Hydroxy anthraquinone pigments

The class of hydroxyanthraquinone pigments comprises two different groups of compounds metal complexes of hydroxyanthraquinones on the one hand and metal salts of hydroxyanthraquinone sulfonic acids on the other hand. Some of the products are metal chelates. 

The first group includes 1,2-dihydroxyanthraquinone, commonly known as alizarin, 1,4-dihydroxyanthraquinone (quinizarin), and 1,2,4-trihydroxyan-thraquinone (purpurin). Alizarin in particular has been known and appreciated for thousands of years in the form of its lake , i.e., the coordination complex of 1,2-hydroxyanthraquinone 88 with aluminum and calcium (Madder Lake, Turkey Red). 

The chemical structure of 88 was first described in 1963 [12], Aluminum and calcium are equally important constituents of other hydroxyanthraquinone compounds, although iron salts are also known. 

The calcium lake of 1,2-dihydroxyanthraquinone is marketed as Pigment Red 83, 58000 1. It is produced commercially by treating a slightly basic alizarin solution with aqueous calcium chloride. 

All of these compounds were originally precipitated onto aluminum oxide hydrate, which served as a carrier. This applies accordingly to hydroxyanthraquinone sulfonic acids. 

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Hydroxy- and 1,8-dihydroxy-anthraquinones form neutral 2 1 complexes (209) with divalent metals such as copper and zinc, whilst 1,4- and 1,5-dihydroxyanthraquinones form polymeric complexes (210) and (211), respectively. The latter are virtually insoluble in a wide range of organic solvents and have been used for the mass pigmentation of synthetic polymers. The parent compounds have also found application as mordant dyestuffs, e.g. (212), (213) and (214). 

Acidic anthraquinones in which both hydroxy groups in the C-ring have been methylated occur in Cortinarius armillatus and C. miniatopus, while cortinaric acid (313) and its 8-0-methyl ether (314) have been isolated from Cortinarius elegantior where they are partly responsible for the characteristic pink colour produced in the stem base with ammonia. The structures of the latter pigments have been confirmed by synthesis involving chromium trioxide oxidation of the peracetyl derivatives of endocrocin (303) and dermolutein (305), respectively 387). 

Among toadstools belonging to Cortinarius subgenus Phlegmacium the principal pigments are derivatives of 3,4-dihydroanthracen-l (2//)-one and, in contrast to the situation in Dermocybe, anthraquinones themselves play only a subordinate role (617). The pivotal biosynthetic intermediate atrochrysone (321) is present in Cortinarius atrovirens and C. odoratus (Table 28) where it is accompanied by the 4-hydroxy derivatives (322) and (323) of varying stereochemistry. The dimethyl ether... 

The rare bird s nest fungus Cyathus intermedius produces the yellow xanthone pigment (391) in small amounts when grown in laboratory fermentations 56). The structure (391) was confirmed by synthesis involving regioselective hydroxylation of l-hydroxy-6,8-dimethylxan-thone (Scheme 65). It seems likely that the xanthone (391) arises in the fungus from an octaketide derived anthraquinone precursor. 

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