Pyrazolo[2′,3′:3,4][1,3]oxazino[5,6-b]quinoxaline, a novel tetracyclic ring system

Reaction of 2-chloro-3-(3-chloro-1H-pyrazol-5-yl)quinoxaline and aldehydes does not afford the corresponding N-(α-hydroxyalkylated) derivatives but results in a cyclisation reaction to give a derivative bearing the hitherto undescribed pyrazolo[2′,3′:3,4][1,3]oxazino[5,6-b]quinoxaline system.     

Versatile transamination in quinonediimine chemistry: Towards a novel class of water soluble UV/violet chromophores

The transamination reaction of 2,5-diaminobenzoquinonediimine (QDI) with ethylenediamine gave fluorescent 1,2,3,4-tetrahydropyrazino[2,3-g]quinoxaline (1). When the same reaction was carried out with N,N’-bis(aminoethyl)-1,3-propanediamine, a novel cationic quinoxalinium species (2) was isolated, which can be further condensed with p-cyanobenzaldehyde to afford a benzimidazolo-fused quinoxaline dye (3) that is a water-soluble fluorophore in the UV–visible range.     

Quinoxaline-2,3-dithiol in reactions with propargyl bromide and 1,3-dibromopropyne

Propargyl bromide with quinoxaline-2,3-dithiol in DMSO in the presence of K₂CO₃ affords 2,3-bis-(2-propynylsulfanyl)quinoxaline in good yield whereas in absolute methanol in the presence of sodium methoxide at 20°C a 1:2 mixture of 2,3-bis(2-propynylsulfanyl)quinoxaline and 3-(2-propynylsulfanyl)-2(1H)-quinoxalinethione is formed. Individual 3-(2-propynylsulfanyl)-2(1H)-quinoxalinethione was obtained by crystallization of this mixture from ether. The reaction of 1,3-dibromopropyne with quinoxaline-2,3-dithiol in ethanol in the presence of NaOH at heating results in 2-bromomethylidene-1,4-(3H)-dithiino[2,3-b]quinoxaline in 77% yield. Performing this reaction in methanol in the presence of sodium methoxide during long heating (16 h) led to 2,3-bis[(3-bromo-2-propynyl)sulfanyl]quinoxaline in 72% yield.     

Synthesis of new donor-acceptor polymers containing thiadiazoloquinoxaline and pyrazinoquinoxaline moieties: low-band gap, high optical contrast, and almost black colored materials

Two low band gap conjugated polymers, poly[4,9-bis(4-hexylthien-2-yl)-6,7-di(thien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline] (PHTTQ) and poly[5,10-bis(4-hexylthien-2-yl)-2,3,7,8-tetra(thien-2-yl)pyrazino[2,3-g]quinoxaline] (PHTPQ), consisting of alternating electron-rich 3-hexylthiophene and electron-deficient 6,7-di(thien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TTQ) and 2,3,7,8-tetra(thien-2-yl)-2,3-dihydropyrazino[2,3-g]quinoxaline (TPQ) units were synthesized electrochemically. The structures of the π-conjugated monomers were tailored using thiophene as the pendant group on the acceptor units (TTQ and TPQ). The electrochemical and optical properties of the polymers were investigated by cyclic voltammetry and UV-vis-NIR spectroscopy. The absorption spectra of PHTPQ, revealing a 1.0 eV band gap, exhibited three maxima at 352 nm, 535 nm, and 750 nm. Consequently, its absorption spectra cover the region between 400 and 800 nm, which make the polymer almost black in appearance. PHTTQ shows a λ_max value of 820 nm and a band gap of 0.8 eV which is very low among other [1,2,5]thiadiazolo[3,4-g]quinoxaline-containing donor-acceptor type polymers.     

Push–Pull Derivatives Based on 2,4′-Biphenylene Linker with Quinoxaline, [1,2,5]Oxadiazolo[3,4-B]Pyrazine and [1,2,5]Thiadiazolo[3,4-B]Pyrazine Electron Withdrawing Parts

A series of novel V-shaped quinoxaline, [1,2,5]oxadiazolo[3,4-b]pyrazine and [1,2,5]thiadiazolo[3,4-b]pyrazine push–pull derivatives with 2,4′-biphenylene linker were designed and their electrochemical, photophysical and nonlinear optical properties were investigated. [1,2,5]Oxadiazolo[3,4-b]pyrazine is the stronger electron-withdrawing fragment as shown by electrochemical, and photophysical data. All compounds are emissive in a solid-state (from the cyan to red region of the spectrum) and quinoxaline derivatives are emissions in DCM solution. It has been found that quinoxaline derivatives demonstrate important solvatochromism and extra-large Stokes shifts, characteristic of twisted intramolecular charge transfer excited state as well as aggregation induced emission. The experimental conclusions have been justified by theoretical (TD-)DFT calculations.     

