Synthesis Of Benzoxazoles Research Articles

Continuous flow/waste-minimized synthesis of benzoxazoles catalysed by heterogeneous manganese systems

Herein, we present our results on the development of a waste minimized protocol for the synthesis of 2-arylbenzoxazoles in continuous flow by combining safe solvent and heterogeneous manganese-based catalyst.

Oxidative NHC Catalysis for the Generation of Imidoyl Azoliums: Synthesis of Benzoxazoles.

N-Heterocyclic carbene (NHC)-catalyzed intramolecular cyclization of aldimines generated from 2-amino phenols and aromatic aldehydes leading to the synthesis of 2-arylbenzoxazoles under mild conditions is presented. The reaction proceeds via the generation of the aza-Breslow intermediates from imines and NHC, which under oxidative conditions form the key imidoyl azoliums and a subsequent intramolecular cyclization furnishes the product. The reaction tolerates a broad range of functional groups, and the products are formed in generally good yields.

A Facile, Efficient and Convenient One Pot Synthesis of Benzoxazoles from 1,2-diols and 2-aminophenols with Pb(OAc)4 Reagent

Background: In this work, a series of benzoxazole derivatives has been synthesized using 1,2-diols and 2-aminophenols in the presence of lead tetraacetate. This one pot cyclocondensation involves the breaking of a C-C bond and formation of C-O and C-N bonds in a single synthetic operation. Methods: The synthesis of benzoxazole derivatives is developed through the reaction of 1,2-diols and 2-aminophenols in the presence of lead tetraacetate in ethanol under reflux conditions. Results: A series of benzoxazole derivatives was synthesized in good yields (82-96%) under reflux for 2-3.5 hours in ethanol solvent. The structural assignments of these compounds were made on the basis of elemental analysis and spectroscopic data. Conclusion: This domino reaction implies oxidation, condensation, cyclization and dehydrogenation. This procedure is an alternative to the existing procedures for the synthesis of these compounds. Keywords: Benzoxazole, oxazole, lead tetraacetate, one pot synthesis, 1, 2-diols, ethanol solvent.

NH3(CH2)5NH3BiCl5: an efficient and recyclable catalyst for the synthesis of benzoxazole, benzimidazole and benzothiazole heterocycles

In this work, the condensation of aromatic aldehydes with different two-substituted aniline catalyzed by NH3(CH2)5NH3BiCl5 as heterogeneous and recyclable catalyst was reported. It was demonstrated that NH3(CH2)5NH3BiCl5 can act as an efficient and active catalyst and is reusable for six runs without a significant loss of their catalytic activity. Simple preparation of the catalyst, high catalytic activity and good reusability are noteworthy advantages of this catalytic system in the synthesis of benzoxazole, benzimidazole and benzothiazole heterocycles at room temperature under solvent-free conditions.

CuNiFe a Magnetic Nano‐Catalyst: an Efficient Catalyst for the Selective Synthesis of Benzoxazoles

AbstractAn efficient protocol for the synthesis of benzoxazoles from o‐nitrophenols and benzyl alcohols using CuNiFe as a magnetic nano‐catalyst via hydrogenation transfer has been described. The CuNiFe magnetic nano‐catalyst was prepared by an operationally simple method, without the use of any toxic reagents. The prepared catalyst was characterized by XRD, FEG‐SEM, EDAX, TEM, and FT‐IR techniques. No additional additives such as oxidants and reductants were required for this reaction. CuNiFe magnetic nano‐catalyst showed good air stability and could be recycled up to six times without any significant loss in its catalytic activity.

