Blue Light-Emitting Diode-Induced Direct C-H Functionalization of 1,4-Quinones with Aryl and Alkyl Boronic Acids

A direct functionalization of numerous 1,4-quinones with various aryl boronic acids is reported under blue light-emitting diodes (LEDs). This reaction occurs at room temperature in an open flask without any catalysts, base, and oxidants in acetonitrile (ACN) and is scalable in grams. With diverse 1,4-quinones like 1,4-benzo-, naphtho-, anthra-, and 4-bromonaphthoquinones as substrates, facile cross coupling reactions occur with aryl and alkyl boronic acids without assistance from any photocatalysts. 2-Alkylated cyclohexene-1,4-diones were obtained when the 1,4-quinones were reacted with alkyl boronic acids under standard reaction conditions. However, slight warming of the reaction mixture afforded the desired alkylated 1,4-quinones. The reaction is believed to proceed through the blue LED-induced radical formation of the aryl rings assisted by the 1,4-quinones.

Menadione: a platform and a target to valuable compounds synthesis

Naphthoquinones are important natural or synthetic compounds belonging to the general class of quinones. Many compounds in this class have become drugs that are on the pharmaceutical market for the treatment of various diseases. A special naphthoquinone derivative is menadione, a synthetic naphthoquinone belonging to the vitamin K group. This compound can be synthesized by different methods and it has a broad range of biological and synthetic applications, which will be highlighted in this review.

Enoyl-Coenzyme A Respiration via Formate Cycling in Syntrophic Bacteria

Syntrophic bacteria play a key role in the anaerobic conversion of biological matter to methane. They convert short-chain fatty acids or alcohols to H2, formate, and acetate that serve as substrates for methanogenic archaea. Many syntrophic bacteria can also grow with unsaturated fatty acids such as crotonate without a syntrophic partner, and the reducing equivalents derived from the oxidation of one crotonate to two acetate are regenerated by the reduction of a second crotonate. However, it has remained unresolved how the oxidative and reductive catabolic branches are interconnected and how energy may be conserved in the reductive branch. Here, we provide evidence that during axenic growth of the syntrophic model organism Syntrophus aciditrophicus with crotonate, the NAD+-dependent oxidation of 3-hydroxybutyryl-CoA to acetoacetyl-CoA is coupled to the reduction of crotonyl-CoA via formate cycling. In this process, the intracellular formate generated by a NAD+-regenerating CO2 reductase is taken up by a periplasmic, membrane-bound formate dehydrogenase that in concert with a membrane-bound electron-transferring flavoprotein (ETF):methylmenaquinone oxidoreductase, ETF, and an acyl-CoA dehydrogenase reduces intracellular enoyl-CoA to acyl-CoA. This novel type of energy metabolism, referred to as enoyl-CoA respiration, generates a proton motive force via a methylmenaquinone-dependent redox-loop. As a result, the beneficial syntrophic cooperation of fermenting bacteria and methanogenic archaea during growth with saturated fatty acids appears to turn into a competition for formate and/or H2 during growth with unsaturated fatty acids. IMPORTANCE The syntrophic interaction of fermenting bacteria and methanogenic archaea is important for the global carbon cycle. As an example, it accomplishes the conversion of biomass-derived saturated fatty acid fermentation intermediates into methane. In contrast, unsaturated fatty acid intermediates such as crotonate may serve as growth substrate for the fermenting partner alone. Thereby, the reducing equivalents generated during the oxidation of one crotonate to two acetate are regenerated by reduction of a second crotonate to butyrate. Here, we show that the oxidative and reductive branches of this pathway are connected via formate cycling involving an energy-conserving redox-loop. We refer to this previously unknown type of energy metabolism as to enoyl-CoA respiration with acyl-CoA dehydrogenases serving as cytoplasmic terminal reductases.

