Robinia pseud-acacia, Flowers (CHAPTER VII. Flavonol 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.

Robinin was first isolated from the flowers of the white Azalea by Zwengerand Dronke (Annalen, Supp., 1861, 1, 263), who considered that it was a glucoside of quercetin. Perkin (Chem. Soc. Trans., 1902, 81, 473) has shown that this is not the case.

To prepare the glucoside the flowers are exhausted with boiling alcohol, the solution concentrated by evaporation and poured into water. The mixture is extracted with ether, and the aqueous liquid distilled down to a small bulk. On standing, crystals of robinin separate, which are purified by crystallisation from water.

Robinin, according to Perkin, consists of pale yellow needles, sintering at 190 and melting at 196-197°, and when air-dried it possesses the formula C₂₃H₄₂O₂₀, 8H₂O.

Boiling dilute sulphuric acid hydrolyses robinin with formation of kaempferol, 2 molecules of rhamnose and 1 of glucose, according to the following equation:
C₂₃H₄₂O₂₀ + 4H₂O = C₁₅H₁₀O₆ + 2C₆H₁₄O₆ + C₆H₁₂O₆

Schmidt (Chem. Zentr., 1901, ii., 121), who examined robinin at almost the same time, also obtained by its hydrolysis a colouring matter C₁₅H₁₀O₆, the acetyl compound of which melts at 182-183° (evidently kaempferol), and Waljascko (J. Russ. Phys. Chem. Soc., 1904, 36, 421), again, no doubt, also unaware of the communication of Perkin, terms this colouring matter C₁₅H₁₆O₆, H₂O, robigenin. Robinin he considered to possess the formula C₃₃H₄₀O₁₉.7½H₂O, and the sugars that it yields by hydrolysis to consist of galactose (1 mol.) and rhamnose (2 mols.).

Robinin is a most interesting glucoside, and with the exception of xanthorhamnin is the only known substance of this class which yields three sugar nuclei. It is practically devoid of tinctorial property.

Interesting is the fact that whereas the bark of this plant contains acacetin, the monomethyl ether of the trihydroxyflavone, apigenin, its flowers yield the glucoside of the trihydroxyflavonol kaempferol. Whether the occurrence of distinct flavones in various portions of the same plant is exceptional or otherwise, has been little studied, and appears to have only been observed elsewhere in the cases of the yellow cedar (Rhodesphaera rhodanthema), the leaves of which contain quercetin and the stem fisetin, and the Venetian sumach, the stem of which contains fisetin and the leaves myricetin.