242 CAROTENOIDS CONTAINING CARBONYL GROUPS XII 



pigment contains ten double bonds'^, which must be conjugated in view of the 

 long- wavelengths location of the absorption maxima. Zerewitinoff deter- 

 minations and esterification''* proved that only two of the three oxygen atoms 

 are present as hydroxyl groups. (This result was confirmed by Zechmeister 

 and VON Cholnoky'^^). The third oxygen atom belongs to a carbonyl group 

 which is directly attached to the system of conjugated double bonds and cannot be 

 oximated. The presence of a carbonyl group was indirectly proved by Zechmeis- 

 ter and VON Cholnoky'^^ by showing that perhydrocapsanthin contains three 

 hydroxyl groups which can be acetylated. This is in agreement with the results 

 of microhydrogenation'^^ during which capsanthin takes up ii mols of hydrogen. 

 The presence of a carbonyl group and its position in the conjugated system was 

 confirmed by the investigations of Karrer and Hubner in the course of which 

 capsanthin was converted into the corresponding alcohol, capsanthol, by 

 reduction with aluminium isopropoxide and isopropyl alcohol. Capsanthol is a 

 triol of the formula €401157(011)3, the longest wavelength absorption band of 

 which is displaced by only 35 m/< towards shorter wavelengths as compared with 

 capsanthin. This shows that the carbonyl group in capsanthin must be ter- 

 minally conjugated rather than cross-conjugated, otherwise the displacement 

 of the absorption maxima on reduction would be larger owing to the break 

 in conjugation. 



Karrer and co-workers ^® subjected capsanthin to permanganate degra- 

 dation and obtained a:a-dimethylmalonic acid and a:a-dimethylsuccinic acid. 

 No a:a-dimethylglutaric acid was formed, so that the presence of an unsubsti- 

 tuted j3-ionone ring is excluded. These findings are in agreement with the results 

 of biological assay'^ which show that capsanthin has no vitamin A activity. 

 A further conhrmation of the open-chain formula of capsanthin was provided 

 by an investigation of Karrer and Jucker^", in the course of which a caro- 

 tenoid containing the chromophoric system present in capsanthin was obtained 

 b}' a rational partial synthesis (cf. p. 250). 



Properties 



Crystalline form: Capsanthin crystallises from carbon disulphide in deep 

 carmine-red spheres. From petrol, the pigment is obtained in needles, and from 

 methanol in prisms. 



Melting point: 176° (uncorr.)^^, 175-176° (corr.)^^^ 



Solubility: Capsanthin is readily soluble in acetone and chloroform, less 

 soluble in methanol, ethanol, ether and benzene, only sparingly soluble in car- 

 bon disulphide, and almost insoluble in petroleum ether^^. 

 References p. 253-255. 



