Structural Modifications Anthraquinones

One hydroxyl is missing, and a new hydroxyl has been incorporated adjacent to the methyl. Without any evidence for the sequence of such reactions, the structure of intermediate 2 shows the result of three aldol condensations and reduction of a carbonyl. A dehydration reaction, two oxidations, and a decarboxylation are necessary to attain the islandicin structure. In chrysophanol, aloe-emodin, and rhein, the same oxygen function is lost by reduction as in islandicin, and decarboxylation also occurs. The three compounds 

Note that many other natural anthraquinone structures are not formed via the acetate pathway, but by a more elaborate sequence involving shiki-mate and an isoprene unit (see page 158). Such structures do not contain the characteristic meta oxygenation pattern, and often have oxygenation in only one aromatic ring (see page 164). 

R = C02H, rhein anthrone R = CH2OH, aloe-emodin anthrone 

The principal purgative activity originates from the cascarosides, the C-glycosides barbaloin and chrysaloin being less active when taken orally. As with the sennosides, the actual purgative 

Aloes and rhubarb have found considerable use as purgatives in the past, but they both have a rather drastic action and their use for this purpose has largely been abandoned. 

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The quinones are a rather heterogeneous collection of compounds with structures based on an unsaturated system of cyclic diketones. The biologically important plastoquinones, ubiquinones, and vitamin K are included in this group however, as pigments the most widespread and important quinones are the 1,4-naphthaquinones (Fig. 18) and the 9,10-anthraquinones (Fig. 19, Table 10). Methyl, methoxyl, and hydroxyl groups are the most common substituents, and O- and C-glycosides (see Fig. 20) are frequently present in the anthraquinone group. Several structural modifications exist due to reduction, dimerization (Fig. 21), and addition of side chains. 

The new Colour Index volume Pigments and Solvent Dyes lists some 350 solvent dyes and gives their chemical structures, unlike earlier editions which named 800 dyes but included few structures. This fall in numbers is not because of any decreased use but rather the general contraction in numbers of all dyes used in the textile industry. Solvent dyes have been introduced not by attempts to synthesise new colorants but by selection and in some cases modification of known disperse dyes to meet the technical requirements. The majority of solvent dyes are azo compounds but among the blue dyes there are anthraquinones. The aqueous solubility of some of the parent sulphonated dyes has been reduced to acceptable levels by formation of their salts with heavy metals or long-chain alkylamines. 

In a DTA study of 14 anthraquinone dyes, most had high flash points (225—335°C) and ignition points (320—375°C). Purpurin dianilide [107528-40-5] was exceptional with the much lower values of 110 and 155°C, respectively [1]. A similar study of indigo type dyes and vat solubilised modifications is reported. The basic dyes decompose over 350°C, destabilised to around 200°C for solubilised dyes. The relation between functional groups, structure and flammability is discussed [2]. Sulfonyl azides have been employed for attachment of reactive dyes, it is claimed they are safer used in supercritical carbon dioxide than in water [3]. 

With respect to the above requirements for DET, laccase and BOx have been shown to be useful bioelectrocatalysts for O2 reduction. For both en mes, the substrates to be oxidized or reduced interact at different locations within the enzyme structure thus it is possible to orientate these enzymes without physically blocking access for the second substrate. In addition to the above-mentioned orientation of BOx by carb-ojq late groups at the surface of electrodes, the modification of electrodes with phenolic-type heterocycles (such as anthracene, anthraquinone and naphthoquinone derivatives) has been shown to significantly enhance the orientation of both enzymes to the electrode surface via their T1 Gu center, resulting in increased bioelectrocatalytic O2 reduction at the TNG. ° The phenolic modifications of the electrode constructs mimic the natural substrates of the enzymes, which results in docking of the enzymes to the electrode surface at their T1 Gu center in BFGs, this electrode then acts as the biocathode of the device, utilizing O2 as the oxidant and final electron acceptor. 

Anthraquinone-2-diazonium tetrafluoroborate has been adsorbed onto an edge plane pyrolytic graphite electrode to form a thin unreacted submonolayer film. After transfer to a buffer solution containing no diazonium salt, the adsorbed material was thermally decomposed at room temperature and a grafted anthra-quinone film was obtained with a surface coverage rs ,f < 2 x 10 ° mol car [240]. Nitroazobenzene submonolayers have been obtained by spontaneous modification of PPF surfaces. The apparent thickness was th = 0.6 nm and Raman spectra confirmed the structure of the flhn [241]. On gold, 0.06 was found,... 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

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