S bond-forming process from the readily available starting materials. The reaction of arylglyoxal, indole, and aryl thioamides in the acetic acid medium under sealed heating conditions provided 3-(2,4-diarylthiazol-5-yl)-1H-indoles (4) in good to excellent yields. Using a similar reaction strategy, the reaction of arylglyoxal, aryl thioamide, and 2,5-dihydroxy-1,4-benzoquinone provided structurally interesting bis-thiazoles having dihydroxy-1,4-benzoquinone linker (9). All of the products were fully characterized by spectroscopic techniques. We also recorded single-crystal X-ray diffraction (XRD) of compounds 4b and 9a for unambiguous structure determination. Indole-linked trisubstituted thiazoles (4) exhibit prominent fluorescence properties. The relative fluorescence quantum yields of all of the thiazole-linked indoles were measured in the dimethyl sulfoxide (DMSO) medium with respect to quinine sulfate in 0.1 M H₂SO₄ as reference. The scope of this reaction was further explored by preparing novel polymers 11a and 11b using naphthalene/benzene-1,4-bis(carbothioamide) in multicomponent polymerization.

Regioselective synthesis of 2,5- and 4,5-disubstituted thiazoles via cyclization of 2-oxo-2-(amino)ethanedithioates with isocyanides

The regioselective synthesis of 2-(methylthio)-N-aryl/alkylthiazole-5-carboxamides and ethyl-5-(aryl/alkyl carbamoyl)thiazole-4-carboxylates by base induced cyclization of methyl-2-oxo-2-(amino)ethanedithioates with TosMIC and ethylisocyanoacetate respectively in high yield. The regioisomeric product was confirmed by X-ray diffraction...

Double C-S bond formation via multiple Csp3-H bond cleavage: synthesis of 4-hydroxythiazoles from amides and elemental sulfur under metal-free conditions

A novel and efficient approach for the synthesis of 4-hydroxythiazoles from amides and elemental sulfur has been developed. In the presence of P₂O₅, DMSO and HMPA, this metal-free protocol proceeds smoothly and tolerates a spectrum of functional groups. Furthermore, this strategy involves the process of double Csp³-S bond formation through the cleavage of multiple Csp³-H bonds for the first time.

Base-Catalyzed Three-Component Reaction between Chalcones, Isothiocyanates, and Sulfur: Access to Thiazole-2-thiones

A convenient synthesis of thiazole-2-thiones was developed based on base-catalyzed three-component reactions between chalcones, isothiocyanates, and elemental sulfur.

Synthesis and Structural Characterization of Isostructural 4-(4-Aryl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazoles

4-(4-Chlorophenyl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazole (4) and 4-(4-fluorophenyl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazole (5) have been synthesized in high yields. Crystallization of 4 and 5 from dimethylformamide solvent produced samples suitable for structure determination by single crystal diffraction. The materials are isostructural with triclinic, PĪ and symmetry and comprise two independent molecules in the asymmetric unit. The two independent molecules in the asymmetric unit assume similar conformation. The molecule is essentially planar apart from one of the two fluorophenyl groups, which is oriented roughly perpendicular to the plane of the rest of the molecule.

NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

Thiazole Ring-A Biologically Active Scaffold

BACKGROUND

Thiazole is a good pharmacophore nucleus due to its various pharmaceutical applications. Its derivatives have a wide range of biological activities such as antioxidant, analgesic, and antimicrobial including antibacterial, antifungal, antimalarial, anticancer, antiallergic, antihypertensive, anti-inflammatory, and antipsychotic. Indeed, the thiazole scaffold is contained in more than 18 FDA-approved drugs as well as in numerous experimental drugs.

OBJECTIVE

To summarize recent literature on the biological activities of thiazole ring-containing compounds Methods: A literature survey regarding the topics from the year 2015 up to now was carried out. Older publications were not included, since they were previously analyzed in available peer reviews.

RESULTS

Nearly 124 research articles were found, critically analyzed, and arranged regarding the synthesis and biological activities of thiazoles derivatives in the last 5 years.

Metal-Promoted Heterocyclization: A Heterosynthetic Approach to Face a Pandemic Crisis

The outbreak of SARS-CoV-2 has drastically changed our everyday life and the life of scientists from all over the world. In the last year, the scientific community has faced this worldwide threat using any tool available in order to find an effective response. The recent formulation, production, and ongoing administration of vaccines represent a starting point in the battle against SARS-CoV-2, but they cannot be the only aid available. In this regard, the use of drugs capable to mitigate and fight the virus is a crucial aspect of the pharmacological strategy. Among the plethora of approved drugs, a consistent element is a heterocyclic framework inside its skeleton. Heterocycles have played a pivotal role for decades in the pharmaceutical industry due to their high bioactivity derived from anticancer, antiviral, and anti-inflammatory capabilities. In this context, the development of new performing and sustainable synthetic strategies to obtain heterocyclic molecules has become a key focus of scientists. In this review, we present the recent trends in metal-promoted heterocyclization, and we focus our attention on the construction of heterocycles associated with the skeleton of drugs targeting SARS-CoV-2 coronavirus.

