Benzoquinone derivatives

I.I.3. From 1,4-Benzoquinone Derivatives with Ethane-1,2-diamine... 

Benzo(b)fIuoranthrene la 39, 85 Benzo(k)fluoranthrene la 39, 86 Benzo(ghi)perylene la 39,85 Benzophenone derivatives lb 282 Benzo(a)pyrene la 39,85,103 p-Benzoquinone derivatives la 72 Benzothiazoles lb 237 Benzoyl chloride la 70 Benzoylecgonine lb 32,34,35 3,4-Benzpyrene la 60 Benzthiazide lb 188 Benztriazole, 2-(2-hydroxy-5-methyl-phe-nyl)- la 282... 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

A polyprenylated benzoquinone derivative that is a key component in mitochondrial electron transport, and the... 

An interesting entry to functionalized dihydropyrans has been intensively studied by Tietze in the 1990s using a three-component domino-Knoevenagel Hetero-Diels-Alder sequence. The overall transformation involves the transient formation of an activated heterodienophile by condensation of simple aldehydes with 1,3-dicarbonyls such as barbituric acids [127], Meldrum s acid [128], or activated carbonyls. In situ cycloaddition with electron-rich alkenes furnished the expected functionalized dihydropyrans. Two recent examples concern the reactivity of 1,4-benzoquinones and pyrazolones as 1,3-dicarbonyl equivalents under microwave irradiation. In the first case, a new three-component catalyst-free efficient one-pot transformation was proposed for the synthesis of pyrano-1,4-benzoquinone scaffolds [129]. In this synthetic method, 2,5-dihydroxy-3-undecyl-1,4-benzoquinone, paraformaldehyde, and alkenes were suspended in ethanol and placed under microwave irradiations to lead regioselectively the corresponding pyrano-l,4-benzoquinone derivatives (Scheme 38). The total regioselectivity was... 

Scheme 38 Three-component synthesis of pyrano-l,4-benzoquinone derivatives...

Scheme 39 Example of heptacyclic bis-pyrano-l,4-benzoquinone derivative...

This sequence has been successfully extended to the regioselective multicomponent construction of bis- pyrano-l,4-benzoquinone derivatives when unsubstituted 2,5-dihydroxy-l,4-benzoquinone is used (Scheme 39) [130], Depending on the alkene moiety, the reaction yielded only the linear tri-, penta-, or heptacyclic product in a 1 1 diasteromeric ratio, as illustrated with indene. 

The intramolecular nitrile oxide-alkene cycloaddition sequence has been used for the assembly of a great variety of natural products. A target that has received special attention is that of taxol (156), undoubtedly due to its unique structural features, its potent anticancer activity, and its hmited availability from natural sources (318,319). In 1984 Kozikowski et al. found that the treatment of nitro dienone 158 (obtained from the p-benzoquinone derivative 157) with p-chlorophe-nyl isocyanate under Mukaiyama conditions afforded the unexpected eight-mem-bered ring 159, which is related to ring B of taxol (156) (Scheme 6.79). 

In the ESR spectra of some related trimethylsilyl benzoquinone derivatives, as with the ketyls, the spin density in the quinone ring increases for trimethylsilyl substitution and decreases for alkyl substitution, consistent with the electron-accepting ability of the trimethylsilyl group. This ability is also manifested in the reduction potentials of the compounds (65). The ESR data for the trimethylsily ketyls and for the trimethylsilyl benzoquinone anion radicals are summarized in Table IX. 

Fluorinated phenols generally undergo oxidation to cyclohexadienone and benzoquinone derivatives, and to the products of ring rearrangement and ring degradation. 

Solutions of the nitrosodisulfonate salts are most stable in weakly alkaline solutions (pH 10) and decompose rapidly when the solution is acidic or strongly alkaline.3 The solid dipotassium nitrosodisulfonate (Fremy s salt) has been reported to decompose spontaneously2,3 suggesting that procedures involving the use of substantial quantities of the dry solid salt may be hazardous. In the present procedure, separation and use of the solid salt is avoided since the disodium nitrosodisulfonate is formed and used in aqueous solution. In this procedure, 2 moles of the preformed nitrosodisulfonate salt are consumed in the oxidation of one mole of the phenol to the benzoquinone derivative.8 The submitters report that only one molar equivalent of the nitrosodisulfonate salt is required if the electrochemical oxidation is carried out in the presence of a heptane solution of the phenol. 

Such a second-order dependence of kel on [HC104] has also been reported for the acid-catalyzed electron transfer from cis-diethylcobalt(III) complex, c/s-[Co(bpy)2]+, to p-benzoquinone derivatives in H20-EtOH (5 1 v/v) under high acidic conditions such that two t otons are involved for the one-electron reduction of p-benzoquinone derivatives [45] ... 

FIGURE 5.26 pH-rate constant profile for the specific acid-base catalyzed hydrolysis of a water-soluble diaziridinyl benzoquinone derivative. [Graph reconstructed from data by Jain et al., AAPS National Meeting, Paper No. 2268, Indianapolis, IN, Oct. 29, 2000.]... 

Coenzyme Q (alternatively known as ubiquinone or CoQ) is a benzoquinone derivative with a long hydrocarbon side chain made up of repeating isoprene units. The number of units generally... 

Imaeda, K., Enoki, T., Mori, T., Inokuchi, H., Sasaki, M., Nakasuji, K. and Murata, I. (1989). Electronic properties of new organic conductors based on 2,7-bis(methylthio)-l,6-dithiapyrene (MTDTRY) with TCNQ and / -benzoquinone derivatives. Bull. Chem. Soc. Jpn, 62, 372-9. [191]... 

The simplest substance which can act as a catalyst in the electron transfer reduction of an electron acceptor may be a proton (C = H" "), since the radical anion of an electron acceptor (A ) becomes a much stronger base as compared with the neutral form (A). The substrates first described here are / -benzoquinone derivatives (Q), since the redox and acid-base properties of Q and the reduced forms (Q and as the one-electron and two-electron reduced form, respectively) have well been established and they exhibit important thermodynamic parameters in biological redox systems [75, 76], The variations of the reduction potentials with pH are governed by the acid-base properties of the reduced species. Semiquinone radical anion (Q ) is not only singly protonated but also doubly protonated, as shown in Eqs. 2 and 3 [75, 76]. 

The acid catalysis starts to operate under acidic conditions, pH < p ai (Figure 3) [77], Under more acidic conditions such that pH < pA"a2, the slope changes from — 1 to —2, showing the second-order dependence of on [H+], agreeing with the expectation based on Eq. 6 [77], The pATai and pA a2 values of each / -benzoquinone derivative determined from the pH dependence of kgt in Figure 3 agree well with the known values in the literature [75, 77], Thus, the acceleration of the rate of electron transfer is achieved by the protonation of Q in Eqs. 2 and 3, and the number of protons which can form the chemical bond with Q determines the kinetic order... 

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