Unexpected Decarboxylation‐Triggered o‐Hydroxyl‐Controlled Redox Condensation of Phenylglycines with 2‐Nitrophenols in Aqueous Media

AbstractA decarboxylation‐triggered and o‐hydroxyl‐controlled hydrogen‐transfer strategy for the one‐pot synthesis of benzoxazoles from readily available amino acids and 2‐nitrophenols is reported. On the basis of this autoredox reaction, the C−N bond can be efficiently constructed to afford the desired products in moderate to good yields under transition‐metal‐free conditions in aqueous media.magnified image

Synthesis and characterization of amino glucose-functionalized silica-coated NiFe2O4 nanoparticles: A heterogeneous, new and magnetically separable catalyst for the solvent-free synthesis of 2,4,5–trisubstituted imidazoles, benzo[d]imidazoles, benzo[d] oxazoles and azo-linked benzo[d]oxazoles

Amino glucose-functionalized silica-coated NiFe2O4 nanoparticles (NiFe2O4@SiO2@amino glucose) were chemically synthesized and characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), energy dispersive X-ray analysis (EDX), vibrating sample magnetometer (VSM), Zetasizer and Fourier transform infrared spectroscopy (FT-IR) instruments. NiFe2O4@SiO2@amino glucose supply an environmentally friendly procedure for the synthesis of 2,4,5-trisubstituted imidazoles through one-pot multicomponent condensation of benzil or benzoin, ammonium acetate with aryl aldehydes and for the synthesis of benzoxazoles using condensation reaction of 2-aminophenol with aryl aldehydes under solvent free condition. In the other study, this synthesized magnetically reusable catalyst was introduced as a new avenue for the synthesis of benzo[d]imidazoles using the reaction between aryl aldehydes and 1,2-diaminobenzene. These compounds were obtained in high yields and short reaction times. The catalyst could be easily recovered and reused for five cycles with almost consistent activity. Synthesized compounds were characterized by their physical constant, comparison with authentic samples, FT-IR, 1H NMR, 13C NMR spectroscopy and elemental analysis.

Palladium(II) Immobilized Onto the Glucose Functionalized Magnetic Nanoparticle as a New and Efficient Catalyst for the One‐pot Synthesis of Benzoxazoles

Palladium(II) have been immobilized into the nano magnetic Fe3O4 which was functionalized with glucose in order to achieve a one‐pot synthesis of 2‐substituted benzoxazole derivatives with high yields in the diverse range of organic solvents. The nano catalyst is highly dispersive in polar solvents and can be easily recovered and reused for 6 runs without significant loss of its activity. Finally, the catalyst was fully characterized by FT‐IR, TGA, CHN, SEM, EDX and atomic absorption spectroscopy.

Synthesis and characterization of a novel oxo-bridged binuclear iron(iii) complex: its catalytic application in the synthesis of benzoxazoles using benzyl alcohol in water

A novel oxo-bridged binuclear iron(iii) complex was found to be an efficient catalyst in the synthesis of benzoxazoles from benzyl alcohols.

Synthesis of benzoxazoles via an iron-catalyzed domino C-N/C-O cross-coupling reaction.

An eco-friendly and efficient method has been developed for the synthesis of 2-arylbenzoxazoles via a domino iron-catalyzed C–N/C–O cross-coupling reaction. Some of the issues typically encountered during the synthesis of 2-arylbenzoxazoles in the presence of palladium and copper catalysts, including poor substrate scope and long reaction times have been addressed using this newly developed iron-catalyzed method.

A novel approach to sesquiterpenoid benzoxazole synthesis from marine sponges: nakijinols A, B and E-G.

Nakijinols A, B and analogues E through G, which are structurally unique and biologically significant sesquiterpenoid benzoxazoles, can be efficiently obtained in a highly unified manner from the sesquiterpenoid quinone, smenospongine. The starting material is accessible from the (+)-5-methyl Wieland-Miescher ketone. The synthetic method features strategic construction of the requisite dihydroxylated benzoxazole substructure via the ring closure of the N-(2-hydroxyphenyl)-formamide or -acetamide moiety. The synthesis of nakijinols is reported here for the first time. The absolute configurations of nakijinols A and E were also established.

OMS-2/H2O2/Dimethyl Carbonate: An Environmentally-Friendly Heterogeneous Catalytic System for the Oxidative Synthesis of Benzoxazoles at Room Temperature

A manganese octahedral molecular sieve (OMS-2) was found to be an efficient and recyclable heterogeneous catalyst for the oxidative synthesis of benzoxazoles in the gram-scale from phenolic imines at room temperature. H2O2 and biobased reagent dimethyl carbonate (DMC) were successfully employed as the environmentally friendly oxidant and solvent, respectively, in an OMS-2-catalysted redox reaction for the first time. Benzoxazoles could also be obtained from N-substituted 2-aminophenols via Cu(OH)x/OMS-2-catalyzed sequential oxidative transformation at elevated temperatures.