Preparation and spectroscopic and electrochemical properties of complexes of dibenzo-and dipyrido-substituted 1,4-diazines

The spectroscopic and electrochemical properties of palladium ethylenediamine complexes [Pd(N^N)En]Cl₂ with 1,4-diazine derivatives of o-phenanthroline [(N^N) = dipyrido[a,c]phenazine (dppz), dipyrido[f,h]quinoxaline (dpq)] were studied in comparison with those of the free diimine (N^N) ligands, dibenzo-substituted 1,4-diazines [dibenzo[f,h]quinoxaline (dbq), dibenzo[a,c]phenazine (dbpz)], and cyclometallated dichloride [Pd(C^N)(μ-Cl)]₂ and ethylenediamine [Pd(C^N)En]Cl complexes derived from dibenzo-substituted 1,4-diazines.     

Selective synthesis and optical properties of diimidazo[1,2-a:5′,1′-c]quinoxaline derivatives

Novel diimidazo[1,2-a:5′,1′-c]quinoxaline derivatives were synthesized by a Pd-catalyzed intramolecular coupling reaction of 1,2-bis(2-iodo-1H-imidazol-1-yl)benzene. This method was applied to the selective introduction of an aryl substituent at the 10 position by the addition of a boronic ester. Moreover, a stepwise introduction of the aryl substituent could be achieved by the further coupling reaction at the 3 position. Focused on the optical properties, a more efficient substituent effect at the 10 position was obtained compared with the analogous structure, diimidazo[1,2-a:2′,1′-c]quinoxaline derivatives. In addition, we revealed that the diimidazo[1,2-a:5′,1′-c]quinoxaline skeleton potentially has an electron-donating character on the optical properties by introducing an electron-withdrawing substituent.     

Synthesis of chiroptical properties of some N-(3-methyl-2-quinoxaloyl) l-amino acids and their dioxides

A number of N-(3-methyl-2-quinoxaloyl) l-α-amino acids and esters, and their 1,4-dioxides have been prepared. The quinoxaline derivatives of aliphatic and aromaticl-α-amino acids exhibits enantiomorphic CD spectra in ethanol as well as in ethanolic KOH. However, the corresponding quinoxaline-1,4-dioxide derivatives of the l-α-aliphatic and l-α-aromatic amino acids show, in organic solvents, similar CD spectra. This behaviour is attributed to differences in conformational equilibria in both the quinoxaline and the quinoxaline-1,4-dioxide series NMR and mass spectra of these compounds are discussed.     

Spectroscopic and electrochemical properties of mixed-ligand cyclopalladinized complexes of deprotonated forms of 2-(2-thienyl)pyridine and 2-phenylpyridine with 1,10-phenantroline and its 1,4-diazine derivatives

A comparative study of [Pd(N?C)(N?N)]⁺ complexes {(N?C)^? are deprotonated forms of 2-phenylpyridine and 2-(2-thienyl)pyridine and (N^N) is 1,10-phenantroline, dipyrido[f,h]quinoxaline, dipyrido[a,c]phenazine or 6,7-dicyanodibenzo[f,h]quinoxaline} was carried out by the methods of ¹H NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Optical and electrochemical properties of dipyrido-substituted 1,4-diazines are conditioned by the electron transfer involving π* orbitals localized on weakly interacting phenantroline (π*_phen) and diazne (π*_diaz) components.     

Efficient regioselective heterocyclisation leading to fluorescent fused pyrazine derivatives

The regioselective syntheses of substituted pyrrolo[2,3-b]quinoxaline, pyrido[2,3-b]pyrrolo[2,3-e]pyrazine, pyrido[2,3-b]pyrrolo[3,2-e]pyrazine and pyrido[3,4-b]pyrrolo[3,2-e]pyrazine are reported. Differential reactivity between two amino groups in ortho-diaminopyridine can be exploited to obtain new regio-defined unsymmetrical pyridopyrrolopyrazine derivatives. Weak electron-donating methyl or moderately electron-withdrawing carboxylic groups attached to the aromatic ortho-diamines reduce the regioselectivity of obtaining unsymmetrical substituted pyrrolo[2,3-b]quinoxaline. The fluorescence properties of the resultant 1-alkyl pyridopyrrolopyrazine and substituted pyrrolo[2,3-b]quinoxaline derivatives are presented.     

