Anthraquinone ring

Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation (electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone is rather more susceptible to electrophilic attack than is the 2-position, as indicated by jt-electron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonic acid (6.3). The severity of the reaction conditions ensures that the thermodynamically favoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group, is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained by sulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). It appears that mercuration first takes place at the 1-position followed by displacement. Some disulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8-disulphonic acids, respectively. Separation of the various compounds can be achieved without too much difficulty. Sulphonation of anthraquinone derivatives is also of some importance. 

Nucleophilic displacement of halogen by amines is an important method of introducing amino groups into the anthraquinone ring system. In the Ullmann reaction the displacement is catalysed by metallic copper or by copper ions so that relatively mild conditions can be used. Mechanistic studies suggest that copper(I) ions exert a catalytic effect via complex formation. Derivatives of 1,4-diaminoanthraquinone are of considerable industrial significance. Many compounds are prepared from the reduced form of quinizarin (6.6). 

The molecular structure of the anticancer drug daunomycin is shown in Fig. 1. It consists of an anthraquinone ring system (aglycon), the portion which intercalates into the DNA [15], and an amino sugar group (daunosamine), which stays in the DNA minor groove. 

The chemistry associated with the vatting process is illustrated in Fig. 13.24. For the reduction step, a mixture of sodium hydroxide (caustic) and sodium hydrosulfite (hydro, Na2S204) is used. Depending upon the amount of caustic and hydro employed one or both of the anthraquinone rings may undergo reduction. 

Disperse dyes containing substituents in both anthraquinone rings are often prepared from dinitroanthrarufin (DNA) and dini-trochrysazin (DNC), the structures of which are shown in Fig. 13.116. Examples of these dyes are C.I. Disperse Blue 56 and Blue 77. The former dye is made by reduction of... 

One can see from Table 7.1 that the quantum yield of photolysis of alkyl-substituted anthraquinones only slightly depended on the replacement of the methyl group by the ethyl or isopropyl group.26 However, it decreased drastically after the introduction of electron-donor substituents in 3- or 4-positions of the anthraquinone ring.17 Quantum yields of phototransformations of l-arylcyanomethyl-9,10-anthra-quinones were found to be substantially lower (by two orders of magnitude) than for 1-methylanthraquinone.36... 

The substituents in the anthraquinone ring affected the rate constant of thermal bleaching of derivatives of 1-methylphenoxyanthraquinone (Table 7.1).17,18,26 The substituents that increased the Tt-electron density on the hydrogen atom of the methide group decreased the lifetime of the photoinduced form (Table 7.1).18,26 The introduction of methoxy and piperidine substituents stabilized the photoinduced form (Table 7.1). 

Quantum yields of photoproduction of ana-quinones of acetoxy-substituted anthraquinones with amino substituents in the anthraquinone ring as well as with an acetyl group proved to be the lowest (Table 7.2) As in the case of photochromic alkylanthraquinones, the reverse photoreaction, from ana-quinone to para-quinone for derivatives of acetoxyanthraquinone, proved to be impossible.21 The transition from ana-quinone to para-quinone occurred during freezing out of a sample owing to thermal bleaching. 

Many other syntheses incorporate such nucleophilic additions to lactones with subsequent reduction to C-glycosyl compounds because of the facile reduction of the tertiary anomeric hydroxyl. Condensation of acetylenic carbanions with lactones, followed by reduction, has been used as a starting point to construct the anthraquinone ring system of vineomycinone B2 [225]. 

The grafting of the anthraquinone ring to the 5a-cholestanol moiety afforded an effective gelator of several alcohols (2.91 wt% in 4-heptanol) and alkanes (1.53 wt % in dodecane) in the case of the naturally occmring epimer (S)-20 (Scheme 8) [31]. Interestingly, the a-epimer, (R)-20, failed to gel the same Uquids under the same conditions [36], which illustrates the importance of the configuration at C-3 of the steroid skeleton. In general, an ALS with /3-stereochemistry can preferentially adopt an extended conformation with... 

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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