Synthesis of Naphthoquinone Derivatives: Menaquinones, Lipoquinones and Other Vitamin K Derivatives

Menaquinones are a class of isoprenoid molecules that have important roles in human biology and bacterial electron transport, and multiple methods have been developed for their synthesis. These compounds consist of a methylnaphthoquinone (MK) unit and an isoprene side chain, such as found in vitamin K₁ (phylloquinone), K₂, and other lipoquinones. The most common naturally occurring menaquinones contain multiple isoprene units and are very hydrophobic, rendering it difficult to evaluate the biological activity of these compounds in aqueous assays. One way to overcome this challenge has been the application of truncated MK-derivatives for their moderate solubility in water. The synthesis of such derivatives has been dominated by Friedel-Crafts alkylation with BF₃∙OEt₂. This attractive method occurs over two steps from commercially available starting materials, but it generally produces low yields and a mixture of isomers. In this review, we summarize reported syntheses of both truncated and naturally occurring MK-derivatives that encompass five different synthetic strategies: Nucleophilic ring methods, metal-mediated reactions, electrophilic ring methods, pericyclic reactions, and homologation and side chain extensions. The advantages and disadvantages of each method are discussed, identifying methods with a focus on high yields, regioselectivity, and stereochemistry leading to a detailed overview of the reported chemistry available for preparation of these compounds.

Unified Approach toward Syntheses of Juglomycins and Their Derivatives

A unified and common intermediate strategy for syntheses of juglomycins and their derivatives is reported. The use of a 1,4-dimethoxynaphthalene derivative as a key intermediate enabled easy access to various juglomycin derivatives. In this study, juglomycins A-D, juglomycin C amide, khatmiamycin and its 4-epimer, and the structure proposed for juglomycin Z were synthesized from this intermediate. The absolute configuration of natural khatmiamycin has been established to be 3R,4R through our synthesis. Unfortunately, the spectroscopic data for synthetic juglomycin Z were not consistent with the data reported for the natural one, strongly suggesting a structural misassignment.

Total Synthesis of (Nor)illudalane Sesquiterpenes Based on a C(sp3)-H Activation Strategy

Three (nor)illudalane sesquiterpenes were synthesized from a common intermediate in racemic and enantioenriched forms using Pd⁰-catalyzed C(sp³)-H arylation as a key step. The configuration of the isolated, highly symmetric quaternary stereocenter of the target molecules was controlled through a matched combination of chiral substrate and catalyst. Moreover, the recently developed Ir-catalyzed C-H borylation/Cu-catalyzed methylation method was employed to install the methyl group on the benzene ring. This strategy allowed the efficient synthesis of both racemic and (S)-configured puraquinonic acid, deliquinone, and russujaponol F.

Tuning the redox potential of vitamin K3 derivatives by oxidative functionalization using a Ag(i)/GO catalyst

We propose herein initial results to develop optimum redox mediators by the combination of computational simulation and catalytic functionalization of the core structure of vitamin K₃. We aim to correlate the calculated energy value of the LUMO of different vitamin K₃ derivatives with their actual redox potential. For this, we optimized the catalytic alkylation of 1,4-naphthoquinones with a designed Ag(i)/GO catalyst and synthesized a series of molecules.

Design, Synthesis, and Some Aspects of the Biological Activity of Mitochondria-Targeted Antioxidants

This review summarizes for the first time data on the design and synthesis of biologically active compounds of a new generation - mitochondria-targeted antioxidants, which are natural (or synthetic) p-benzoquinones conjugated via a lipophilic linker with (triphenyl)phosphonium or ammonium cations with delocalized charge. It also describes the synthesis of mitochondria-targeted antioxidants - uncouplers of oxidative phosphorylation - based on fluorescent dyes.

Pharmacomodulation of the Antimalarial Plasmodione: Synthesis of Biaryl- and N-Arylalkylamine Analogues, Antimalarial Activities and Physicochemical Properties

With the aim of increasing the structural diversity on the early antimalarial drug plasmodione, an efficient and versatile procedure to prepare a series of biaryl- and N-arylalkylamines as plasmodione analogues is described. Using the naturally occurring and commercially available menadione as starting material, a 2-step sequence using a Kochi-Anderson reaction and subsequent Pd-catalyzed Suzuki-Miyaura coupling was developed to prepare three representative biphenyl derivatives in good yields for antimalarial evaluation. In addition, synthetic methodologies to afford 3-benzylmenadione derivatives bearing a terminal -N(Me)₂ or -N(Et)₂ in different positions (ortho, meta and para) on the aryl ring of the benzylic chain of plasmodione were investigated through reductive amination was used as the optimal route to prepare these protonable N-arylalkylamine privileged scaffolds. The antimalarial activities were evaluated and discussed in light of their physicochemical properties. Among the newly synthesized compounds, the para-position of the substituent remains the most favourable position on the benzyl chain and the carbamate -NHBoc was found active both in vitro (42 nM versus 29 nM for plasmodione) and in vivo in Plasmodium berghei-infected mice. The measured acido-basic features of these new molecules support the cytosol-food vacuole shuttling properties of non-protonable plasmodione derivatives essential for redox-cycling. These findings may be useful in antimalarial drug optimization.

