322 CAROTENOIDS OF UNCERTAIN STRUCTURE XIV 



sorbed on the uppermost part of the column and can be eluted with a mixture of 

 ether and methanol. It is purified by precipitation with water from methanol 

 solution covered with petroleum ether and subsequent crystallisation from metha- 

 nol. About 40 mg of taraxanthin are obtained from 200 mg of phytoxanthin 

 mixture. 



Taraxanthin can be prepared more simply from spring cabbage^®. The blossoms 

 are dried, finely ground, and extracted with petroleum ether at room temperature. 

 The phytoxanthin esters are saponified with ethanolic potassium hydroxide and the 

 products are separated into an epiphasic and an hypophasic fraction. The free 

 phytoxanthins are precipitated from the latter by the addition of water under a 

 layer of petroleum ether. The pigment thus obtained is again dissolved in methanol, 

 precipitated with water and finally recrystallised from methanol. In this way 4 mg 

 of pure taraxanthin are obtained from about 500 blossoms. 



Chemical Constitution 



The constitution of taraxanthin is still unknown. The molecular formula 

 C4oH5g04 shows that the pigment is isomeric with violaxanthin (cf. p. 193). 

 The similar spectral properties of the two pigments indicate that they contain 

 similar chromophoric systems. 



KuHN and Lederer^' obtained values corresponding to 3-4 active hydrogen 

 atoms in Zerewitinoff determinations. On catalytic hydrogenation, tara- 

 xanthin absorbs 11 mols of hydrogen and the composition of the perhydro- 

 derivatives shows that the pigment contains two isocyclic rings. 



Properties 



Crystalline form: Taraxanthin crystallises from methanol in fine lustrous 

 prisms or plates. 



Melting point: 185-186° (uncorr.)6o, 184-185° (corr.)«i. 



Solubility: The pigment is readily soluble in benzene, ether, ethanol and 

 methanol, but insoluble in petroleum ether. 



Spectral properties: 



Solvent: Absorption maxima:* 



Carbon disulphide 501 469 441 m// 



Petrol 472 443 m// 



(cf. Fig. 24, p. 358) 



Optical activity: [a]g^ § = +200° in ethyl acetate solution. 



Partition test: Taraxanthin exhibits entirely hypophasic properties. 



Chromatographic behaviour: The pigment is well adsorbed on zinc carbonate 

 from benzene solution (cf. Kuhn and Brockmann^^). 



Quantitative extinction measurements are reported by K. W. Hausser and A. 

 Smakula, Z. angew. Chem. ^7 (1934) 663; 48 (1935) 152. — K. W. Hausser, Z. techn. 

 Pkys. 15 (1934) 13- 

 References p. 341-243. 



