Quinone Constitution

As presented in Table II, no quinones are obtained with NADPH for dibenz[a,h]anthracene and benz[a]anthracene, whereas with cumene hydroperoxide a trace amount of benz[a]anthracene quinone is observed. For the PAH with low IP, quinones are formed in the presence of both cofactors. The relationship between IP and formation of quinones constitutes further evidence that these metabolites are obtained by an initial one-electron oxidation of the PAH with formation of its radical cation. 

Quinones constitute one of the most easily reducible systems. They can be reduced to hydroquinones, to ketones, to diols, or to hydrocarbons. 

ABSTRACT Quinones constitute a structurally diverse class of phenolic compounds with a w ide range of pharmacologial properties, which are the basis for different applications in the broad field of pharmacy and medicine. In traditional medicine all over the world, plants which are rich in quinones are used for the treatment of a variety of diseases. Besides the classical applications of these plants in industry (dyestuffs) and pharmaceutical (laxatives) practice, the relatively new field of biologically active quinones will be discussed. This review gives an account of the work done on naturally occurring bioactive quinones from 1992 to the present date. The biological activity detected in quinones from natural and synthetic sources has been discussed in relation to chemical structure under the respective titles. 

Quinones constitute an extremely important group of natural products. They are one of the oldest known classes of compounds, have achieved importance as dyestuffs, and in addition possess many different biological activities many quinones are active as anti-... 

Quinones constitute a unique class of carbonyl compounds. They are cyclic conjugated diketones. The simplest example is 1,4-benzoquinone (eq. 7.44). All quinones are colored, and many are naturally occurring pigments that are used as dyes. Alizarin is an orange-red quinone that was used to dye the red coats of the British army during the American Revolution. Vitamin K is a quinone that is required for the normal clotting ofblood. 

Organic phase enzyme electrodes (OPEEs) have attracted considerable interest for their applications in environmental and clinical monitoring. The use of organic solvents facilitates, indeed, the detection of compounds poorly soluble in water, prevents microbial contamination, and may circumvent side reactions leading to enzyme deactivation or electrode fouling. Among the various enzymes successfully applied to the fabrication of OPEEs, polyphenol oxidase (PPO) was widely used because Kazandjian and Klibanov demonstrated, 30 years ago, the possibility for this enzyme to work in chloroform. Thanks to its excellent activity in both aqueous and organic solvents, this enzyme which catalyzes the oxidation of phenols and o-diphenols to o-quinone constitutes a convenient enzyme model for the concept and the development of new procedures of OPEE construction. 

Reactions of quinones with radicals have been explored, and alkylation with diacyl peroxides constitutes an important synthetic tool (68). Although there are limitations, an impressive range of substituents can be introduced in good yield. Examples include alkyl chains ending with functional groups, eg, 50% yield of (70) [80632-67-3] (69,70). 

Tile constitution of the hydrociiioride is doulitfui but the so-cailed gi/i/w/wi/l structure, by which the substance is lepre-sented as a derivative of quinone, is generally accepted,... 

Of more intrinsic interest are processes involving two electrons, since these constitute the great bulk of organic reductive processes, especially in protonic solvents. The reason for this is that the first electron transfer will generate an unstable radical species, whereas the second can regenerate a stable closed-shell product. One example that we have already encountered is C02 reduction but a case that has been well studied is that of quinone reduction as ... 

The bleaching powder reaction is peculiar to aniline the dye which is formed is a complex quinone derivative the constitution of which is not yet known with complete certainty. The other experiments, about to be described, are group reactions for primary aromatic amines. 

Compounds [54] and [55] have been shown to complex group 1 and 2 metal cations and also ammonium and alkylammonium cations by nmr and UV/Vis spectroscopies and also by a number of solid-state X-ray crystallographically determined structures. The quinone moieties in these molecules constitute not only the coordination site but also the redox-active centre. The complexation... 

A rather difficult double Wittig reaction (Eq. 3.24) has been effected with enhanced efficiency under sonication [123], The process constitutes a novel type of annelation of an aromatic ring when applied to o-quinones. 

Hydroxybenzenediazonium tetrafluoroborates constitute a special case the meta-isomer has the structure of a true diazonium salt, while the ortho- and para-isomers are in ready equilibrium with the corresponding quinone-diazide dimers which have been isolated as com-... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Tanshen (Salvia miltiorrhiza Bung), a medicinal plant, has been used in traditional Chinese medicine for its tranquilizing, sedative, circulation-promoting and bacteriocidal effects. [73], It has proven to be a rich source of abietane o-quinone diterpenoids. Miltirone (197) is a tricyclic diterpenoid quinone which has been isolated from the roots of salvia miltiorrhiza Bung. The isolation of miltirone constitutes a new addition to naturally occurring quinines related to tanshinones [74,75] isolated from the same source. 

Human rhino viruses (HRVs) are the single most significant causative agents of the common cold. HRV 3C-protease, a cysteine protease, plays a critical role in the replication cycle of HRVs and thus constitutes a potential therapeutic target for the control of HRVs and common cold. Singh et al established the structure-activity relationships of various naphthoquinones as HRV 3C-proteasa inhibitors and compared the activities of ortho- versus -para- quinones. The results indicated that the mode of action of the two classes of compounds is different [207]. 

Study of the oxidation chemistry of exifone (25a) constitutes a perfect illustration of the above statement [59, 60]. This pyrogallol benzophenone (Adlone ) was commercialized in France for the treatment of cognitive disorders in the elderly, but it was withdrawn from the market because of its hepatotoxicity, which was attributed to its 3,4-quinone metabolite 26a. Fleury and co-workers found that blocking of 26 in the form of l,4-benzoxazin-8-one ortho-quinols such as 28 led to a twenty-fold decrease in toxicity, together with a five-fold activity enhancement (Figure 9) [59, 60]. 

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