Leuco-quinizarin

In the benzene and naphthalene series there are few examples of quinone reductions other than that of hydroquinone itself. There are, however, many intermediate reaction sequences in the anthraquinone series that depend on the generation, usually by employing aqueous "hydros" (sodium dithionite) of the so-called leuco compound. The reaction with leuco quinizarin [122308-59-2] is shown because this provides the key route to the important 1,4-diaminoanthtaquinones. 

In 1963, Bloom and Hutton suggested5 the structure of leuco quinizarin in solution as 9,10-dihydroxy-2,3-dihydro-l,4-anthraquinone (9a). In 1981, Kikuchi and colleagues6 confirmed the structure by means of H- and... 

Two structures 9a or 9b are possible for leuco quinizarin (9). Bloom and Hutton5 have proposed the structure of 9 to be 9a by comparing the chemical shift of methylene protons with those of leuco naphthazarin (3.05 ppm) and leuco naphthoquinone (3.08 ppm). However, based on UV spectra and chemical reactivity, Egerton and CO-workers7 and Greenhalgh8 independently suggested an equilibrium mixture of 9a and 9b in solution. 

The reduction of quinizarin using sodium hydrosulfite produces leuco quinizarin, which, in turn, undergoes condensation with alkyl- or arylamines and reoxidation to produce blue and green disperse and solvent dyes. Although chemical oxidation can be used, air oxidation is normally sufficient. See steps A and B in Fig. 13.112 for the general reaction scheme. The use of boric acid in the reduction step follows the course outlined in Fig. 13.H3, where the synthesis of C.I. Solvent Green 3 is given as an example.56... 

Quinizarin green base from leuco quinizarin and p-toluidine. 317... 

Some other organic reagents have been also proposed for determination of Mg, e.g., Eriochrome Cyanine R [45], Alizarin S [46], o-cresolphthalein [47] and its derivatives [48], 1,2,7-trihydroxyanthraquinone [49], 1,8-dihydroxyanthraquinone (e = 1.2-10 at 510 nm) [50], and leuco-quinizarin [51]. Mg has been determined also with the use of emodin... 

Further work has been reported on using sugars to elaborate chiral anthracyclines. The xylo-dialdose derivative (9) can be condensed with leuco-quinizarin leading to the derivative (10),which was then cyclized by the Marschlak procedure to the optically pure... 

In the main product Ri=CH3 and R2=—CH2CH2OH although small quantities of the products where Ri=R2=CH3 and Ri=R2=CH2CH20H are present, as will be seen from the synthetic method used in manufacture. Quinizarine is reduced with sodium dithionite to give leuco quinizarin which is probably an equilibrium mixture (Figure 2.69). 

It is well known that quinizarin (22) is alkylaminated in air to give a mixture of l-alkylamino-4-hydroxyanthraquinone (23), l,4-bis(alkylamino)-anthraquinone (24), and 2-alkylaminoquinizarin (25) (Scheme 7). The reaction conditions affect the ratio of these products. In a nitrogen atmosphere, or in the presence of sodium dithionite as reducing agent, the main amination product is 24. The solvent effects of the reaction of leuco... 

Blue Anthraquinone Dyes. All the important blue anthraquinone disperse dyes contain at least two amino groups in either the 1,4- or 1,5-positions, often with two additional hydroxy groups in the 5,8- or 4,8-respectively. The 1,4-substituted compounds are obtained by condensing the reduction product of quinizarin, 1,4-dihydroxyan-thraquinone, often called the leuco form, with the desired amines as shown in Figure 2.12. It should be noted that most anthraquinone disperse dyes are mixtures of products and not single compounds as drawn, a fact beneficial to their dyeing performance on polyester. 

Manchalk reaction. Some years ago Marschalk et al. found that leucoquinizarin (1), prepared in situ by reduction of quinizarin with alkaline dithionite, reacts with aldehydes to form 2-alkylquinizarins (2). It is not possible to obtain 2,3-disubstituted quinizarins in this way. Lewis found that pyridinium acetate with isopropyl alcohol as solvent is superior to a base or an acid catalyst for this reaction yields as high as 90% of 2 can be obtained. In addition, aromatic aldehydes can be used successfully. The leuco forms of 2 can be alkylated in this way to give 2,3-dialkylquinizarins in 40-70% yields. 

Annelation of hydroxyanthraquinones. Leucoquinizarin (2), the reduction product of quinizarin with sodium dithionite, reacts with sucdndialdehyde (excess) to give, after oxygenation, the naphthacenequinone derivative 3 in 62% yield. This annelation is related to the Marschalk reaction by which an alkyl group is introduced into the 2-position of a 1-hydroxyanthraquinone by reaction of an aldehyde with the leuco derivative of the quinone. 

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