Modifying the Oxidative Potentials of Imines in a Dye Loaded Capsule for Photocatalytic Cyclization with Hydrogen Evolution

Modifying redox potential of substrates and intermediates to balance pairs of redox steps are important stages for multistep photosynthesis but faced marked challenges. Through co-clathration of iridium photosensitizer and imine substrate within one packet of a metal-organic capsule to shift the redox potentials of substrate, herein, we reported a multiphoton enzymatic strategy for the generation of heterocycles by intramolecular C-X hydrogen evolution cross-couplings. The cage facilitated a pre-equilibrium substrate-involving clathrate that cathodic shifts the oxidation potential of the substrate-dye-host ternary complex and configuration inversion of substrate via spatial constraints in the confined space. The new two photon excitation strategy enabled the precise control of the multistep electron transfer between each pair (photosensitizer, substrate and the capsule), endowing the catalytic system proceeding smoothly with an enzymatic fashion. Three kinds of 2-subsituted (-OH, -NH2 , and -SH) imines and N-aryl enamines all give the corresponding cyclization products efficiently under visible light irradiation, demonstrating the promising of the microenvironment driven thermodynamic activation in the host-dye-substrate ternary for synergistic combination of multistep photocatalytic transformations.

Application of Vitreoscilla Hemoglobin as a Green and Efficient Biocatalyst for the Synthesis of Benzoxazoles in Water

We report an efficient and eco-friendly method for the Vitreoscilla hemoglobin (VHb)-catalyzed synthesis of benzoxazoles in water at room temperature. tert-Butyl hydroperoxide and 2,2,6,6-tetramethyl-1-piperidinyloxy were used as oxidant and radical scavenger, respectively. A total of 27 functionally diverse benzoxazoles were prepared in moderate to high yields (62 %-94 %) by the annulation reaction of phenols with amines in the presence of VHb in 1 h. Thus, this method is highly viable for practical applications. This work broadens the application of hemoglobin to organic synthesis.

Design, synthesis and in vitro antiproliferation activity of some 2-aryl and -heteroaryl benzoxazole derivatives

Background: Phortress produces reactive electrophilic metabolites that form DNA adducts only in sensitive tumor cells. The authors converted the 2-phenylbenzothiazole nucleus in phortress to 2-aryl and -heteroaryl benzoxazole derivatives (11 new and 14 resynthesized). All synthesized compounds were studied for antitumor activity in various cancer cells. Materials & methods: Cytotoxicity, cell morphology, flow cytometry and cell-cycle analyses of compounds were performed and more active derivatives were tested in the MCF-7 cell line. Conclusion: Methyl 2-(thiophen-2-yl)benzo[d]oxazole-6-carboxylate (BK89) has a higher effect than fluorouracil to induce apoptotic cell death (apoptosis value of 49.44%). Cell-cycle analysis shows that the compounds BK89 and methyl 2-(furan-2-yl)benzo[d]oxazole-6-carboxylate (BK82) can be used as potential cell-cycle blockers by arresting MCF-7 cells in G0/G1 phase at rates of 63% and 85%, respectively.

One-Pot Multicomponent Reaction of Catechols, Ammonium Acetate, and Aldehydes for the Synthesis of Benzoxazole Derivatives Using the Fe(III)-Salen Complex

The Fe(III)-salen complex has been applied successfully as a catalyst for the novel, simple, efficient, and one-pot multicomponent synthesis of benzoxazole derivatives from catechols, ammonium acetate as the nitrogen source, and aldehydes (nontoxic and cheap alternatives of amines) for the first time. Using this procedure, a wide range of benzoxazoles was successfully synthesized in the presence of a catalyst in EtOH under mild conditions, and all products were obtained in excellent yields. To the best of our knowledge, this method is the first example of the multicomponent synthesis of benzoxazole derivatives using these starting materials. The notable features such as the use of air that is considered as a benign oxidant and EtOH as a green solvent, ease of product separation, readily available and inexpensive aldehydes, and mild conditions make our procedure more efficient and practical for organic synthesis. Moreover, the current protocol is successfully applied to synthesize desirable products on a large scale.