Recent advances in the copper-catalyzed aerobic Csp3-H oxidation strategy

The interplay between copper catalysts and molecular oxygen provides the opportunity to control the promiscuous catalytic behaviors in aerobic Csp3-H bond oxidations without using stoichiometric amounts of oxidants. This mini-review aims to cover the Cu-catalyzed aerobic benzylic and α-carbonyl Csp3-H oxidations and that of the carbon next to an amine group in the past five years. The development of tandem multicomponent reactions employing aerobic Csp3-H bond oxidations will be discussed to highlight the controlled catalyst behaviors and the catalyst interactions between multiple reaction components.

Oxidation of the inert sp3 C-H bonds of tetrahydroisoquinolines through C-H activation relay (CHAR): construction of functionalized isoquinolin-1-ones

A TBN/O₂-initiated oxidation of the relatively inert 3,4-C-H bonds of THIQs was accomplished, in which the existence of an α-phosphoric ester group is crucial to enable dioxygen trapping and intramolecular HAT (C-H activation relay, CHAR), realizing the synthesis of a series of isoquinolin-1-ones in high yields. The mechanistic study confirmed that the formation of the 3,4-double bond is mediated by the CHAR process. This work provides a new strategy to achieve remote C-H bond activation.

Metal-catalyzed C-S bond formation using sulfur surrogates

Sulfur-containing compounds are present in a wide range of biologically important natural products, drugs, catalysts, and ligands and they have wide applications in material chemistry. Transition metal-catalyzed C-S bond-forming reactions have successfully overcome the obstacles associated with traditional organosulfur compound syntheses such as stoichiometric use of metal-catalysts, catalyst-poisoning and harsh reaction conditions. One of the key demands in metal-catalyzed C-S bond-forming reactions is the use of an appropriate sulfur source due to its odor and availability. The unpleasant odor of many organic sulfur sources might be one of the reasons for the metal-catalyzed C-S bond-forming reactions being less explored compared to other metal-catalyzed C-heteroatom bond-forming reactions. Hence, employing an appropriate sulfur surrogate in the synthesis of organosulfur compounds in metal-catalyzed reactions is still of prime interest for chemists. This review explores the recent advances in C-S bond formation using transition metal-catalyzed cross-coupling reactions and C-H bond functionalization using diverse and commercially available sulfur surrogates. Based on the different transition metal-catalysts, this review has been divided into three major classes namely (1) palladium-catalyzed C-S bond formation, (2) copper-catalyzed C-S bond formation, and (3) other metal-catalyzed C-S bond formation. This review is further arranged based on the different sulfur surrogates. Also, this review provides an insight into the growing opportunities in the construction of complex organosulfur scaffolds covering natural product synthesis and functional materials.

Benign Synthesis of Fused-thiazoles with Enone-based Natural Products and Drugs for Lead Discovery

In an effort to synthesize a library of bioactive molecules, we present an efficient synthesis of fused-thiazole derivatives of natural products and approved drugs by using an environmentally usable solvent, acetic acid, and without any external reagent. Cholestenone, ethisterone, progesterone, and nootkatone-derived epoxyketones have been utilized to synthesize 50 novel compounds. The plausible mechanism of the reaction has been determined by theoretical calculation using M06-2X/6-31+G(d,p). These novel molecules have been tested against cancer cell lines and pathogenic bacterial strains. Several ethisterone-based fused-thiazole compounds are found to be potent growth inhibitors of cancer cell lines at submicromolar concentrations.

An Overview of the Synthesis and Antimicrobial, Antiprotozoal, and Antitumor Activity of Thiazole and Bisthiazole Derivatives

Thiazole, a five-membered heteroaromatic ring, is an important scaffold of a large number of synthetic compounds. Its diverse pharmacological activity is reflected in many clinically approved thiazole-containing molecules, with an extensive range of biological activities, such as antibacterial, antifungal, antiviral, antihelmintic, antitumor, and anti-inflammatory effects. Due to its significance in the field of medicinal chemistry, numerous biologically active thiazole and bisthiazole derivatives have been reported in the scientific literature. The current review provides an overview of different methods for the synthesis of thiazole and bisthiazole derivatives and describes various compounds bearing a thiazole and bisthiazole moiety possessing antibacterial, antifungal, antiprotozoal, and antitumor activity, encouraging further research on the discovery of thiazole-containing drugs.

