Products of reaction between tetrabromo-substituted ortho- and para-hydroxybenzoic acids and sodium nitrite in CH3COOH

The reaction of 2,3,5,6-tetrabromo-4-hydroxybenzoic acid with a 10-fold excess of NaNO₂ in the glacial acetic acid at 20°C affords tetrabromonitrosophenols whose further transformations under the reaction conditions leads to the formation of a mixture of 2,4,5,6-tetrabromo-p-quinone diazide and tetrabromo-p- and -o-nitrophenols in the molar ratio 37: 2: 1. Under similar conditions the 3,4,5,6-tetrabromo-2-hydroxybenzoic acid is converted into a mixture of 3,4,5,6-tetrabromo-o-quinone diazide with the same nitrophenols in the ratio 13: 1: 3. The reaction of sodium 2,3,5,6-tetrabromo-4-hydroxy-benzoate with NaNO₂ in dilute acetic acid resulted in a quantitative yield of tetrabromo-p-quinone monooxime.     

The effects of biologically important divalent and trivalent metal cations on the cyclization step of dopamine autoxidation reaction: A quantum chemical study

Dopamine is the most essential monoaminergic neurotransmitter involved in the pathophysiology of neurodegenerative disorders, and its autoxidation has been recognized as one of the potential trigger factors for dopaminergic neuron loss. The cyclization of dopamine o-quinone was shown to be the irreversible and rate-limiting step of the autoxidation reaction at physiologic pH values. Furthermore, various metal ions such as Al³⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Mn²⁺ have been clinically associated with neurodegeneration, especially Parkinsonism and dementia. It has been proposed that these metal ions could increase the rate of the dopamine autoxidation reaction; however, the exact mechanism has not yet been fully understood. Using advanced quantum chemical calculations with the inclusion of solvent effects we showed that except for Mn²⁺, the studied metal cations could form complexes with dopamine o-quinone and significantly increase the dopamine o-quinone cyclization rate in aqueous solution; first, by enabling the cyclization to proceed spontaneously without the attack of the unprotonated amino group by hydroxide ion; second, by decreasing the intrinsic activation energy; and third, by decreasing the free energy of protonated amino group deprotonation. The latter also decreases the protective effect of acidic pH on dopamine autoxidation found in synaptic vesicles. The results are fully consistent with experimental data and provide deeper understanding of the effects of metal cations on the dopamine autoxidation reaction at physiologic pH values.     

Highly selective synthesis of triptycene o-quinone derivatives and their optical and electrochemical properties

A convenient and efficient method for the selective synthesis of a series of triptycene o-quinone derivatives is described. The triptycene o-quinones, especially the ones containing the methoxy group(s) (electron donor) and the o-quinone group(s) (electron acceptor) simultaneously, show interesting intramolecular CT interactions and electrochemical properties. Moreover, DFT calculations display that introducing a strong electron-donor group into triptycene o-quinone results in an increasing of the HOMO energy level, which subsequently decreases the HOMO-LUMO energy gap.     

Reaction of 2-chloromethylphenols with enaminones

The reaction of o-(chloromethyl)phenols with enamino ketones afforded a series of 3-acyl- and 3-formyl-4H-chromenes as a result of cascade transformation including [4 + 2]-cycloaddition of enamino ketone to o-quinone methide generated in situ and subsequent elimination of secondary amine.     

Synthesis, structures, and properties of peripheral o-dimethoxy-substituted pentiptycene quinones and their o-quinone derivatives

A series of peripheral o-dimethoxy-substituted pentiptycene quinones and their o-quinone derivatives have been synthesized. Especially, it was found that if two o-dimethoxybenzene moieties were situated at the same side of the pentiptycene quinones, one of them was only oxidized by excess CAN in aqueous acetonitrile. Moreover, the pentiptycene quinones with unique 3D rigid structure could all self-assemble into a 3D microporous structure in the solid state. For the pentiptycene quinones containing the dimethoxybenzene unit(s) and the quinone group(s) simultaneously, interesting intramolecular charge transfer interactions and electrochemical properties were also shown. These peripheral-substituted pentiptycene quinones and their o-quinone derivatives can be used as new useful building blocks and will find wide applications in material science and host-guest chemistry.     