Spectroscopic and electrochemical properties of dichlorodiimine complexes of Au(III) and Pt(II) with 1,4-diazine derivatives of o-phenanthroline

A series of dichlorodiimine complexes [M(N N)Cl₂]^z of Au(III) and Pt(II) with 1,4-derivatives of o-phenanthroline [(N N) = o-phenanthroline (phen), dipyrido[f,h]quinoxaline (dpq), dipyrido[a,c]phenazine (dppz), 6,7-dicyanodipyrido[f,h]quinoxaline (dicnq)] were prepared and characterized by ¹H NMR, electronic absorption, and emission spectroscopy and by cyclic voltammetry. In all the complexes, the ³(π-π*)-type transition is the spin-forbidden transition of the lowest energy, responsible for the luminescence. The longest wave bands in the absorption spectra of the Au(III) and Pd(II) complexes were assigned in accordance with the results of the electrochemical studies to the ¹(d*)-and ¹(d*)-type transitions, respectively.     

Quinoxaline Moiety: A Potential Scaffold against Mycobacterium tuberculosis

Background. The past decades have seen numerous efforts to develop new antitubercular agents. Currently, the available regimens are lengthy, only partially effective, and associated with high rates of adverse events. The challenge is therefore to develop new agents with faster and more efficient action. The versatile quinoxaline ring possesses a broad spectrum of pharmacological activities, ensuring considerable attention to it in the field of medicinal chemistry. Objectives. In continuation of our program on the pharmacological activity of quinoxaline derivatives, this review focuses on potential antimycobacterial activity of recent quinoxaline derivatives and discusses their structure—activity relationship for designing new analogs with improved activity. Methods. The review compiles recent studies published between January 2011 and April 2021. Results. The final total of 23 studies were examined. Conclusions. Data from studies of quinoxaline and quinoxaline 1,4-di-N-oxide derivatives highlight that specific derivatives show encouraging perspectives in the treatment of Mycobacterium tuberculosis and the recent growing interest for these scaffolds. These interesting results warrant further investigation, which may allow identification of novel antitubercular candidates based on this scaffold.     

S/Se-embedded acenaphthylene-imide-containing polycyclic heteroaromatic hydrocarbon

Acenapththylene-imide(An I), similar to naphthalene diimide(NDI), is an outstanding building block for organic functional materials and has gained a lot of research attention. Herein, Sulphur and Selenium-embedded AnI-containing polycyclic aromatic hydrocarbon molecules, AnI-SQ and AnI-SeQ,with [1,2,5]thiadiazolo[3,4-g]quinoxaline(SQ) and [1,2,5]selenadiazolo[3,4-g]quinoxaline(Se Q) are designed and synthesized with low-lying LUMO energy levels. The absorption and emission of AnI-SQ and An I-Se ...     

A new and versatile Ugi/SNAr synthesis of fused 4,5-dihydrotetrazolo[1,5-a]quinoxalines

A combinatorial synthetic route yielding fused tetrazolo[1,5-a]quinoxalines is described. The use of 2-fluorophenylisocyanide in the Ugi-tetrazole reaction (tetrazole-U-4CR) followed by a nucleophilic aromatic substitution (S_NAr) affords the tricylic tetrazolo[1,5-a]quinoxaline moiety in good yields and with high diversity.     

Synthesis and electronic properties of expanded 5-(hetero)aryl-thien-2-yl substituted 3-ethynyl quinoxalines with AIE characteristics

Expanded 5-(hetero)aryl-thien-2-yl substituted 3-ethynyl quinoxaline dyes with variable substitution pattern on the peripheral thiophene ring were synthesized in moderate to very good yields by Suzuki and Buchwald-Hartwig coupling of the corresponding brominated 3-ethynyl quinoxalines. Dumbbell-shaped bis(thienyl 3-ethynyl quinoxalines) are also accessible by the Suzuki protocol. The photophysical properties were investigated by UV and fluorescence spectroscopy. Most of the obtained compounds display fluorescence in solution and some of them also in the solid state. Additionally, tuning of the emission color of the quinoxaline based chromophores can be conveniently accomplished by the remote substituent group. The determined absorption and emission maximum as well as the Stokes shifts strongly correlate with Hammett σp+parameters. Besides,photophysical properties of selected derivatives in the solid state, biphasic solutions, and PMMA films, along with their relationships, are comparatively investigated. Moreover, two 5-(hetero)aryl-thien-2-yl substituted 3-ethynyl quinoxaline dyes are aggregation induced emission(AIE) chromophores indicated by restriction of molecular motions. A covalently restricted 3-ethynyl quinoxaline supports that the inhibition of molecular rotation is responsible for the significant enhancement of fluorescence in acetonitrile/water mixtures.     