Copper catalyzed synthesis of benzoxazoles and benzothiazoles via tandem manner

A useful protocol for the preparation of substituted 2-aminobenzoxazoles and 2-aminobenzothiazoles was presented. Under the catalysis of copper, 2-aminophenols or 2-aminothiophenols reacted with thiocarbamoyl chlorides via a tandem manner, furnishing a series of 17 benzoheterocycles smoothly with good to excellent yields (70–91%). The broad substrate scope, short reaction time, mild react conditions, easy performance and nice yields make this approach attractive, showing its practical synthetic value for the preparation of some biologically or pharmaceutically active compounds.

Synthesis of Benzoxazoles Using Electrochemically Generated Hypervalent Iodine.

The indirect ("ex-cell") electrochemical synthesis of benzoxazoles from imines using a redox mediator based on the iodine(I)/iodine(III) redox couple is reported. Tethering the redox-active iodophenyl subunit to a tetra-alkylammonium moiety allowed for anodic oxidation to be performed without supporting electrolyte. The mediator salt can be easily recovered and reused. Our "ex-cell" approach toward the electrosynthesis of benzoxazoles is compatible with a range of redox-sensitive functional groups. An unprecedented concerted reductive elimination mechanism for benzoxazole formation is proposed on the basis of control experiments and DFT calculations.

OMS-2-Supported Cu Hydroxide-Catalyzed Benzoxazoles Synthesis from Catechols and Amines via Domino Oxidation Process at Room Temperature.

In the presence of manganese oxide octahedral molecular sieve (OMS-2) supported copper hydroxide Cu(OH)x/OMS-2, aerobic synthesis of benzoxazoles from catechols and amines via domino oxidation/cyclization at room temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the efficient oxidation of catechols over Cu(OH)x/OMS-2 tolerates a variety of substrates, especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl, ester, and even acetyl groups) and heterocycles, which affords functionalized benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol % Cu). The characterization and plausible catalytic mechanism of Cu(OH)x/OMS-2 are described. The notable features of our catalytic protocol such as the use of air as the benign oxidant and EtOH as the solvent, mild conditions, ease of product separation, being scalable up to the gram level, and superior reusability of catalyst (up to 10 cycles) make it more practical and environmentally friendly for organic synthesis.

Synthesis of benzoxazole derivatives as interleukin-6 antagonists

A growing number of studies have demonstrated that interleukin (IL)-6 plays pathological roles in the development of chronic inflammatory disease and autoimmune disease by activating innate immune cells and by stimulating adaptive inflammatory T cells. So, suppression of IL-6 function may be beneficial for prevention and treatment of chronic inflammatory disease. This study reports that a series of synthetic derivatives of benzoxazole have suppressive effects on IL-6-mediated signaling. Among 16 synthetic derivatives of benzoxazole, the compounds 4, 6, 11, 15, 17, and 19 showed a strong suppressive activity against IL-6-induced phosphorylation of signal transducer and activator of transcription (STAT) 3 by 80–90%. While the cell viability was strongly decreased by compounds 11, 17, 19, the compounds 4, 6, and 15 revealed less cytotoxicity. We then examined the effects of the compounds on inflammatory cytokine production by CD4+ T cells. CD4+ T cells were induced to differentiate into interferon (IFN)-γ-, IL-17-, or IL-4-producing effector T cells in the presence of either the compound 4 or the compound 7. While the inactive compound 7 had no significant effect on the cytokine production by effector T cells, the active compound 4 strongly suppressed the production of inflammatory cytokines IFN-γ and IL-17, and also inhibited allergic inflammatory cytokines IL-4, IL-5, and IL-13 produced by effector Th2 cells. These results suggest that a benzoxazole derivative, compound 4 effectively suppresses IL-6-STAT3 signaling and inflammatory cytokine production by T cells and provides a beneficial effect for treating chronic inflammatory and autoimmune disease.

