Synthesis of Anthraquinones

The outer rings of the anthraquinone molecule (52) are aromatic in nature and as such are capable of undergoing substitution reactions. The reactivity of the rings towards substitution is determined by the fact that 

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A method for synthesis of anthraquinones is by reaction of cyanophthalide carban-ions with benzynes [162], It is particularly useful for the access of 2 aza-l,3,8-trimeth-oxyanthraquinone because of the high regioselectivity imposed by the methoxy groups and the nitrogen atom of the pyridine. 

The synthesis of anthraquinone is instructive. Phthalic anhydride condenses with benzene in the presence of aluminium chloride (slightly more than two equivalents, see 1 above) to yield o-benzoylbenzoic acid, and the latter is heated with concentrated sulphuric acid ... 

A new potential synthesis of anthraquinones based on l-acetoxybenzo[c]furan (470) has been reported. The s-tetrazine reaction used to generate the benzo[c]furan proceeded in the presence of N-methylmaleimide to give the expected adduct (471). The reaction takes a different path in the presence of quinones to yield a novel phthalide (472), the structure of which was confirmed by X-ray diffraction (81AJC1223). 

Methyl anthraquinone has been obtained by the oxidation of /3-methyl anthracene by several investigators 1 and material of the same origin, obtained by the benzene-extraction of crude commercial anthraquinone,2 has been fully described. As regards the synthesis from phthalic anhydride and toluene, both the preparation and properties of />-toluyl-o-benzoic acid 3 and the complete synthesis 4 have been the subject of several papers. This acid has also been prepared from o-carbomethoxy benzoyl chloride and toluene.5 The phthalic anhydride synthesis of anthraquinone derivatives in general has received considerable attention. An account of this work, together with extensive references, is given by Barnett.6... 

The addition-protonation procedure maintains the arene-chromium bond and allows further application of the activating effect of the metal. In an approach to the synthesis of anthraquinone antibiotics, the dihydronaphthalene complex (79) was allowed to react with a cyanohydrin acetal anion and then quenched with acid.129 The resulting tetralin complex (80) could be metallated effectively and carried on to a key intermediate (81) in anthraquinone construction (equation 54)... 

Due to the wide range of reactions for the synthesis of anthraquinone dyes and to remain within the scope of this book, this section concentrates on the most important basic reactions and refers to the literature for more details. For further information, see [38],... 

The commercial preparation of anthraquinone dyes begins with the synthesis of anthraquinone itself. In this regard, the three-step synthesis involves (1) the oxidation of naphthalene to phthalic anhydride, (2) Friedel-Crafts acylation of benzene to give a keto acid, and (3) cyclodehydration using H2S04. See Fig. 13.110. The preparation of 1,4-disubstituted anthraquinones utilizes the intermediates... 

A key intermediate in the synthesis of anthraquinone acid dyes is bromamine acid. 

Under forcing conditions, alkyl aryl ethers are cleaved by Grignard reagents by displacement of aryloxide [A]. Such reactions are not generally useful in synthesis, but the displacement of methoxide from positions ortho to alkoxycarbonyl [2] and oxazolinyl groups [3] has been applied to the synthesis of orffco-substituted carboxylic acids, and displacement ortho to diphenylphosphino groups has also been reported [4], The following example describes the key step in a synthesis of anthraquinones. Recently, asymmetric induction by a chiral oxazolinyl substituent has been reported [5] ... 

The direct synthesis of anthraquinone from phthalic anhydride and benzene has been reported to proceed over zeolite Beta [50] in a shape selective manner. In a conventional anthraquinone synthesis, anthracene is used as a feedstock for oxidation. Once there is a shortage of it in the market, additional anthracene could be produced by isomerization of its isomer, viz. phenanthrene. This, however, is not possible by direct isomerization of the trinuclear aromatic system but involves the partially (symmetrically) hydrogenated species. Consequently, isomerization of symmetrical octahydrophenanthrene to symmetrical octahydro-anthracene was studied by Song and Moffatt [51]. As sketched in Figure 3, a high yield of symmetrical octahydroanthracene can be obtained over zeolite H-mordenite (ngj/nyy = 8) at 250 °C (liquid phase, decalin as solvent). These examples show that (shape selective) catalysis on zeolites is more and more expanding into the conversion of polycyclic aromatics, and we foresee continued interest and success in this field of zeolite catalysis. 

Friedel-Crafts acylation is used, for example, in the synthesis of anthraquinone from benzene and phtalic anhydride. 

A recent synthesis of anthraquinones involves the base-catalyzed addition of phthalides to arynes (generated from the corresponding aryl bromide and lithium diisopropylamide), as outlined in Scheme 6.54... 

Yamaguchi has reported a related biomimetic synthesis of anthraquinones, summarized in Scheme Reaction of diester (165) with methyl acetoacetate dianion provides a tetraketo diester which is cy-clized by calcium hydroxide to obtain anthrone (166). Air oxidation in basic medium affords the anthra-quinone (167). 

In the conversion of anthracene into anthraquinone, the para bond between the two carbon atoms involved in the oxidation is broken, but no change takes place in the benzene rings present in the compound. The synthesis of anthraquinone from phthalic anhydride confirms this view of its structure. When the anhydride is heated with benzene in the presence of a dehydrating agent, such as aluminium chloride, one molecule of water is lost and anthraquinone is formed —... 

Trimethylsilylated benzamide was reacted with benzaldehyde to give the 1,2-adduct, which was treated with AcOH-toluene to give the phthalide in 76% yield. Phthalides have been used as precursors for the synthesis of anthraquinones [57] (Scheme 5.37). 

Electrophilic substitution and other reactions of naphthalenes (alkylation, acylation, condensation and migration in acidic ionic liquids have been reported. Anthracene undergoes photochemical [4+4] cycloaddition reactions - in acidic chloroaluminate(III) ionic liquids. One interesting study ineluded a one-pot synthesis of anthraquinone from benzene giving a 94% yield. In general a much wider range of redox products are formed than occur in conventional solvents the strong Bronsted acidity of die ionic liquid induces protonation of anthracene, by residual traces of HCl, to form an anthracenium species which couples readily via photochemically driven electron transfer mechanisms. 

The development of synthetic dyestuffs provided further impetus for advances in naphthalene chemistry. Towards the end of the last century, its oxidation into phthalic anhydride (PA) attained particular importance as a step in a successful route for the economical synthesis of indigo. In the 20 th century, the classic uses of naphthalene have been extended into new areas, for example, the development of the naphthol AS dyes from the parent naphthols, and the production of PA-based plasticizers and pesticides. The most recent developments in the field of industrial naphthalene chemistry concern the production of alkylnaphthalene-derivatives as solvents for use in carbonless copy papers, as well as the manufacture of naphthoquinone for the synthesis of anthraquinone. 

Singh, R. Geetanjali. Isolation and synthesis of anthraquinones and related compounds of Rubia cordifolia. J. Serb. Chem. Soc. 2005, 70, 937-942. 

The synthesis of anthraquinone colorants may effectively be envisaged as involving two general stages. The first stage involves the construetion of the anthraquinone ring system and in the second phase the anthraquinone nucleus is elaborated to produce the desired strueture. Frequently, the latter involves substitution reactions, but group interconversion and further cyclisation reaetions may also... 

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