Lawsone in organic synthesis

Lawsone has been used as the starting material for the synthesis of a variety of biologically active compounds and materials.

Synthesis and characterization of mitoQ and idebenone analogues as mediators of oxygen consumption in mitochondria

Analogues of mitoQ and idebenone were synthesized to define the structural elements that support oxygen consumption in the mitochondrial respiratory chain. Eight analogues were prepared and fully characterized, then evaluated for their ability to support oxygen consumption in the mitochondrial respiratory chain. While oxygen consumption was strongly inhibited by mitoQ analogues 2-4 in a chain length-dependent manner, modification of idebenone by replacement of the quinone methoxy groups by methyl groups (analogues 6-8) reduced, but did not eliminate, oxygen consumption. Idebenone analogues 6-8 also displayed significant cytoprotective properties toward cultured mammalian cells in which glutathione had been depleted by treatment with diethyl maleate.

Effects of thymoquinone-fatty acid conjugates on cancer cells

4-Acylhydrazones and 6-alkyl derivatives of thymoquinone (TQ) were tested for growth inhibition of human HL-60 leukemia, 518A2 melanoma, KB-V1/Vbl cervix, and MCF-7/Topo breast carcinoma cells. Unsaturated side chains conferred greater activities than equally long saturated chains. The number of C==C bonds was less decisive than chain length. The 6-hencosahexaenyl conjugate 3 e was most active in all resistant tumor cells, with IC(50) (72 h) values as low as 30 nM in MCF-7/Topo cells. The conjugates are likely to operate by mechanisms different from that of TQ. For instance, 3 e induced distinct caspase-independent apoptosis in HL-60 and 518A2 cells concomitant with a loss of mitochondrial membrane potential and a subsequent rise in the levels of reactive oxygen species.

Pyranonaphthoquinone lactones: a new class of AKT selective kinase inhibitors alkylate a regulatory loop cysteine

The naturally occurring pyranonaphthoquinone (PNQ) antibiotic lactoquinomycin and related aglycones were found to be selective inhibitors of the serine-threonine kinase AKT. A set of synthetic PNQs were prepared and a minimum active feature set and preliminary SAR were determined. PNQ lactones inhibit the proliferation of human tumor cell lines containing constitutively activated AKT and show expected effects on cellular biomarkers. Biochemical data are presented supporting a proposed bioreductive alkylation mechanism of action.

Acid-induced rearrangement reactions of reduced benzoquinone cyclopentadiene cycloadducts

Several Diels-Alder adducts between benzoquinones and cyclopentadiene were reduced to the corresponding diols 7a-c and 11. Treatment of these diols with strong acid triggered a skeletal rearrangement reaction resulting in compounds 8a-c and 12 that contain a 4,8-methanoazulene substructure. In addition, a dyotropic-like rearrangement of the tetracyclic lactone 13 to the spiro-lactone 18 was observed. Five of the structures were supported by X-ray analysis.

Synthesis of puraquinonic acid ethyl ester and deliquinone via a common intermediate

Both compounds were prepared via a common intermediate. The key features included the direct synthesis of the indan skeleton and the radical addition to a quinone.

Concerted action of DT-diaphorase and superoxide dismutase in preventing redox cycling of naphthoquinones: an evaluation

