Quinones system

Quinonoid compounds have been thoroughly reviewed (4,6). More recent trends in quinone addition and substitution chemistry were reviewed in 1993 (7). The quinone system in natural products has also been covered (8) there has been a fascinating discussion of the quinone problem (9). 

A. Lipski, S. Klatte, B. Bendinger, and K. Altendorf, Differentiation of gram-negative, non-fermentative bacteria isolated from biofilters on the basis of fatty acid composition, quinone system, and physiological reaction profile, Appl. Environ. Microbiol. 58 2060 (1992). 

M. Antolovich, P. J. Keyte, A M. Oliver, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, S. A Jonker, J. M. War-man, Modelling Long-Range Photosynthetic Electron Transfer in Rigidly Bridged Porphyrin-Quinone Systems ,/. Phys. Chem. 1991, 95,1933-1941. 

Isoviolanthrone (109) is an highly anellated polycyclic quinone system. It is derived from the chemical structure of isodibenzanthrone, which may be visualized as being obtained by unsymmetrical condensation of two benzanthrone molecules. The compound itself affords an intense blue shade. 

Investigation of photoinduced intramolecular ET processes in such systems has in general shown that, compared to analogous porphyrin/quinone systems, i. charge separation occurs with higher efficiency and ii. charge-separated states have longer lifetimes (Scheme 9.4). Such a behavior is the result of the combination of two... 

ABSTRACT This chapter will focus on three classes of naturally occurring compounds that we have considered as beginning points for mechanistic studies in drug design. Each compound class is dependent for its biological activity on the presence of the redox-sensitive catechol-o-quinone system. 

Early workers recognized some of the similarities between isatogens and quinones.5 They described 2-(2-pyridyl)isatogen as the first example of a me/a-quinone system.93 However the suggestion that methyl... 

Modelling of the intermediate step of the charge separation process during photosynthesis with porphyrin-quinone systems... 

A number of covalently bound porphyrin-quinone systems have recently been synthesized as models for carefully spaced donor-acceptor systems. These are designed to test the possibility of controlling electron transfer rates by spatial separation of donor-acceptor pairs. Among these exceedingly clever studies (30) are molecules designed to separate the donor and acceptor by rigid insulating molecules, for example, 2 (31),... 

Another rare kind of 6-electron ionic cycloaddition is that between a pentadienyl cation and an alkene. A telling example is the key step 2.66 — 2.67 in a synthesis of gymnomitrol 2.68, where the nature of the pericyclic step is heavily disguised, but all the more remarkable for that. Ionization of the acetal gives the cationic quinone system 2.66. That this is a pentadienyl cation can be seen in the drawing of a canonical structure on the left, with the components of the pericyclic cycloaddition emphasized in bold. Intramolecular [4+2] cycloaddition takes place, with the pentadienyl cation as the 4-electron component and the cyclopentene as the 2-electron component. Th is reaction is an excellent example of how a reaction can become embedded in so much framework that its pericyclic nature is obscured. 

The first reaction converts protected hydroquinone system 12 into a yj-quinone system. Should reducing or oxidizing conditions be selected ... 

The determination of the strength of the Lewis acids MF , has been carried out in various solvents using the conventional methods. Numerous techniques have been applied conductivity measurements, cryoscopy, aromatic hydrocarbon extraction,53,84 solubility measurements,85-87 kinetic parameters determinations,52,88,89 electroanalytical techniques (hydrogen electrode),90-93 quinones systems as pH indicators,94-97 or other electrochemical systems,98 99 IR,100,101 and acidity function (//,) determinations with UV-visible spectroscopy,8 9 14 19 102-105 or with NMR spectros-copy.20-22,44-46,106-108 Gas-phase measurements are also available.109-111 Comparison of the results obtained by different methods shows large discrepancies (Table 1.2). 

Modelling of the Intermediate Step of the Charge Separation Process During Photosynthesis with Porphyrin-Quinone Systems... 

The list of publications in the obituary of the organic chemist Otto Dimroth (1872-1940)139 has eleven papers that are classified as physikalisch-organische Chemie. They are mainly about tautomerism, intramolecular rearrangements, and solvent effects on tautomeric equilibria. One paper,140 published in 1933, deals with relationships between reaction velocities and oxidation-reduction potentials for quinone systems and it is evident that Dimroth was a pioneer in developing linear free-energy relationships. He was Professor in Wurzburg from 1918 to 1940. 

In these devices, the acceptor is joined to two donors which can in principle transfer electrons to it with comparable efficiencies. From the point of view of long-lived charge separation, molecules of this type offer no apparent advantages over simple D-A systems. However, as mentioned above, they may be useful in some other situations, especially as components of larger devices when A is a 2-electron acceptor. There are a few examples of porphyrin-quinone systems with this structure in the literature. 

Treatment of indoles with 2,5,8-quinolinetriones in the presence of a catalytic amount HC1 provided the 3-vinylindole derivatives 48a and 48b, which could be transformed to polyheterocyclic quinone systems through Diels-Alder reactions (Scheme 14) [65]. 

