196 CAROTENOIDS CONTAINING HYDROXYL GROUPS XI 



of the fact that the epoxide nature of violaxanthin was not realized at the time, 

 some of the hydrogenations were carried out in glacial acetic acid solution, 

 resulting in a partial isomerisation of the pigment (cf. p. 6i). 



Violaxanthin di-p-nitrohenzoate C54H620ioN2^^ : The ester is obtained by 

 treating violaxanthin dissolved in pyridine with p-nitrobenzoyl chloride. 

 M.p. 208° (with decomposition, uncorr., evacuated capillary). 



Violaxanthin dibenzoate Cg4Hg40g^^ : The dibenzoate is prepared in a manner 

 analogous to that employed for the compound described above. M.p. 217° 

 (uncorr. evacuated capillary). In agreement with the formula of violaxanthin, 

 neither ester contains any active hydrogen atoms. 



7. AUROXANTHIN C4oH5g04 



History and Occurrence 



During the chromatographic separation of carotenoids from Viola tricolor, 

 Karrer and Rutschmann^'' isolated a new pigment which they termed auro- 

 xanthin in view of the beautiful golden-yellow colour of its crystals. Up to the 

 present time this phytoxanthin has only been found in the blossoms of Viola 

 tricolor, but it is probable that it occurs more widely together with its isomer, 

 violaxanthin*. 



Preparation 



Dried blossoms of Viola tricolor are extracted with petroleum ether, the com- 

 bined extracts are strongly concentrated in vacuum and the phytoxanthin esters 

 are saponified with methanolic potassium hydroxide. After the saponification has 

 been completed, the free phytoxanthins are dissolved in methanol, petroleum ether 

 is added, and the pigments are precipitated by careful addition of water. The 

 crude carotenoid mixture thus obtained is recrystallised once from methanol. 

 Auroxanthin remains in the mother liquors. The pigments from the mother liquors 

 are extracted with benzene and chromatographed on zinc carbonate. Auroxanthin 

 is eluted with a mixture of ether and methanol from the upper part of the absorption 

 column. After two recrystallisations from methanol, the pigment is obtained 

 analytically pure. 



Chemical Constitution^^ 



C C 



CH2 C CH CH3 C1XI3 CH3 HgC CH — C CH2 



HOCH C GH-C=CHCH=CH-C=GHCH=CHCH=C-CH=CHCH=C-CH C GHOH 



Auroxanthin 



For the relationships between auroxanthin and violaxanthin, cf. below. 

 References p. 214— 21J. 



