Naphthalene 1,4-quinone reduction

In the benzene and naphthalene series there are few examples of quinone reductions other than that of hydroquinone itself. There are, however, many intermediate reaction sequences in the anthraquinone series that depend on the generation, usually by employing aqueous "hydros" (sodium dithionite) of the so-called leuco compound. The reaction with leuco quinizarin [122308-59-2] is shown because this provides the key route to the important 1,4-diaminoanthtaquinones. 

Reaction LVIIL (a) Reduction of Phenols and Quinones by Distillation with Zinc Dust. (A., 140, 205.)—When certain aromatic oxygen compounds (phenols, naphthols, quinones, etc.), are heated with zinc dust, they are reduced to the corresponding hydrocarbons. Thus, phenol yields benzene, the naphthols naphthalene while anthracene can be obtained from anthraquinone or its hydroxy derivatives, alizarin, or quinizarin. In this way alizarin was first proved to be an anthracene derivative. (B., 1, 43.) For catalytic reduction of phenols, see C. r. 193, 1023. 

In support of this view the p-amino-phenols themselves readily yield quinones. Also most p-substituted primary amines, e.g., p-diamines, p-alkylamines, such as p-toluidine, sulphanilic acid and its derivatives, behave similarly. In fact, the reaction can be used as a test for p-substi-tuted primary amines. p-Benzoquinone is usually made from aniline for the other p-quinones the p-amino-phenols, which are easily obtained by reduction of the p-nitroso-phenols and of azo-phenols, are employed. These reactions also apply, but not so widely, in the naphthalene series. 

Quinones of the more reactive, polycyclic, aromatic systems can usually be obtained by direct oxidation, which is best carried out with chromium(vi) compounds under acidic conditions. In this way 1,4-naphthoquinone, 9,10-anthraquinone and 9,10-phenanthraquinone are prepared from naphthalene, anthracene and phenanthrene respectively (Expt 6.128). Also included in this section is the reduction of anthraquinone with tin and acid to give anthrone, probably by the sequence of steps formulated below. 

Direct reduction of naphthalene 1,4-quinone with diisobutylaluminum hydride produces the free cis-1,4-dihydrodiol. C -naphthalene dioxide 153 and compound 158 have been obtained by the photooxidation of l,6-imino[10]annulene, followed by thermal isomerization and reaction with nitrosylchloride. An inter-... 

Anodic methoxylation of aromatic ethers via the EEQCp mechanism has found its widest application in the synthesis of quinone bisketals (LXX) from para-dimethox-ybenzenes (LXIX) [79]. Yields are generally excellent, and the reaction is conveniently carried out at constant current in a single cell [42,80]. The reaction has also been shown to work well for naphthalenes [81] and benzothiophenes [82]. As shown in Table 4, a variety of substituents may be present on the ring. When substituents sensitive to cathodic reduction such as -CHO and -CH CHCO Me are present, a divided cell apparatus may be required to obtain reasonable yields of the corresponding bisketals. 

Anthracene and phenanthrene are even less resistant toward oxidation or reduction than naphthalene. Both hydrocarbons are oxidized to the 9,10-quinones and reduced to the 9,10-dihydro compounds. Both the orientation of these reactions and the comparative ease with which they take place are understandable on the basis of the structures involved. Attack at the 9- and 10-positions leaves two... 

Derivatives of anthracene are seldom prepared from anthracene itself, but rather by ring-closure methods. As in the case of naphthalene, the most important method of ring closure involves adaptation of Friedel-Crafts acylation. The products initially obtained are anthraquinones, which can be converted into corresponding anthracenes by reduction with zinc and alkali. This last step is seldom carried out, since the quinones are by far the more important class of compounds. 

Give the products resulting from the following reactions (a) 2-naphthol + HCN + ZnCl, + HCl (b) anthracene + c/ -butene-dioic anhydride (c) naphtho-1,4-quinone + hexa-2,4-diene followed by reaction with CrOj (d) naphthalene + phthalic anhydride + AlCl followed by reaction with polyphosphoric acid and then reduction and dehydrogenation (e)... 

The synthesis of Gates and Tschudi (Scheme 13.44) began with 2,6-dihydroxy-naphthalene, which was converted to its monobenzoate and nitrosated. Reduction of the nitroso (H2, Pd/C) yielded the corresponding a-aminophenol, the oxidation of which, with iron(III) chloride, produced a quinone, and reduction of the quinone with sodium hydrosulfite followed by methylation (dimethyl sulfate [(CH3)2S04]) yielded a dimethoxybenzoate. Removal of the benzoate protecting group and repetition of the entire sequence outlined above produced a dimethoxyquinone. 

One of Clar s most notable early successes of the decade was pentacene 33. dar and John isolated 33 via dehydrogenation of a dihydropentacene isomer, several of which had been laboriously produced since 1911 when Phillipi first claimed one as 33 [28]. The combination of two groups efforts provided a far simpler route to pentacene two decades later. The highly efficient route began with condensation of o-phthalaldehyde and cydohexane-l,4-dione to afford quinone 34 (Scheme 1.10) [29]. Reduction with A1 powder afforded 33 in two steps [30]. Clar and coworkers also synthesized several pentacene derivatives in the 1940s 1.2-benzopentacene from pseduocumene and three additional dibenzopentacene derivatives from naphthalene and/or phenanthrene starting materials [la]. The syntheses focused on condensation cydization reactions of keto-adds to form the penultimate quinones and ultimate polycydes. 

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