HYDROCARBON OAROTENOIDft OF THE C40 SERIES 533 



aluminum isopropoxide in isopropanol. Up to now it has been obtained 

 only as an oil. 



H5C CHj 



\ / 



C 



CH, CH3 CH3 



/\ HHlHHHiHHHHl 



HeC c-c:c-c:C'C:C'C:c-c:C'C:C- CH^OH 



1 II 



HjC C 



\ / \ 

 C CH3 



Hz 



^-Apo-4-carotenol 



(r) Dihydrocarotenes. Polgar and Zechmeister"" prepared a new com- 

 pound by treating j8-carotene in petroleum-ether solution with a commercial 

 concentrated hydrogen iodide without the application of heat. This was 

 isolated on the chromatographic column and was shown to be 5,6-dihydro- 

 /3-carotene. Other double bonds were not affected, as was indicated by 

 the fact that these were still able to undergo trans-cis rearrangement when 

 heated, on melting, or by acid or iodine catalysis. 



d. Properties. Such properties as solubility, absorption maxima, 

 melting point, and stereoisomerism of /3-carotene and its derivatives are 

 discussed in Section 6 of this chapter. 



(3) a-Carotene 



a. Structure. Although a-carotene was discovered almost simultane- 

 ously, in 1931, by Kuhn and Lederer*^""*^ and Karrer and associates,^"-^^ it 

 was not until several years later that it was prepared in pure form. Karrer 

 and Walker '^^ succeeded in accomplishing this purification by the use of 

 new adsorbents — calcium oxide and calcium carbonate — which they demon- 

 strated to be effective in the separation of the epiphasic polyene pigments. 

 It was at first thought that a-carotene possessed neither a /3- nor an a-ionone 

 ring, since on ozonization neither geronic nor isogeronic acid could be 

 detected."^ However, it was later shown by Karrer, Morf, and Walker"* 

 that geronic and, to a slightly lesser extent, isogeronic acids, did originate 

 when a-carotene was treated with ozone. Presumably the geronic acid is 

 formed from the /3-ionone residue which is present at one end of the mole- 

 cule, while the isogeronic acid results from the degradation of an a-ionone 

 structure which occurs on the opposite end of the aliphatic chain. a-Caro- 

 tene also contains a series of conjugated double bonds. On catalytic 



'■« A. Polgdr and L. Zechmeister, J. Am. Chem. Soc, 65, 1528-1534 (1943). 

 '"' P. Karrer and O. Walker, Hdv. Chim. Acta, 16, 641-643 (1933). 

 1" P. Karrer, R. Morf, E. von Krauss, and A. Zubrvs, Helv. Chim. Acta, 15, 490-493 

 (1932). 



'"'P. Karrer, R. Morf, and O. Walker, Helv. Chim. Acta, 16, 975-977 (1933). 



