Other Quinones

Other Quinones. The isoprenoid nature of the C30 side-chain of caldariellaquinone (161) has been established by incorporation studies with [ Cjacetate.  

This chapter reviews the year s published work on physical and analytical aspects of steroid chemistry. No attempt has been made to survey the enormous number of routine applications of spectroscopic methods to structure determination. Attention has been concentrated mainly upon those developments of a fundamental nature which increase our understanding of the physical techniques and the phenomena which they explore. The major advances reported this year in the area of spectroscopy lie in the interpretation and applications of Cn.m.r. tritium n.m.r. has made its appearance as a method for the analysis of labelled steroids. The short sections on analytical methods give the Reviewer s selection of significant advances in radioimmunoassay and chromatographic methods of interest to chemists. 

The deformed-chair conformation of ring A in a 4,4-dimethyl-3-oxo-5or-steroid has been confirmed by an X-ray study of a 17 -benzoyloxyandrostane derivative. The results agree with those of an earlier study of the 17-iodoacetate, and with the geometry indicated by force-field calculations. Dipole moments calculated by the application of molecular mechanics to 5a-androstane-3,17-dione and its distorted 4,4-dimethyl derivative are larger than those observed the reasons for these deviations are not yet clear. 

The reported (in 1969) isolation of two stable rotamers of 20-methyl-20-(2-hydroxyethoxy)pregn-5-ene-3j8,17a-diol (4) is contradicted by AT-ray analysis of the diacetate of the major rotamer , which shows it to be the D-homoandrostane derivative (5).  

evidence has indicated that vitamin D3 exists in solution as a mixture of the alternative chair conformers in ring A. X-Ray data now show that the same is true of the crystalline vitamin, which comprises an equimolar ratio of the con-formers with 3-OH equatorial and axial, respectively. Strong hydrogen-bonding between hydroxy-groups results in an extended helix of molecules about an axis 

When p-benzoquinone or toluquinone are treated with nickel carbonyl in the presence of cycloocta-l,5-diene only Ni(II)-hydroquinone or quin-hydrone salts, respectively, are formed. When the somewhat less strongly oxidizing 2,5- or 2,6-dimethyl quinones are treated with nickel carbonyl under the same conditions cycloocta-l,5-diene-dimethylquinone-Ni(0) complexes are obtained. These substances resemble their duroquinone 

C=0 Stretching Frequencies and Decomposition Points of Cycloocta-1,5-diene-Nickel Complexes of Some Quinones  

It is worth mentioning that the first transition metal complex of a natural product was obtained by treating tocopherylquinone (vitamin-E-quinone) with nickel carbonyl in the presence of cycloocta-1,5-diene. A study of its physiological properties promises to be interesting (59). 

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Volatility in steam. Add about 0 1 g. of benzoquinone to 3 ml. of water in a test tube and boil gently. The benzoquinone dissolves to give a yellow solution, which rapidly darkens in colour. Note the irritating and characteristic odour of benzoquinone which has volatilised in the steam. Also given by />-toluquinone and 1,4 naphthoquinone but not by the other quinones mentioned above. 

In the eadier Hterature, the 1,4,5,8 positions were known as alpha (a) and the 2,3,6,7 as beta (P). The 9,10-positions are known as meso- or ms-. Other quinones of anthracene are known, but only the 9,10-quinone is of technical importance. 

The procedure described is essentially that of Belleau and Weinberg and represents the only known way of obtaining the title compound. One other quinone acetal, 1,4,9,12-t6traoxadispiro[4.2.4.2]tetradeea-6,13-diene, has been synthesized by a conventional method (reaction of 1,4-cyclohexanedione with ethylene glycol followed by bromination and dehydrobromination ) as well as by an electrochemical method (anodic oxidation of 2,2-(l,4-phenylenedioxy)diethanol ). Quinone acetals have been used as intermediates in the synthesis of 4,4-dimethoxy-2,5-cyclohexadienone,. syw-bishomoquinone, - and compounds related to natural products. ... 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

Scheme 3.34 Adduct formation with other quinones.

What distinguishes QR1 and QR2 from other quinone-reducing enzymes, such as cytochrome P450, is that they are a direct two-electron reductant, i.e., quinone is reduced... 

Over the years, there have been numerous reports of oxidase preparations that contain polypeptide components, additional to those described above. As yet no molecular probes are available for these, and so their true association with the oxidase is unconfirmed. There are many reports in the literature describing the role of ubiquinone as an electron transfer component of the oxidase, but its involvement is controversial. Quinones (ubiquinone-10) have reportedly been detected in some neutrophil membrane preparations, but other reports have shown that neither plasma membranes, specific granules nor most oxidase preparations contain appreciable amounts of quinone, although some is found in either tertiary granules or mitochondria. Still other reports suggest that ubiquinone, flavoprotein and cytochrome b are present in active oxidase preparations. Thus, the role of ubiquinone and other quinones in oxidase activity is in doubt, but the available evidence weighs against their involvement. Indeed, the refinement of the cell-free activation system described above obviates the requirement for any other redox carriers for oxidase function. 

Other Quinones.—Look up o-quinones and the three naphthoquinones the best method of preparing a- and / -naphthoquinone is froml 4-andl 2-aminonaphthol respectively, which are obtained from azo-dyes of the two naphthols by reduction (see p. 302). 