Facile synthesis of 2-alkynyl oxazoles via a Ce(OTf)3-catalyzed cascade reaction of alkynyl carboxylic acids with tert-butyl isocyanide

We developed an efficient and novel protocol to synthesize 2-alkynyl oxazoles from tert-butyl isocyanide and alkynyl carboxylic acids. This method allowed the synthesis of diversely functionalized oxazoles under mild reaction conditions, coupled with operational simplicity and these functionalized oxazoles showed a certain degree of biological activity. Moreover, compounds 2b, 2h, 2k, 2n, 2p and 2t exhibited good anticancer activities in human gastric cancer cells (MGC803) and human bladder tumor cells (T24), with IC₅₀ below 20.0 μM.

Efficient synthesis of NIR emitting bis[2-(2'-hydroxylphenyl)benzoxazole] derivative and its potential for imaging applications

Unassymetric bis[2-(2'-hydroxyphenylbenzoxole)] bis(HBO) derivatives with a DPA functionality for zinc binding have been developed with an efficient synthetic route, using the retrosynthetic analysis. Comparison of bis(HBO) derivatives with different substitution patterns allows us to verify and optimize their unique fluorescence properties. Upon binding zinc cation, bis(HBO) derivatives give a large fluorescence turn-on in both visible (λ_em ≈ 536 nm) and near-infrared (NIR) window (λ_em ≈ 746 nm). The probes are readily excitable by a 488 nm laser, making this series of compounds a suitable imaging tool for in vitro and in vivo study on a confocal microscope. The application of zinc binding-induced fluorescence turn-on is successfully demonstrated in cellular environments and thrombus imaging.

Thiol substrate-promoted dehydrogenative cyclization of arylmethyl thiols with ortho-substituted amines: a universal approach to heteroaromatic compounds

A thiol substrate-promoted one-pot synthesis of a library of heteroaromatic compounds was developed.

Visible light photoredox catalysis: conversion of a mixture of thiophenols and nitriles into 2-substituted benzothiazoles via consecutive C–S and C–N bond formation reactions

A novel, visible-light-mediated method for the synthesis of 2-substituted benzothiazoles from thiophenols and alkyl/aryl nitriles in the presence of eosin Y under air atmosphere is disclosed.

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.

Implications of dynamic imine chemistry for the sustainable synthesis of nitrogen heterocycles via transimination followed by intramolecular cyclisation

An application of dynamic imine chemistry to the sustainable synthesis of N-heterocycles from N-aryl benzylamines or imines and ortho-substituted anilines is described.

Supported gold-catalyzed and ammonia-promoted selective synthesis of quinazolines in aqueous media

A highly efficient and selective synthesis of 2,4-disubstituted quinazolines via a hydrogen-transfer strategy was developed under the catalysis of nano-Au/TiO₂ and the assistance of nitrogen source-promotion.

Discovery of a metalloenzyme-like cooperative catalytic system of metal nanoclusters and catechol derivatives for the aerobic oxidation of amines

We have discovered a new class of cooperative catalytic system, consisting of heterogeneous polymer-immobilized bimetallic Pt/Ir alloyed nanoclusters (NCs) and 4-tert-butylcatechol, for the aerobic oxidation of amines to imines under ambient conditions. After optimization, the desired imines were obtained in good to excellent yields with broad substrate scope. The reaction rate was determined to be first-order with respect to the substrate and catechol and zero-order for the alloyed Pt/Ir NC catalyst. Control studies revealed that both the heterogeneous NC catalyst and 4-tert-butylcatechol are essential and act cooperatively to facilitate the aerobic oxidation under mild conditions.

Synthesis of Benzoxazoles by the Copper Triflate Catalysed Reaction of Nitriles and O-Aminophenols

An efficient procedure for the synthesis of benzoxazoles by the heteroannulation of nitriles and o-aminophenols in the presence of a catalytic amount of copper(II) trifluoromethanesulfonate, has been developed. This method represents a practical and attractive alternative for the synthesis of both 2-arylbenzoxazoles and 2-alkylbenzoxazoles.

Oxidative transformation of N-substituted 2-aminophenols to 2-substituted benzoxazoles catalyzed by polymer-incarcerated and carbon-stabilized platinum nanoclusters

The preparation of 2-substituted benzoxazoles was achieved through a sequential aerobic oxidation – enolization – oxidative cyclization reaction of N-substituted 2-aminophenols catalyzed by platinum nanoclusters supported on a polymer / carbon black composite material.