Synthesis, spectroscopic properties and crystal structure of mononuclear tricarbonylrhenium(I) chloride complexes carrying 6-functionalised quinoxalines

Two quinoxaline derivatives pqCH₃ and pqCl (where pq stands for 2-(2′-pyridyl)quinoxaline) were prepared by condensation of 2-acetyl pyridyl with 2-amino-4-methylphenylamine or 2-amino-4-chlorophenylamine, correspondingly and were studied spectroscopically and electrochemically, in correlation with the originally reported pq. Their novel corresponded complexes namely, fac-[Re(CO)₃Cl(L)] (where L = pqCH₃2 and pqCl 3) were synthesized, characterized, studied and compared to Re(CO)₃Clpq, 1. Complex 2 crystallizes in space group C2/c with a = 20.4476(17) Å, b = 15.4521(13) Å, c = 15.2887(13) Å, β = 126.1210(11)°, Z = 8 and V = 3902.0(6) Å³. The substitution of -H by -CH₃ or -Cl at 6-position of pq has a minor electronic effect on the pyridyl ring of the ligands, but seems to influence the quinoxaline moiety enough to alter the spectroscopical and electrochemical features.     

Synthesis and spectroscopic studies of highly fluorescent,solvatochromic diastereomers with differentially stacked bithiophene-substituted quinoxaline rings

Diastereomeric C-shaped molecules containing closely stacked bithiophene-substituted quinoxaline rings were synthesized and characterized by NMR, UV–vis absorption, and fluorescence spectroscopy. The unique geometry of each diastereomer resulted in different degrees of π-overlap between the bithiophene-substituted quinoxaline ring chromophores, modulating their spectroscopic properties. The donor-acceptor nature of this chromophore gave rise to its positive solvatochromism. ¹H NMR and UV–vis absorption spectroscopy confirmed the existence of π-π interactions in the ground state between the quinoxaline rings in both molecules but between the bithiophene rings only in the syn isomer. They exhibited significant emission maxima bathochromic shifts, a strong, positive solvatochromism, increased band broadening, and larger Stokes shifts when compared to a compound with an unstacked chromophore. Additionally, the syn isomer consistently showed λ_max,em value red-shifts and larger band broadening and Stokes shifts compared to the anti isomer due to the greater π-overlap in the syn isomer.     

A synthetic route to 11-(1H-pyrrol-1-yl)-11H-indeno[1,2-b]quinoxaline derivatives exploiting a three-component coupling strategy under microwave irradiation

11H-Indeno[1,2-b]quinoxalin-11-ones generated in situ from ninhydrin and various 1,2-phenylenediamines, catalysed by montmorillonite K10 under microwave irradiation, condense with 4-hydroxyproline to produce 11-(1H-pyrrol-1-yl)-11H-indeno[1,2-b]quinoxaline derivatives in good yields.     

Hydrothermal synthesis and structure of a novel 3D framework based on ξ-octamolybdate chains: [Cu2(quinoxaline)2Mo4O13]n

An unprecedented three-dimensional (3D) polymer [Cu₂(quinoxaline)₂Mo₄O₁₃]_n, formed from ξ-octamolybdate chains as building units and pairs of 1D [Cu(quinoxaline)]_nⁿ⁺ polymeric chains as linkers, provides the first example of an extended higher dimensional structure based on octamolybdate chain. The basic building block of the octamolybdate chain included in the title compound is first reported to be constructed from ξ-isomer of octamolybdate unit.     

Anionic ring opening of norbornenes fused to heterocycles

The 2-hydroxy and 2-oxo derivatives of 1,2,3,4-tetrahydro-1,4-methanophenazine were prepared and found to evolve in basic media through the opening of their bicyclo[2.2.1]heptene moiety, affording 2,3-dihydro-1H-cyclopenta[b]quinoxaline derivatives with two-carbon 1-substituents that depend on the starting compound. In the case of 2-hydroxy starting compounds, ring-opening occurs regardless of the orientation of the hydroxyl group, and in methanolic solution is spontaneous, though slow, even in the absence of added base (at least in the case of the endo derivative). It is presumably favoured by the steric strain of the heteroaryl-fused bicyclo[2.2.1]heptene moiety, and is hypothesized to involve the base-promoted formation of anionic intermediates that are stabilized by the π-deficient nature of the quinoxaline system.