The surface-confined Fc+/Fc system behaves ideally (34) at all values of pH investigated (0-10). The surface-confined Q/QH2 is not ideal, in that there is a large difference in the potential for the anodic and cathodic current peaks. Such behavior is well documented for other quinones (35-36). Despite the non-ideality, the effect of pH on the electrochemical response of Q/QH2 is reproducible. Both the anodic and cathodic current peaks for the quinone system shift to more positive potentials at lower pH. 

The electrochemical properties of (40)-(47) in the presence and absence of stoichiometric amounts of Na+ and K+ guest cations were investigated in acetonitile solution by cyclic voltammetry. Table VI shows that addition of alkali metal salt in 1 1 molar ratio produces anodic shifts (AE) in the original redox couple of 40-320 mV in the reduction potentials of the respective host s molybdenum redox center. Comparing (45)-(47) with the organic redox-active quinone systems described earlier (see Table I), in the case of Na+ guest cation these AE... 

As noted above, a possible objection to our assignment of the excited-state reaction to H-atom transfer in these perylene quinone systems is the observation of mirror image symmetry between the absorption and the emission spectra, which indicates minimal structural changes between the absorbing and the emitting species. Our first attempt to explain this symmetry was to suggest that the... 

The reaction of sulfinyl naphthoquinones 114a-c with cyclopentadiene afforded mixtures of two exo-sulfinyl adducts (the endo-orientating character of the quinonic system is clearly predominant). Compound exo(c)-115 was the major product in CH2C12 at -20 °C (de ranged between 80 and 90% depending on the dienophile). The 7r-facial selectivity of the process was reversed in the presence of ZnBr2, exo(t) 115 becoming predominant or exclusive [110]. These catalyzed cycloadditions required shorter reaction times and took place with... 

Formal potentials can be defined on different levels of conditions Thus the formal potential of the -> quinhydrone electrode may be defined (I) as including (a) the standard potential of the hydroquinone di-anion/quinone system, (b) the two acidity constants of the hydroquinone, and (c) the activity coefficients of the hydroquinone dianion and quinone, or, (II), it may also include (c) the pH value. In the latter case, for each pH value there is one formal potential, whereas in the first case one has one formal potential for all pH values, and an equation describing the dependence of the electrode potential as a function of that formal potential and the individual pH values. Formal potentials are strictly thermodynamic quantities, and no kinetic effects (e.g., by electrochemical -> irreversibility) are considered. 

An exceptional case of through-bond coupling where the distance dependence has been demonstrated is the electron transfer between singlet porphyrin and an electron-accepting quinone in a series of rigid porphyrin-bicyclo[2.2.2]octane-quinone systems [58] ... 

Dimethyl-5,8-quinoxalinedione gives two reversible polaro-graphic waves at 0.06 and 0.76 volt (SCE), at pH 7. The first wave corresponds to a reduction of the -quinone system to 2,3-dimethyl-5,8-dihydroxyquinoxaline, whereas the second reduction takes place in the heterocyclic ring and leads to the 1,4-dihydroquinoxaline.241... 

In most of the quinone systems we have studied, the triplet reacts not only with the added phenol but also with the isopropanol in the solvent to generate polarized radicals. This can be... 

In the studies of the synthesis of the ansamycin antibiotic rifamycin S (13S), Corey and Clark [76] found numerous attempts to effect the lactam closure of the linear precursor 132 to 134 uniformly unsuccessful under a variety of experimental conditions, e.g. via activated ester with imidazole and mixed benzoic anhydride. The crux of the problem was associated with the quinone system which so deactivates the amino group to prevent its attachment to mildly activated carboxylic derivatives. Cyclization was achieved after conversion of the quinone system to the hydroquinone system. Thus, as shown in Scheme 45, treatment of 132 with 10 equiv of isobutyl chloroformate and 1 eqtuv of triethylamine at 23 °C produced the corresponding mixed carbonic anhydride in 95% yield. The quinone C=C bond was reduced by hydrogenation with Lindlar catalyst at low temperature. A cold solution of the hydroquinone was added over 2 h to THF at 50 °C and stirred for an additional 12 h at the same temperature. Oxidation with aqueous potassium ferricyanide afforded the cyclic product 134 in 80% yield. Kishi and coworkers [73] gained a similar result by using mixed ethyl carbonic anhydride. 

Antibiotic-683 (210) [152] reveals an unusual OH residue at C-4 and is therefore related to elmycin E and X-14881 D [3]. The nearly completely saturated ring C is probably a result of biosynthetic redox reactions leading to an epoxide ring between C-6 a and C-12 a and a reduced quinone system (see also elmycin C). Structure 210 is produced by Streptomyces sp. strain Y-83,30683 and shows antibacterial and extraordinary anticancer (mouse leukemia l-1210, ICjq 3 g ml ) activities. Another recent relative of 210 and elmycin C is angucyclinone D 211,... 

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