A simple and straightforward strategy for expedient access to benzoxazoles using chemically engineered 2D magnetic graphene oxide nanosheets as an eco-compatible catalyst

Two-dimensional (2D) graphene oxide nanosheets serve as an excellent support material for immobilizing metal complexes to deal with the drawbacks of homogeneous catalysis. In this work, we report a magnetically retrievable graphene oxide (MGO) based copper nanocatalytic system that has been efficiently exploited for obtaining a series of pharmaceutically and biologically active benzoxazole scaffolds. The nanocatalyst was designed by covalent immobilization of dehydroacetic acid (DHA) onto a magnetic amino-silanized graphene oxide nanosupport which was accompanied by its metallation with copper acetate. The structure of the synthesized MGO hybrid material (Cu@DHA@APTES@MGO) was characterized by numerous physico-chemical techniques such as transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), elemental mapping, atomic absorption spectroscopy (AAS), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) surface area analysis and energy-dispersive X-ray fluorescence spectroscopy (ED-XRF). The fabricated architectures exhibited high efficiency for cyclization of 2-aminophenols and β-diketones with wide substrate scope, excellent functional group tolerance, a higher conversion percentage (>98%) and a high turnover number (TON). The exceptional catalytic activity could be attributed to the 2D architecture of graphene oxide which provides space for trapping of reactants between 2D graphitic overlayers and metal surfaces and the reaction proceeds to afford benzoxazole products with moderate to excellent conversion percentages. Notably, this nanocomposite could be recovered easily through an external magnetic force and reused for multiple runs without any appreciable loss in its catalytic efficacy.

Cascade Transformation of the Ansamycin Benzoquinone Core into Benzoxazole Influencing Anticancer Activity and Selectivity

The metal-free cascade transformation of geldanamycin benzoquinone core is proposed at relatively mild conditions. This approach yields new benzoxazole ansamycin antibiotics and enables their functionalization in an atom-economic manner, irrespective of the type of amine used. The analysis of the heterocyclization course reveals the dependence of its rate on the nature of the para-substituent within the benzylamine moiety (EDG/EWG) and the strength of the base. The reduction of the ansamycin core enables an increase in anticancer potency and selectivity.

New iron(iii) complex of bis-bidentate-anchored diacyl resorcinol on a Fe3O4 nanomagnet: C-H bond oxygenation, oxidative cleavage of alkenes and benzoxazole synthesis

We have synthesized a novel, bis-bidentate, covalently anchored, 4,6-diacetyl resorcinol (DAR) ligand on silica-coated magnetic Fe₃O₄ nanoparticles and the corresponding bi-metallic iron(iii) complex (Fe₃O₄@SiO₂-APTESFe₂L^DAR). Both the chemical nature and the structure of the homogeneously heterogenized catalyst were investigated using physico-chemical techniques. The results obtained by XPS, XRD, FT-IR, TGA, VSM, SEM, TEM, EDX, ICP and AAS revealed a magnetic core, a silica layer and the grafting of a binuclear iron complex on the Fe₃O₄@SiO₂-APTES, as well as its thermodynamic stability. Despite many reports of metal complexes on different supports, there are no reports of anchored, bi-metallic complexes. To the best of our knowledge, this is the first report of a bi-active site catalyst covalently attached to a support. This study focuses on the catalytic activity of an as-synthesized, bi-active site catalyst for C-H bond oxygenation, the oxidative cleavage of alkenes, and the multicomponent, one-pot synthesis of benzoxazole derivatives with excellent yields from readily available starting materials. Our results indicated high conversion rates and selectivity under mild reaction conditions and simple separation using a magnetic field. The leaching and recyclability tests of the catalyst were investigated for the above processes, which indicated that all the reactions proceed via a heterogeneous pathway and that the catalyst is recyclable without any tangible loss in catalytic activity for at least 8, 5 and 5 cycles for C-H bond oxygenation, C[double bond, length as m-dash]C bond cleavage and benzoxazole synthesis, respectively.

One-pot synthesis of phosphorylnaphth[2,1-d]oxazoles and products as P,N-ligands in C-N and C-C formation

Phosphanylnaphtho[2,1-d]oxazoles were synthesized successfully through one-pot phosphonation of naphthoquinone with diaryl(alkyl)phosphine oxides and Cu-catalyzed oxidative condensation with imines, followed by methylation and reduction. Upon applying 4-phosphanylnaphtho[2,1-d]oxazole as a P,N-chelating ligand, Pd-catalyzed C-N formation of amines or nitrobenzene as well as Ni-catalyzed C-C formation and the synthesis of quinoline proceeded successfully. The reaction was facilely scaled up to give N-benzylaniline 15a in a gram scale synthesis. This research provided a facile and convenient protocol to synthesize phosphanylnaphtho[2,1-d]oxazoles, which could be applied as an efficient P,N-ligand in transition-metal-catalyzed C-N and C-C formation to produce the desired products in high yields with wide functional group tolerance under small catalyst loading, solvent-free conditions in many reactions.

Recent Advances in the Tandem Copper-Catalyzed Ullmann-Goldberg N-Arylation–Cyclization Strategies

N‒Aryl bond formation under copper catalysis has been playing a pivotal role and has been extensively used as a key step in the total syntheses of several therapeutic molecules. The...

Copper-catalyzed oxidative cyclization of glycine derivatives toward 2-substituted benzoxazoles

A novel and straightforward intramolecular cyclization of glycine derivatives to 2-substituted benzoxazoles through copper-catalyzed oxidative C-H/O-H cross-coupling was described. A variety of glycine derivatives involving short peptides underwent cross-dehydrogenative-coupling readily to afford diverse 2-substituted benzoxazoles. The synthetic method has the advantages of simple operation, broad substrate scope and mild reaction conditions, thus providing an alternative effective approach for benzoxazole construction.

Iron-catalyzed synthesis of benzoxazoles by oxidative coupling/cyclization of phenol derivatives with benzoyl aldehyde oximes

An iron-catalyzed oxidative coupling/cyclization reaction for the synthesis of benzoxazoles at room temperature is reported.

N-Heterocyclic carbene copper(i) complex-catalyzed synthesis of 2-aryl benzoxazoles and benzothiazoles

A new an efficient synthesis of 2-aryl benzoxazoles and benzothiazoles by intramolecular cyclization of 2-haloanilides/2-halothioanilides using a copper(i)-NHC complex is described.

Intramolecular oxidative deselenization of acylselenoureas: a facile synthesis of benzoxazole amides and carbonic anhydrase inhibitors

A mild, efficient and one pot procedure to access benzoxazoles using easily accessible acylselenoureas as starting materials has been discovered.