Heterocyclization Reagents for Rapid Assembly of N-Fused Heteroarenes from Alkenes

N-Fused heterocycles are of particular use and upmost importance in multiple fields. Herein, we disclose a conceptually new approach for the rapid assembly of N-fused heteroarenes from alkenes. A portfolio of strategically designed heterocyclization reagents are readily prepared for the cascade reaction. A plethora of N-fused heteroarenes including seven types of heterocyclic core are furnished. The protocol features a broad functional-group compatibility and high product diversity, and provides a practical tool for late-stage heteroarene elaboration.

Elemental-Sulfur-Incorporated Cyclizations of Pyrrolidines Leading to Thienopyrroles

We report, herein, the synthesis of thieno[3,2-b]pyrroles from the direct oxidative [4 + 1] cyclization of 2-alkynyl pyrrolidines with elemental sulfur. This transformation likely originates from electrophilic attack at the β-position of pyrrolidine followed by an intramolecular thienannulation to deliver the desired product. Mechanistic investigation suggests that the present reaction involves the formation of dihydrothieno[3,2-b]pyrrole as an intermediate.

Recent Advances in Sulfur-Containing Heterocycle Formation via Direct C–H Sulfuration with Elemental Sulfur

The synthesis of sulfur heterocycles via the construction of C–S bonds has received considerable attention due to their biological value and extensive pharmaceutical application. While diverse sulfurating agents have been developed over the past few decades, in this regard, elemental sulfur, with advantages of low toxicity, odorless nature and chemical stability, has great potential for the construction of diverse sulfur heterocycles through its direct incorporation into the target molecules in a concise way. Direct functionalization of inert C–H bonds can shorten the number of reaction steps and minimize the amount of waste formed. Hence, heteroannulations via direct C–H sulfuration is considered to be an attractive strategy for the synthesis of sulfur heterocycles. In the last few years, a vast array of concise systems have been reported for the synthesis of some valuable sulfur heterocycles such as thiophenes, thienoindoles, thienothiazoles, thiazoles, benzothiazoles, and thiadiazoles through direct C–H sulfuration/annulations with elemental sulfur. These are discussed in detail in this review.

1 Introduction

2 Thiophenes

3 Thienoindoles

4 Thienothiazoles

5 Other Fused Thiophenes

6 Thiazoles

7 Benzothiazoles

8 Thiadiazoles

9 Others

10 Summary and Outlook

Rhodium(III)-Catalyzed Oxidative Annulation of 4-Aminoquinolines and Acrylate through Two Consecutive C(sp2)-H Activations

The C-H annulation of the five-position of quinolines and acrylates to afford heterocycles is an active field of research in organic synthesis. Herein the annulation of 4-aminoquinolines with acrylates through two consecutive C-H activations catalyzed by Rh(III) is described. The reaction proceeds with high atom efficiency under mild reaction conditions, and this protocol will provide appealing strategies for the synthesis of fused quinoline heterocycles.

Three-component synthesis of 1,4-benzothiazines via iodide-catalyzed aerobic C–H sulfuration with elemental sulfur

Five to Six: Beyond the well-established thiazole formation from elemental sulfur, this method provides the first access to the corresponding six-membered N,S-heterocyclic products via direct functionalization of multiple C–H bonds.

Homo-condensation of acetophenones toward imidazothiones

Direct synthesis of imidazothiones from simple, commercial substrates is not known. We report a method for the condensation of acetophenones, elemental sulfur, and ammonium acetate as a nitrogen source to obtain the hitherto challenging five-membered heterocycles. Functionalities such as halogen, trifluoromethyl, cyano, methylthio, and heteroaryl groups were well tolerated.

Synthesis of imidazothiones from acetophenones is reported for the first time. The use of a wide range of functionalities and heterocycles was well tolerated.

Brønsted acid-promoted thiazole synthesis under metal-free conditions using sulfur powder as the sulfur source

A Brønsted acid-promoted sulfuration/annulation reaction for the one-pot synthesis of bis-substituted thiazoles from benzylamines, acetophenones, and sulfur powder has been developed. One C–N bond and multi C–S bonds were selectively formed in one pot. The choice of the Brønsted acid was the key to the high efficiency of this transformation under metal-free conditions.

A Brønsted acid-promoted protocol for the synthesis of 2,4-disubstituted thiazoles from benzylamines, acetophenones, and sulfur powder under metal-free conditions is described.

Multicomponent synthesis of diphenyl-1,3-thiazole-barbituric acid hybrids and their fluorescence property studies

A series of novel diphenyl-1,3-thiazole linked barbituric acid hybrids (4) were prepared by two catalyst-free methods from readily available starting materials.