HYDKOCAKBON CAKOTKNOIDS OF THE C40 SERIES 539 



(g) 5,Q-Dihydro-a-cnrotene , C40H58. When a petroleum ether sohition of 

 a-carotene is treated with concentrated hj^driodic acid in the cold state, a 

 number of chromatographically separable products are formed. Polgar 

 and Zechmeister™ succeeded in separating one of these products in 

 cr^ystalline form. On the basis of elementary analysis and of other con- 

 siderations the structure of 5,6-dihydro-a-carotene was assigned to it. It 

 melts at 202-203°C., and has no optical rotation. The crj^stals obtained 

 from a benzene-methanol mixture resemble those of a-carotene. 



(3) y-Carotenc 



The presence of a third member of the carotene series was first suggested 

 by the work of Winterstein and Ehrenberg.*^ Kuhn and Brockmann** 

 were able to separate the new pigment by the use of chromatographic 

 analysis. When it was oxidized with ozone, only one eciuivalent of geronic 

 acid was fomied per molecule of pigment. This result was interpreted to 

 mean that this carotenoid possesses only one /3-ionone ring. On ozoniza- 

 tion, 0.85 mole of acetone was produced, presumably arising from the break- 

 down of the end of the aliphatic chain. This obviously represents an open- 

 chain structure.^* On catalytic reduction, 7-carotene was shown to have 

 12 double bonds, °^ 11 of which were conjugated. This result is in contra- 

 distinction to the structure of a- and /3-carotenes, which have only 1 1 double 

 bonds. ^^ The formula suggested by Kuhn and Brockmann^^ is still the 

 generally accepted one. 



H3C CHj H jC CH3 



Q CH3 CH3 CH3 CH3 Q 



/\ hhIhhhIhhhhIhhhIhhh "^ 

 HzC c-c:C-c:c-c:c-C:c-c:C-C;C-C:C-c:c-C:C-C CH 



I II II I 



HzCC CCH2 



\ / \ / \ / 



C CHj HjC C 



Hz Hz 



7-Carotene 



Although rigorous proof has not been brought forward to establish the 

 /3-ionone structure, the fact that 7-carotene is biologically active as a precur- 

 sor of vitamin A would seem to be adequate evidence of the presence of 

 this structure. 



There arc a number of reasons to make one doubt tne homogeneityof the 

 various samples of all-^ra/ts-7-carotene described in the literature. In the 

 first place, variations in the melting point of different samples of the pig- 

 ment have been demonstrated.^^ Secondly, discrepancies in the biological 

 potency have likewise been indicated, Although a number of workers have 

 reported provitamin A activities for 7-carotene which are approximately 



