Polycyclic hydroquinone

Among the many reactions of Fischer-type metal-carbene complexes (see Chap. 9), the most remarkable and most applied to organic synthesis is the Dbtz reaction. " This complex reaction involves an unsaturated Fischer-type carbene complex with an alkyne yielding polycyclic hydroquinones. The principle of the scheme, the mechanism and an example are represented p. 522. 

While the reduction of polycyclic quinones to phenols, hydroquinones,... 

Reduction of polycyclic quinones. This hydride in DMF is the most efficient of various metal hydrides for reduction of polycyclic quinones to hydroquinones (90 95% yield) with the exception of anthraquinone. With this quinone, lithium aluminum tri-t-butoxyhydride is more efficient (757n yield of the hydroquinone diacetate). ... 

Certain polycyclic aromatic hydrocarbons can be converted to their epoxides, as typified by the reaction of phenanthrene with DDO (eq 9). Aromatic heterocycles like furans and benzofurans also give epoxides, although these products are quite susceptible to rearrangement, even at subambient temperatures (eq 10). The oxidation of heavily substituted phenols by DDO leads to quinones, as shown in eq 11, which illustrates the formation of an orthoquinone. 7 The corresponding hydroquinones are intermediates in these reactions, but undergo ready oxidation to the quinones. 

The hydrodeoxygenation reaction (HDO) is carried outin the gas phase in a fixed-bed reactor (400 C, 25 bar of hydrogen), using commercial nickel and molybdenum oxides supported on alumina as catalysts. The HDO allows a quantitative conversion of both catechol and hydroquinone with a 96% selectivity to phenol. The main by-products are heavy condensed polycyclic aromatic hydrocarbons. 

Benzo-l,4-quinoid structures occur in nature as the final oxidation products ofvarious mono- and polycyclic compounds. Most simple benzo-1,4-quin ones occur in microorganisms (moulds), higher fungi and lichens, and less frequently in higher plants and some insects. Common substances are glycosides occurring in colourless reduced forms (such as derivatives of hydroquinone known by the systematic name of benzene-l,4-diol). Coloured quinones are formed from these precursors by hydrolysis catalysed by saccharases and by enzymatic oxidation or autoxidation of aglycones. 

The Scholl reaction is one of the oldest and most useful C-C coupling reactions, and is often used in the synthesis of polycyclic aromatic hydrocarbons (PAHs). In general, a large amount of metal oxidant (such as FeCls) is employed in this reaction, and chlorinated by-products are produced in many cases. Rathore found that DDQ and protic acid effectively promote the Scholl reaction of various 1,2-diarylbenzenes (86) to give triphenylenes (87) in excellent yields (Scheme 8.40). The advantages of this reaction over typical metal oxidant promoted Scholl reactions are (l) an excess of DDQ is not required (2) no chlorinated by-products are produced and (3) recovery and re-use of reduced hydroquinone DDQ-H2 is possible. The reaction of other diaiyls tethered with methylene and propylene, and the intermolecular reaction also proceeds effectively. Furthermore, the treatment of hexaatylbenzene (88) with DDQ quantitatively affords collonene (89). 

Shen, Y.-C., Lo, K.-L., Kuo, Y.-H., and Chakraborlhy, R. (2002) Polycyclic quinones and hydroquinones, antitumor constituents from Taiwanese marine sponge Xestospongia sp. Chin. Pharm. J., 54, 207-213. 

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