It has been found that AAs do not react with 3,4-disubstituted o-benzo-quinones in the expected Strecker degradation reaction instead, a decar-boxylative condensation reaction afforded the corresponding benzoxazoles. A mechanistic explanation has been advanced for this transformation. It should be mentioned, however, that other quinones or diones did not react in the described manner (78JOC509). 

Veratryl Alcohol. Leisola et al. (21,22) recently reported that treatment of veratryl alcohol 45 with lignin peroxidase resulted mainly in the formation of veratryl aldehyde 49, the two 7-lactones 46 and 47 (Fig. 10) and several other quinones not shown. We (23,24) have established that the... 

Other Quinone Types. Although past researchers have examined many coloured quinonoid molecules very few have achieved commercial importance. 

The checkers noted that use of a paper thimble resulted in increased time for extraction. The submitters recommend use of a glass thimble, since prolonged heating can lead to lower yields. It is easier to determine when extraction is complete with a transparent thimble. Other quinones may require longer extraction periods. 

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

Other quinones and and poly phenolic GSH conjugates are also toxic, such as benzohydroquinone. In the latter case, it is the triglutathionyl conjugate of benzohydroquinone that is the most toxic. 

PQQ and the other quinone prosthetic groups described here all function in reactions that would be possible for pyridine nucleotide or flavin coenzymes. All of them, like the flavins, can exist in oxidized, half-reduced semiquinone and fully reduced dihydro forms. The questions to be asked are the same as we asked for flavins. How do the substrates react How is the reduced cofactor reoxidized In nonenzymatic reactions alcohols, amines, and enolate anions all add at C-5 of PQQ to give adducts such as that shown for methanol in Eq. 15-51, step a 444,449,449a Although many additional reactions are possible, this addition is a reasonable first step in the mechanism shown in Eq. 15-51. An enzymatic base could remove a proton as is indicated in step b to give PQQH2. The pathway for reoxidation (step c) might involve a cytochrome b, cytochrome c, or bound ubiquinone.445 446... 

Syntheses of quinones often involve oxidation, because this is the only completely general method. Thus, in several instances, quinones are the reagents of choice for the preparation of other quinones. Oxidation has been especially useful with catechols and hydroquinones as starling materials. The preparative utility of these reactions depends largely on the relative oxidation potentials of the quinones. 

While in DNA, ET mainly occurs from G (preferentially from GG sites, cf. Ito et al. 1993 Saito et al. 1995, 1997 Saito and Takayama 1995 Sugiyama and Saito 1996 Ly et al. 1996 Gasper and Schuster 1997 Nakatani et al. 1998 Kino and Saito 1998), especially with intercalated quinones, non-selective DNA cleavage can occur with free H-abstracting quinones (Breslin and Schuster 1996). It may be mentioned here that 1,4-benzoquinone, and some other quinones, undergo a reaction with water (for details see von Sonntag et al. 2004), and these competitive reactions may have to be taken into account when using quinones as photosensitizer. 

Some other quinone derivatives, although formed from the same pathway, are produced by dimethylallylation of 1,4-dihydroxynaphthoic... 

Pyrroloquinoline quinone, and probably also the other quinone cofactors, can be considered to be medium-potential oxidants (Em PQQ/PQQH2, 0.09 V (NHE) aq., pH 7), as distinguished from high-potential quinones that are of use as oxidants in organic synthesis [13], In addition, PQQ will resist competing nucleophilic 1,4-additions of organic reductants as is observed with less heavily substituted quinones. 

A simple and effective chemical method was developed for quantitatively reducing quinones, based on their reaction with metallic zinc and zinc ions [248]. Comparison of this method with conventional electrochemical reduction [249-252] revealed the chemical method to be considerably superior. A reduction reaction of vitamin Kj and other quinones in the presence of Zn° and Zn2+ eliminates the need to apply large negative potentials and may also be performed in the absence of any applied electrochemical potential. Some quinones used, such as UQ-10, menadione, and vitamin K, of the menaquinone series (MKs 4-10) could all be reduced to their corresponding hydroquinones in these conditions. 

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. 

Calculations performed at the HF/3-21G level indicated smaller energy gaps between the HOMOs of the aforementioned electron-rich dienophiles and the LUMOs of the quinone ketals, as can be expected for inverse electron-demand Diels-Alder reactions under FMO control [141]. Regiochemical controls observed with quinone ketals such as 76a were well corroborated by the relative magnitudes of the atomic coefficients of the frontier orbitals. The highest coefficients at C-5 of the quinone ketal LUMO and at C-2 of the electron-rich alkenes would indeed promote bond formation between these centers. The results of calculations on other quinone ketals were, however, rather vague [141]. 

Quinones have been extensively used for aromatization reactions in addition to the dehydrogenation of steroidal ketones and lactones. Interestingly, whereas chloranil (29) and a number of other quinones oxidize steroidal 4-ene-3-ones (32) selectively to 4,6>dienones (34),DDQ (28) results only in the formation of the 1,4-dienone (36 Scheme 21).This divergent b vior is best exfdained by the intermediacy of the kinetic enolate (35) in the case of the higher potential DDQ, but of the thermodynamic enolate (33) in the case of the less reactive quinones.Addic conditions need to be avoided if the cross-conjugated ketone (36) is the desired inoduct since under these conditions the 3,5-dienol (33) becomes both the kinetic and the thermodynamic enol, resulting only in the formation of the linear di-enone." ... 

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