594 VI. CAROTENOIDS AND RELATED COMPOUNDS 



40 C 



CAROTENES 



8 C 



2 4 



8 C 



M ETHYL 

 HEPTENONE 



B I X I N 



METHYL 

 HEPTENONE 



27 C 



3 C 



Fig. 2. A schematic diagram of the relationship of carotenoid 

 cleavage products to the carotenes.*^ 



and Panzer*^^ suggested the empirical formula of C2BH30O4 on the basis of 

 elementary analyses. 



The nomenclature for the several bixin compounds is somewhat confused. 

 Table 10 (p. 595) lists the different nomenclatures used for the several com- 

 pounds, as well as their melting points and geometrical configurations. 



a. Labile and Stable Bixins. Labile bixin was first recognized as a 

 carotenoid-like substance by Kuhn and Winterstein/^ who described it 

 as a pure aliphatic compound made up of dehydrogenated isoprene residues 

 in which the double bonds are conjugated. An incorrect structural for- 

 mula was first suggested by these workers, who believed that the two car- 

 boxyl groups were not of equal value. More recently it has been shown 

 that the dissimilarity of the molecule is the result of stereoisomerism'^^'^^* 

 and is not due to asymmetry of the carbon skeleton. Bixin contains nine 

 conjugated double bonds, while the positions of the methyl side chains are 

 identical with that of the central section of the C40 carotenoid molecule. 

 The structural formula of bixin now generally accepted is the one suggested 

 by Kuhn and Winterstein." 



CHj CH, CH, 



iiHhIhhhIhhhhIhhhI 

 chjoc •c:c-c:c-c:c-c:c-c:c-c:c-c:c-c:c 



Labile or stable bixin 



OH 5 



i H H ° 

 C : C- C- 



OH 



"2 A. Heiduschka and A. Panzer, Ber., 50, 546-554, 1525-1526 (1917). 

 ^" P. Karrer, A. Helfenstein, R. Widmer, and T. B. van Itallie, Helv. Chim. Acta, 

 12, 741-756 (1929). 