Electrochemical Investigation of some Flavonoids in Aprotic Media

Cyclic voltammerty data for the oxidation of 15 flavonoids in acetonitrile have been presented. Up to three peaks in different potential regions appear in the CV curves, depending on the structure of a flavonoid. The peak in the first potential region (+0.79÷+1.03 V vs SCE) occurs when a flavonoid molecule has an o-dihydroxy moiety in the ring A or B. From the data in the literature, the formation of o-quinone is responsible for the occurrence of the peak. When the o-dihydroxy moiety is in ring B and the hydroxyl group is at position 3, the second peak in the potential region+1.08÷+1.32 V, is present. It is an agreement that the peak is due to oxidation of benzofuranone derivative formed in reaction of o-quinone with water. Molecules with an o-quinone moiety located in ring A seem to be less reactive, and the peak in the second potential region does not occur. However, the existence of an o-quinone in the ring B is not necessary for the peak in the second potential region to be present. The oxidation of kaempferol and morin, which have hydroxyl groups at positions 3 and 4’, takes place at 0.98 V and 0.97 V, respectively, and leads to the formation of p-quinone methide. We have shown that the peak in the second potential region also appears when the hydroxyl group in the flavonoid molecule is only at position 3 or at position 4’. For 3-hydroxyflavone and apigenin, which are oxidized at +1.10 V and 1.49 V respectively, p-quinone methide was detected after controlled potential bulk electrolysis. A mechanism which includes the reaction of the carbon radical with a trace of water has been proposed. For flavonoids that have an o-dihydroxy moiety in ring B and the hydroxyl group at position 3, two oxidation peaks are observed in the CV curves in the second potential region for low scan rates (below 0.1 V s⁻¹). An explanation of this behavior has been proposed.     

Hydrogen and electron transfer in the photoreduction reaction of quinones

The primary products of the photoreduction reaction of 3,6-dit-butyl-o-quinone in single crystals of 3,6-di-t-butyl-pyrocatechol are studied by EPR spectroscopy. Ion radical and neutral radical pairs are identified distinctly. In the case of the same o-quinone in single crystals of 2-6-dit-butyl-4-methylphenol the primary product is the radical pair composed of the two hydroxyphenoxyl radicals of phenol. This indicates the possibility of transfer of two hydrogen atoms in a single elementary photochemical event.     

A new type of angular phenothiazine ring system

The first synthesis of monoaza- and diaza-analogues of benzo[a]phenothiazine ring system is achieved under anhydrous conditions by base-catalysed reaction of o-aminoheterocyclic thiones with 2,3-dichloronaphtho-1,4-quinone. If the reactions were carried out in methanol, 2-chloro-3-methoxynaphtho-1,4-quinone was also isolated. The ease of air oxidation of the Na₂S₂O₄ reduced compounds makes them suitable for consideration as vat dyes.     

Synthesis and some transformations of polybrominated quinone diazides

The reduction of polybrominated o- and p-nitrophenols with granular tin in concentrated aqueous HCl gave polybrominated aminophenols which were diazotized with sodium nitrite in concentrated sulfuric acid at 0°C to obtain polybrominated o- and p-quinone diazides. Their thermolysis with elimination of nitrogen generated ketocarbenes which reacted with acetylacetone to form insertion products at the activated methylene group. Ketocarbenes generated from o-quinone diazides reacted with typical dipolarophiles such as acetonitrile, benzonitrile, styrene, and phenylacetylene to afford the corresponding [3 + 2]-cycloaddition products.     

Functionalization of sterically hindered catechol and <Emphasis Type="Italic">o</Emphasis>-benzoquinone with 2,2,6,6-tetramethylpiperidine 1-oxyl

Sterically hindered 4,6-di-tert-butyl-3-formylcatechol and 3,6-di-tert-butyl-o-benzoquinone react with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl to give new chelate ligands of the o-quinone type bearing a 2,2,6,6-tetramethylpiperidine 1-oxyl neutral radical moiety. Structures of the synthesized compounds were established by ESR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray diffraction analysis.     

Differentiation of planar chiral enantiomers of 〚Cp*M(2-alkyl-Phenoxo)〛 〚BF4〛 {M = Rh,Ir} by the trisphat anion

Precursor oxo-dienyl rhodium and iridium complexes 〚(η⁵-Cp*)M(η⁵-2-alkyl-oxodienyl)〛 〚BF₄〛 (2a–c) were prepared according to literature procedure. Addition of 〚n-Bu₄N〛 〚Δ-trisphat〛 (6) to a CD₂Cl₂ solution of these chiral derivatives has led to the NMR differentiation of the enantiomers. These results pave the way towards the preparation of enantiomerically pure o-quinone methide complexes.     

Synthesis of new carbonyl and fluoroalkyl o-quinone methides from β-lapachone

The synthesis of new carbonyl and fluoroalkyl o-quinone methides from β-lapachone is reported.     

o-Quinone methide with overcrowded olefin component as a dehydridation catalyst under aerobic photoirradiation conditions

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation. The characteristics of this o-QM enable the operation of dehydridative catalysis in the oxidation of benzylic secondary alcohols under aerobic photoirradiation conditions. An experimental analysis and density functional theory calculations provide mechanistic insights; the ground-state zwitterionic intermediate abstracts a hydride and proton simultaneously, and the active oxygen species facilitate catalyst regeneration.

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation.     

