Anthraquinone system

There is a wide diversity of chemical structures of anthraquinone colorants. Many anthraquinone dyes are found in nature, perhaps the best known being alizarin, 1,2-dihydroxyanthraquinone, the principal constituent of madder (see Chapter 1). These natural anthraquinone dyes are no longer of significant commercial importance. Many of the current commercial range of synthetic anthraquinone dyes are simply substituted derivatives of the anthraquinone system. For example, a number of the most important red and blue disperse dyes for application to polyester fibres are simple non-ionic anthraquinone molecules, containing substituents such as amino, hydroxy and methoxy, and a number of sul-fonated derivatives are commonly used as acid dyes for wool. 

Nucleophilic substitution reactions, to which the aromatic rings are activated by the presence of the carbonyl groups, are commonly used in the elaboration of the anthraquinone nucleus, particularly for the introduction of hydroxy and amino groups. Commonly these substitution reactions are catalysed by either boric acid or by transition metal ions. As an example, amino and hydroxy groups may be introduced into the anthraquinone system by nucleophilic displacement of sulfonic acid groups. Another example of an industrially useful nucleophilic substitution is the reaction of l-amino-4-bromoanthraquinone-2-sulfonic acid (bromamine acid) (76) with aromatic amines, as shown in Scheme 4.5, to give a series of useful water-soluble blue dyes. The displacement of bromine in these reactions is catalysed markedly by the presence of copper(n) ions. 

These are, with the exception of some metal complexes, low molecular weight wool dyes that usually have monoazo, disazo, or simple anthraquinone systems. They exhaust from a strongly acid to more neutral bath. The relatively small mol-... 

In contrast, as discussed earlier in Section 3.2.1, studies of the interfacial capacitance allow the effect of the applied potential on the adsorption thermodynamics to be elucidated. For example, as discussed above, cyclic voltammetry reveals that the dependence of the surface coverage T on the bulk concentration of 20H-AQ is accurately described by the Langmuir isotherm over the concentration range 20 nM to 2 iM. However, since adsorption is reversible in the anthraquinone system, the effect of changing the potential at which the monolayer is formed on the surface coverage, or the adsorption thermodynamics, cannot be investigated by ex situ... 

To produce disperse dyes having the brightness of the anthraquinone system and the color strength of the azo system, azo dyes based on heteroaromatic amines were developed.26-28 Examples are C.I. Disperse Red 145, Disperse Blue 148, Disperse Red 156, and C.I. Disperse Blue 339. These dyes employ aminated thiazoles, benzothiazoles, benzisothiazoles, and thiadiazoles in their... 

The 1,4-naphthoquinone system is not only a good acceptor for stabilized carbanions, as exploited by Kraus and Wu [31, 32] (see Scheme 6), but also for radicals. The angular skeleton of the benzo[fl]anthraquinone system is constructed in an elegant one-step transformation by a manganese (Ill)-induced radical addition of a malonic ester derivative 161 to 1,4-naphthoquinone 40 followed by addition of the newly generated radical to the benzene ring to form 162 (Scheme 41) [111], However, the principle has not yet been used in advanced syntheses of angucyclinones. 

The reactivity of (20) is more typicai of anthraquinones in genera) whiie the formation of Dewar structures (21) and (22) is the first observation of this type of reaction for the anthraquinone system. The authors note that (18) and (19) show large perturbations in their U.V. absorption spectra compared with anthraquinone (20). These perturbations include bathochromic shifts and extinction coefficient enhancements of the longest wavelength, presumably n->7r , bands which may reflect steric interactions between one of the anthraquinone carbonyls and the adjacent ferr-butyl group. Anomalous reactivity has also been found for 9-ferf-butyl-lO-cyanoanthracene (23) which upon irradiation at -20 C gave the dibenzobicyclo(2.2. llheptane (24). This is in constrast to the photochemistry of 9-ferf-butylanthracene (25) which forms the Dewar isomer (26). The latter reaction and its reversal have been examined for their... 

An annelation reaction of a,p-unsaturated carbonyl compounds with cyanophtfaalides (34) can be used for the construction of naphthoquinone and anthraquinone systems in biologically active natural... 

Weis, C. D. Meerwein arylation reactions of olefins with anthraquinone diazonium hydrogen sulfates formation of new carbon bonds at the carbon atoms C-1 and at C-1,5 of the anthraquinone system. Dyes Pigm. 1988, 9, 1-20. 

The formation of fused anthraquinone systems in a two-stage photoreaction between 2-bromo-3-methoxy-1,4-naphthoquinone and 1,1-diarylethylenes is well established, and the reaction has been extended to include 1-(heteroaryl)-1-phenylethylenes (e.g., 149). In general the preferred mode of ring-closure follows... 

Fig. 30. Anthracene and anthraquinone derivatives are non-hydrogen-bonding gelators. In anthraquinone systems 2,3-dialkoxy derivatives such as 63 are found to be the most efficient...

At this point, all that remained to complete the targeted natural product (1) was the attachment of the anthraquinone system, which, as discussed above, projected a Diels—Alder reaction with an isobenzofiiran (i. e. 13). Thus, a synthesis of its precursor, phtha-lide 67, was developed as shown in Scheme 9. Since most of these transformations are relatively routine, we will not describe them in any detail here, except to note that the ability to incorporate several different protecting groups for the phenol functions at different stages of the synthesis (cf. 62, 63, 67) was a critical element for the completion of dynemicin A (1). As we shall see, only 67 ultimately proved capable of both participating in the desired Diels—Alder reaction and being elaborated to the complete DE anthraquinone system. 

The 9,10-anthraquinone system is a classic example of an EE mechanism, which includes a synproportionation process. Absorbance versus distance profiles were measured for this reaction and the homogeneous and heterogeneous rate constants were in agreement with those derived from cychc voltammetry [169]. Protonation of the benzophenone anion radical by benzoic add and o-cresol was studied using this technique [170]. A variety of electrode geometries were explored in determining the heterogeneous... 

An important advantage of the anthraquinone system is the stability of its radical anion in the presence of water at neutral pH. When an anthraquinone radical anion podand was added to a two-phase, CH2Cl2-water system containing Li , the cation was transported into the organic phase. This was demonstrated unequivocally using electron spin resonance spectroscopy by detection of a 0.33 G Li-hyperfine splitting. 

The DNA-binding properties of anthraquinones have been studied intensively over the past 25 years because of their clinical potential as anticancer drugs. The anthraquinone system is often found in anti-tumor drugs such as anthracyclines, mitoxantrone (11a), ametantrone (11b) and derivatives thereof [217-220]. Mitoxantrone (11a) and ametantrone (11b) have attracted much interest because of their lower risks of cardiotoxic effects compared with the naturally occurring anthracyclines doxorubicine (adriamycin) (4b) and daunorubicin [221]. 

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