P-Benzoquinone derivatives

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... 

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). 

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] ... 

Kraus and Zhao described the total synthesis of G-2N (48), an angnlarly fused quinone natural product, using a Diels-Alder reaction between an outerring bicyclic diene and a p-benzoquinone derivative. Sahagdn and colleagues reported the synthesis of tetracyclic ketone 49 nsing a Diels-Alder approach. Ketone 49 was intended to be used in the synthesis of new anthracychne analogs. 

This highly substituted p-benzoquinone derivative tends to prevail at high pH values and also at pH 7 when the autoxidation products are checked after several hours [35], It certainly arises from a nucleophilic attack by OH on the quinoneimine C. This attack is obviously increasingly easy as the pH arises, and leads to 2-amino-3,6-dihydroxy-benzoic acid, which is in turn oxidised to the corresponding quinone. This is attacked at the 5 position by a molecule of still unreacted HA, and the corresponding adduct is finally oxidised to the stable final product I, Fig. 

Fig. 36 Plot of the primary kinetic isotope effects for the acid-catalyzed rate constant k ulk i, vs. E (QH /QH2) in the reduction of p-benzoquinone derivatives by ActH2 and ActD2 in H20-Et0H (5 1 v/v) at 298 K. Reproduced from [244] with permission of the Royal Society of Chemistry...

Earlier work on the Co(salen)-catalyzed 02 oxidation of p-substituted phenols to p-benzoquinones has been extended to include substrates that serve as models for lignin phenolic subunits. Lignin is a renewable source of carbon and its oxidation to p-benzoquinone derivatives would allow conversion to useful intermediates. Certain p-substituted phenolics can be oxidized to p-benzoquinones with dioxygen using the Co(salen) complexes A and B in Figure 4 as catalysts ". ... 

Scheme 4.4 Electron transfer from O2 -Sc(HMPA)3 to p-benzoquinone derivatives.

Nature makes use of the benzoquinone-hydroquinone redox couple in reversible oxidation reactions. These processes are part of the comphcated cascade by which oxygen is used in biochemical degradations. An important series of compounds used for this purpose are the ubiquinones (a name coined to indicate their ubiquitous presence in nature), also collectively called coenzyme Q (CoQ, or simply Q). The ubiquinones are substimted p-benzoquinone derivatives bearing a side chain made up of 2-methylbutadiene units (isoprene Sections 4-7 and 14-10). An enzyme system that utilizes NADH (Real Life 8-1 and 25-2) converts CoQ into its reduced form (QH2). 

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