Tin oxidation with 3,6-di-tert-butyl-o-benzoquinone

The kinetics of tin oxidation with 3,6-di-tert-butyl-o-benzoquinone (3,6-Q) in toluene was studied. The scheme of formation of tin o-quinone complexes during metal oxidation with 3,6-Q in different aprotic solvents was suggested on the basis of the kinetic, EPR, and IR data.     

A novel electrochemical sensor based on molecularly imprinted polymers for caffeine recognition and detection

A sensitive and selective electrochemical sensor based on molecularly imprinted polymers (MIPs) was developed for caffeine (CAF) recognition and detection. The sensor was constructed through the following steps: multiwalled carbon nanotubes and gold nanoparticles were first modified onto the glassy carbon electrode surface by potentiostatic deposition method successively. Subsequently, o-aminothiophenol (ATP) was assembled on the surface of the above electrode through Au–S bond before electropolymerization. During the assembled and electropolymerization processes, CAF was embedded into the poly(o-aminothiophenol) film through hydrogen bonding interaction between CAF and ATP, forming an MIP electrochemical sensor. The morphologies and properties of the sensor were characterized by scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry. The recognition and determination of the sensor were observed by measuring the changes of amperometric response of the oxidation-reduction probe, [Fe(CN)₆]^3?/[Fe(CN)₆]^4?, on modified electrode. The results demonstrated that the prepared sensor had excellent selectivity and high sensitivity for CAF, and the linear range was 5.0?×?10^?10?~?1.6?×?10^?7?mol?L^?1 with a detection limit of 9.0?×?10^?11?mol?L^?1 (S/N?=?3). The sensor was also successfully employed to detect CAF in tea samples.     

Convenient one-step synthesis of 4-unsubstituted 2-amino-4H-chromene-2-carbonitriles and 5-unsubstituted 5H-chromeno[2,3-b]pyridine-3-carbonitriles from quaternary ammonium salts

We have reported DBU catalyzed synthesis of 4-unsubstituted 2-amino-4H-chromene-2-carbonitriles in water under reflux. The attractive features of this process are mild reaction conditions, short reaction times, easy isolation of products and good yields. 5H-chromeno[2,3-b]pyridine-3-carbonitriles were obtained by refluxing excess of malononitrile and quaternary ammonium salts in ethanol in the presence of NaOH as catalyst. The mechanisms of these reactions are believed to involve the formation of the o-quinone methide intermediate.     

Carbon-13 NMR of quinone methides

The ¹³C NMR spectra of four ortho- and seven para-quinone methides were assigned using chemical shift and long-range carbon-proton coupling information. The carbonyl shifts are compared with those in ortho- and para-benzoquinones. The chemical shifts of the carbonyls of the p-quinone methides are observed at δ 186.2–186.4 for the three ortho-di-tert-butyl-substituted compounds and at δ 180.7–181.5 for the four ortho-oxy-substituted compounds. In the three o-quinone methides with meta, para-dioxy substituents, the carbonyl signals are at δ 184.2–185.4. The carbonyl signal of the one o-quinone methide with no oxygen substitution is shifted downfield to δ 200.9, apparently as a result of hydrogen bonding to the nearby hydroxyl.     

o-Quinone methide based approach to isoflavans: application to the total syntheses of equol, 3′-hydroxyequol and vestitol

A concise strategy is developed for the synthesis of isoflavans employing a Diels-Alder reaction between o-quinone methides and aryl-substituted enol ethers followed by reductive cleavage of the acetal group. The method is extended towards the total syntheses of equol, 3′-hydroxyequol and vestitol.     

Efficient access to bisphenol A metabolites: Synthesis of monocatechol,mono-o-quinone,dicatechol, and di-o-quinone of bisphenol A

2-Iodoxybenzoic acid (IBX) oxidation of bisphenol A (BPA) is described. The selective production of either the mono-o-quinone or the di-o-quinone can be controlled by IBX stoichiometry. Isolated yields of quinone were greater than 80%. Previous synthesis of BPA-di-o-quinone using a large excess of Fremy’s salt produced only trace amounts of product. In addition to o-quinone products, both mono- and dicatechols of BPA can synthesize in high yield and isolated without chromatography. The more stable catechols can be quantitatively converted back to o-quinones using silver oxide oxidation in either acetone or DMF. These one-pot reactions provide access to four different BPA metabolites in high yield and significant scale.     

Synthesis and oxidation of ‘non-annulated’ vitamin E-like compounds

Two model compounds (3 and 4) having the typical α-tocopherol-type substitution pattern of the aromatic ring, but lacking the annulated pyran ring, have been synthesized. Upon oxidation, the two possible ortho-quinone methides (oQMs) of each are formed in equal ratio. DFT calculations suggest that there is no angular strain in 3 and 4, and each of the oQM pairs is of similar energy. These results prove that the commonly observed regioselectivity in oxidations of vitamin E-type model compounds is not an electronic substitution effect, but a consequence of annulation.