Anthraquinone, structure

The term polycyclic pigments refers to pigments which are derived from the an-thraquinone skeleton, either by chemical structure or by synthesis. The complex fused-ring systems which are discussed in this context are all at least remotely related to the parent anthraquinone structure. 

This class includes polycarbocyclic compounds which are at least formally derived from the anthraquinone structure. The products are considered members of the higher condensed carbocyclic quinone series, which even in the absence of additional substituents provide yellow to red shades. Halogenation is frequently found to afford cleaner shades and improved fastness properties. Heading the list of such derivatives are pyranthrone, anthanthrone, and isoviolanthrone pigments. 

P.Y.99, which is derived from the anthraquinone structure, is produced in Japan. It affords very reddish shades of yellow, even redder but at the same time distinctly duller than those of P.Y.83. P.Y.99 is recommended especially for use in textile printing but is also used in plastics. Although HDPE systems are heat stable up to 300°C, they noticeably lack tinctorial strength. 1/3 SD HDPE samples containing 1% TiOz are formulated at 0.53% pigment. 

The anthraquinone structure occurs in both the plant and animal kingdom. Those natural dyes having this structure surpass all other natural dyes in fastness properties. See also Dyes Anthraquinone. 

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). 

The absorption in UV region is imputed to the jt-jt transitions of the anthraquinonic structure (benzenoid and quinonoid bands). 

The basic anthraquinone structure is depicted in Fig. (6). In the literature two different methods are used for the numbering of the carbon atoms of the parent molecule (Fig. (6)). In this review the method that numbers the carbon atoms from 1 till 14 is chosen. The anthraquinones found in Rubia tinctorum differ in the nature of the substituents and the substitution pattern. These substituents are only found on carbon atoms 1, 2, 3 and 4. Hydroxyl and methoxyl groups are frequently encountered as substituent... 

Fig. (6). Parent anthraquinone structure and the two different numbering systems...

The 1,4-anthraquinone structures of (382) and (383) have been firmly established by extensive n.m.r. investigations, including n.O.e. experiments, and the analysis of proton coupled C-n.m.r. spectra. H-N.m.r. data for rufoolivacin (382) form part of Table 32. It was conveniently analysed by reference to the spectra of known 1,4-anthra-quinones (447) and of torachrysone-8-O-methyl ether (284) (577). From a biogenetic viewpoint it is interesting to note that torachrysone-8-0-methyl ether (284) itself, and several closely related naphthoquinones are also found in extracts of C. rufoolivaceus (see Section 3.4). 

A completely new class of guest-host dyes based on the anthraquinone structures has recently been reported.The chemical structures of these dyes are of the two types illustrated in Fig. 13 where R denotes a general substituent R -OCqHiqCl) for the dye D16 (blue) and -C2H5(II) for the dye D35 (rose). 

A large group of natural and synthetic dyesmffs based on the anthraquinone structure have found use in the formation of lake and other pigments. 

Itokawa et al 74) reported the NMR spectra of some anthraquinones isolated from Rubia species. For one of the compounds a 2-methyl-1,3,6 (or 7)-trihydroxy-9,10-anthraquinone structure was assumed based on various spectral data. Eventually it was stated that the hydroxyl group was in the 6-position rather than in the 7-position because of a 0.6-0.8 ppm upfield shift observed for C-9 if compared with compounds having a 1,3-dihydroxy-2-hydroxymethyl substitution pattern. For a better insight in the validity of using NMR spectrometry for distinguishing 6/7 or 2/3 substitution in anthraquinones bearing several substituents, further NMR studies of such multiply-substituted anthraquinones seem desirable. 

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