Unified Synthesis of 1,2-Oxy-aminoarenes via a Bio-inspired Phenol-Amine Coupling

Summary 1,2-Aminophenols and their related nitrogen-containing heterocycles are ubiquitous in medicine, agriculture, and materials science. Here, we describe a bio-inspired and unified approach for their synthesis through a direct, catalytic aerobic coupling of phenols and amines. Key to the formation of the aromatic C–N bond is a facile condensation between the amine and an ortho -quinone, which triggers redox isomerization to an ortho -iminophenol before a series of downstream events. We discuss the factors that govern regioselectivity during aerobic ortho -oxygenation of the phenol and condensation of the amine, and we demonstrate synthetic utility by preparing a structurally diverse family of immunosuppressive heterocycles and the indole alkaloid cephalandole A. Our conditions involve a wide range of simple, un-activated substrates, occur at room temperature with an earth-abundant copper-catalyst and a commercially available diamine ligand, and produce water as the only stoichiometric by-product.

Microwave-Assisted Synthesis of Benzoxazoles in Glycerol

Microwave-Assisted Synthesis of Benzoxazoles in Glycerol

TiO2ZrO2 composite: Synthesis, characterization and application as a facile, expeditious and recyclable catalyst for the synthesis of 2-aryl substituted benzoxazole derivatives

The activity of an efficient mesoporous TiO2ZrO2 composite catalyst for the synthesis of 2-aryl substituted benzoxazole derivatives using 2-aminophenol and substituted benzaldehydes/heterocyclic aldehydes at moderate temperature was studied. The catalyst was prepared by a co-precipitation method and characterized by X-ray diffraction, BET surface area and scanning electron microscopy. The effect of temperature, solvents and catalyst concentration on the synthesis of benzoxazole derivatives was systematically investigated. Short reaction times, green-reaction profiles, good to excellent yields, reliable cost efficiency, simple workup conditions and reusability of an eco-friendly catalyst are the noteworthy highlights of the reported method. The catalyst could be easily recovered and reused several times without any significant loss in the yield. The use of the present catalytic system to mediate the title chemical synthesis in a synthetic operation is important for the development of new atom-economic strategies and this is efficient in building complex structures from simple starting materials in an environmentally benign fashion.

A Decade Update on Benzoxazoles, a Privileged Scaffold in ­Synthetic Organic Chemistry

In 2014, more than 80% of the marketed drugs in the US contained heterocyclic compounds, which play a major role in the drug-discovery process, as one of the primary components. Benzoxazole, consisting of an oxazole ring fused with a benzene ring, has shown profound impact in drug discovery research owing to its important bioactivity. This moiety has also shown important properties in material science. The synthesis of this privileged scaffold from readily available chemicals remains a primary focus for the synthetic chemistry community. In this review, efforts have been made to focus on the latest information available on the different synthetic strategies such as solution phase, multicomponent, green and solid phase chemistry and applications of different benzoxazole derivatives. Furthermore, the updated synthetic information incorporated in this review article will help to improve future synthesis of this scaffold. 1 Introduction 2 Solution-Phase Synthetic Approach to Substituted Benzoxazoles 2.1 Condensation of o-Aminophenol with Carboxylic Acids 2.2 Condensation of o-Aminophenol with Aldehydes 2.3 Condensation of o-Aminophenol with Orthoesters, ortho-Haloamides, or 1,1-Dihaloalkenes 2.4 Synthesis of Benzoxazoles from Schiff Bases 2.5 Miscellaneous 3 Multicomponent Synthetic Approach to Substituted Benzoxa­zoles 4 Solid-Phase Synthetic Approach to Substituted Benzoxazoles 5 Green Synthetic Approach to Substituted Benzoxazoles 5.1 Condensation of o-Aminophenol with Aldehydes 5.2 Condensation of o-Aminophenol with ortho-Substituted Benz­amides 5.3 Condensation of o-Aminophenol with Carboxylic Acid and its Derivatives 5.4 Condensation of o-Aminophenol with Alcohols or Amines 5.5 Miscellaneous 6 Bioactive Derivatives of Benzoxazoles 7 Conclusion and Outlook