It has been suggested that the enzymes DT-diaphorase and superoxide dismutase act in concert to prevent redox cycling of naphthoquinones and thus protect against the toxic effects of such substances. Little is known, however, about the scope of this process or the conditions necessary for its operation. In the presence of low levels of DT-diaphorase, 2-methyl-1,4-naphthoquinone was found to undergo redox cycling. This was very effectively inhibited by SOD, and in the presence of both enzymes the hydroquinone was maintained in the reduced form. The inhibitory effect of the enzyme combination was overcome, however, at high concentrations of the quinone, or by small increases in pH. Furthermore, redox cycling was re-established by addition of haemoproteins such as cytochrome c and methaemoglobin. DT-diaphorase and SOD strongly inhibited redox cycling by 2,3-dimethyl- and 2,3-dimethoxy-1,4-naphthoquinone, but not that of 2-hydroxy-, 5-hydroxy- or 2-amino-1,4-naphthoquinone. Inhibition of redox cycling by a combination of DT-diaphorase and SOD is therefore not applicable to all naphthoquinone derivatives, and when it does occur, it may be overwhelmed at high quinone concentrations, and it may not operate under slightly alkaline conditions or in the presence of tissue components capable of initiating hydroquinone autoxidation.

Reactivity of ubiquinones and ubiquinols with free radicals

The reactivity of quinones 1-4 and of the corresponding quinols 5-8 towards carbon- and oxygen-centred radicals were studied. All quinones bearing at least one nuclear position free, readily react with alkyl and phenyl radicals to afford the alkylated quinones 12-24; however, quinones 1 and 3 reacted with 2-cyano-2-propyl radical to yield products (the mono- and di-ethers 9-11) derived from the attack on the carbonylic oxygen. The reactions carried out on quinones with the benzoyloxy radical led to no reaction products and in the case of Q10, the isoprenic chain also remained unchanged. Quinols 5-8 reacted only with oxygen-centred radicals (benzoyloxy and 2-cyano-2-propyl-peroxy radicals) to give the corresponding quinones. The isoprenic chain of Q10 did not undergo attack even with peroxy radicals. Carbon-centred radicals resulted unable to abstract hydrogen from the studied quinols.

Heterocyclic ortho-quinones, a novel type of Photosystem II inhibitors

Members of the new chemical class of 7-substituted 6-bromo-benzo[4,5]imidazo[1,2alpha]pyridin-8,9-diones were found to be excellent inhibitors at the Q(B) site of the photosystem II D1 reaction center protein. The best inhibitors with pI(50)-values of >7 are: dimethyl-propyl, 7.05; i-pentyl, 7.36; t. butyl, 7.47; and i-propyl, 7.51. Displacement experiments with [14C]atrazine revealed that the 8,9-diones behave non-competitively in respect of Photosystem II herbicides and, hence, have to be considered as a new type of Photosystem II inhibitors. This notion is further corroborated by their inhibitory activity in D1 mutants of Chlamydomonas reinhardtii.

Effects of modulation of tissue activities of DT-diaphorase on the toxicity of 2,3-dimethyl-1,4-naphthoquinone to rats

The enzyme DT-diaphorase mediates the two-electron reduction of quinones to hydroquinones. It has previously been shown that the toxicity of 2-methyl-1,4-naphthoquinone to rats is decreased by pre-treatment of the animals with compounds that increase tissue levels of this enzyme. In contrast, the severity of the haemolytic anaemia induced in rats by 2-hydroxy-1,4-naphthoquinone was increased in animals with high levels of DT-diaphorase. In the present experiments, the effect of alterations in tissue diaphorase activities on the toxicity of a third naphthoquinone derivative, 2,3-dimethyl-1,4-naphthoquinone, has been investigated. This compound induced severe haemolysis and slight renal tubular necrosis in control rats. Pre-treatment of the animals with BHA, a potent inducer of DT-diaphorase, diminished the severity of the haemolysis induced by this compound and abolished its nephrotoxicity. Pre-treatment with dicoumarol, an inhibitor of this enzyme, caused only a slight increase in the haemolysis induced by 2,3-dimethyl-1,4-naphthoquinone, but provoked a massive increase in its nephrotoxicity. Modulation of DT-diaphorase activity in animals may therefore not only alter the severity of naphthoquinone toxicity, but also cause pronounced changes in the site of toxic action of these substances. The factors that may control whether induction of DT-diaphorase in animals will decrease or increase naphthoquinone toxicity are discussed.

