Rhubarb(CHAPTER I. The Anthraquinone Group.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Probably no natural product has been so frequently examined as rhubarb root (Rheum officinale), but only those communications which appear to be of sufficient importance will be dealt with here. Geiger (Annalen, 1823, 8, 47; 1824, 9, 91) isolated from the root a substance, "rhabarberin," which he termed its most important constituent, in the form of small wart-like crystals, and this was later shown by Schlossberger and Dopping (ibid., 1844, 50, 214) to be identical with the chrysophanic acid described by Rochleder and Heldt (Ann. Chem. Pharm., 1843, 48, 12) as present in the wall lichen (Parmelia parietina). W. de la Rue and Miiller by extracting the root (Chem. Soc. Trans., 1857, 10, 298) with benzene and treating the residue with sodium carbonate solution, separated it into two portions, namely, chrysophanic acid, which remains undissolved, and emodin, which passes into solution.

Liebermann and Seidler (Annalen, 212, 36) by distilling chrysophanic acid with zinc-dust obtained methylanthracene, and gave it the constitution of a dihydroxymethylanthraquinone, and whereas emodin consisted ofa trihydroxymethylanthraquinone (Liebermann, Annalen, 183, 176), it appeared that these compounds were related in the same way as purpurin and alizarin. Much more recently Hesse (Annalen, 1899, 309, 32) found that rhubarb contains rhein, and in addition to this isolated rhabarberon, isomeric with emodin, and a third compound containing a methoxy group presumably chrysophanic acid methyl ether.

Eyken (Pharm. Weekblad, 1904, 41, 177), again, besides chrysophanic acid, emodin, and rhein, obtained iso-emodin, evidently identical with Hesse's rhabarberon, whereas Oesterle and Johann (Arch. Pharm., 1910, 248, 476) indicated that Hesse's third compound, containing a methoxy group, was the emodin methyl ether previously isolated by Tutin and Clewer from the Rumex ecklonianus (Meissner). This in reality was first described by Perkin and Hummel as present in the root-bark of the Ventilago madraspatana (Chem. Soc. Trans., 1894, 65, 940), and that of the Polygonum cuspidatum (ibid., 67, 1084). That rhubarb contains glucosides was shown by Tschirch and Heuberger (Arch. Pharm., 1902, 204, 596), and one of these compounds, chrysophanein, a glucoside of chrysophanic acid, has been described by Gilson (Arch. Internat. de Pharmakadynamie et de Therapie, 1905, 487).

The latest and most systematic examination of rhubarb root was carried out by Tutin and Clewer (Chem. Soc. Trans., 1911, 99, 946), who have shown that the iso-emodin of Tschirch and Eyken and the rhabarberon of Hesse consist of aloe-emodin, and moreover, in addition to these a fifth anthraquinone derivative, rheinolic acid, is also present. Rhein, emodin, aloe-emodin, emodin monomethyl ether, and chrysophanic acid exist in the root partly in the form of glucoside, though it was not found possible by these authors to separate the individual constituents of this glucoside mixture. More recently the interesting discovery has been made by Müller (Trans. Chem. Soc., 1911, 96, 967) that alizarin is present in this drug.

Rhubarb root contains also gallic acid, cinnamic acid, and a non-glucosidic resin, to which latter the purgative actions of this drug are mainly due. On the other hand, emodin and chrysophanic acid possess this property in a mild degree.