Autoxidation of naphthohydroquinones: effects of pH, naphthoquinones and superoxide dismutase

The rates of autoxidation of a number of pure naphthohydroquinones have been determined, and the effects of pH, superoxide dismutase (SOD) and of the parent naphthoquinone on the oxidation rates have been investigated. Most compounds were slowly oxidised in acid solution with the rates increasing with increasing pH, although 2-hydroxy-, 2-hydroxy-3-methyl- and 2-amino-1,4-naphthohydroquinone were rapidly oxidised at pH 5 and the rates of oxidation of these substances were comparatively unresponsive to changes in pH. At pH 7.4, autoxidation rates decreased in the order 2,3-dichloro-1,4-naphthohydroquinone > 5-hydroxy > 2-bromo > 2-hydroxy-3-methyl > 2-amino > 2-hydroxy > 2-methoxy > 2,3-dimethoxy > 2,3-dimethyl > 2-methyl > unsubstituted hydroquinone. The autoxidation rates of the alkyl, alkoxy, hydroxy and amino derivatives were decreased in the presence of SOD, but this enzyme had no effect on the rate of autoxidation of the 2,3-dichloro and 2-bromo derivatives while that of the 5-hydroxy derivative was increased. The rates of autoxidation of all compounds except the halogen derivatives and 5-hydroxy-1,4-naphthohydroquinone were increased by addition of the parent naphthoquinone, and quinone addition partially or completely overcame the inhibitory effect of SOD. There is evidence that the reduction of quinones to hydroquinones in vivo may lead either to detoxification or to activation. This may be due to differences in the rate or mechanism of autoxidation of the hydroquinones that are formed, and the data gained in this study will provide a framework for testing this possibility.

Inhibition of 2,3-dimethyl-1,4-naphthohydroquinone auto-oxidation by copper and by superoxide dismutase

2,3-Dimethyl-1,4-naphthohydroquinone undergoes auto-oxidation to the corresponding quinone at pH 7.4, with stoichiometric consumption of oxygen and formation of hydrogen peroxide. In an unpurified buffer, the rate of oxidation was low, but it increased nearly 9-fold when trace metals were removed from the buffer by treatment with Chelex resin. A similar increase in rate was achieved by addition of DTPA or bathophenanthroline sulfonate to unpurified buffer, whereas EDTA and desferal were less effective. Addition of copper to purified buffer led to inhibition of oxidation, with a 50% decrease in rate being observed at a metal concentration of 7.1 nM, and it is likely that the low auto-oxidation rate recorded in unpurified buffer was due to copper contamination of the latter. The auto-oxidation of 2,3-dimethyl-1,4-naphthohydroquinone was exceptionally sensitive to inhibition by superoxide dismutase, with a concentration of only 4.5 ng/ml being sufficient for a 50% decrease in rate, and the inhibitory effect of copper may be due to the ability of this metal to catalyse the dismutation of superoxide. Previous studies have shown that the rates of auto-oxidation of 1,4-naphthohydroquinone and 2-methyl-1,4-naphthohydroquinone are influenced by copper contamination of buffer and the present study shows that this is also true for a di-substituted naphthohydroquinone. For accurate assessment of rates of naphthohydroquinone auto-oxidation, it is important that purified buffers or appropriate chelating agents, are employed.

Manifestations of Bridgehead-Bridgehead Interactions in the Bicyclo[1.1.1]pentane Ring System

A series of 3-halo-substituted bicyclo[1.1.1]pentane-1-carboxylic acids 1 (Y = COOH; X = F, Cl, Br, I, and CF(3)) as well as the parent compound 1 (Y = COOH, X = H) have been prepared, and a study of some of their properties have been made. It was found that their reactions with xenon difluoride cover a wide range of reactivities. On one hand, the fluoro acid 1 (Y = COOH, X = F) displayed no apparent reaction at all while, on the other, the bromo acid 1 (Y = COOH, X = Br) and parent compound 1 (Y = COOH, X = H) underwent ready reaction with complete disintegration of the ring system. A possible explanation is advanced based on polar kinetic and thermodynamic effects governing the lifetime of an intermediate acyloxy radical species. The relative ease of oxidation of the carboxylates 1 (Y = COO(-); X = H, F, Cl, Br, I, CF(3), and COOCH(3)), as mirrored by their peak oxidation potential values (E(p)) determined by cyclic voltammetry, also covers a wide range. These data coupled with the dissociation constants (pK(a)) of some of the acids 1 (Y = COOH; X = H, F, Cl, and CF(3)) reflect significantly on the modes of transmission of electronic effects acting through the bicyclo[1.1.1]pentane ring system.