516 VI. CAROTENOIDS AND RELATED COMPOUNDS 



— CHo— C=CH— 



I 

 CH, 



It was therefore concluded that a chain containing four isoprene groups 

 separates the two ionone residues. On the other hand, when oxidation is 

 carried out by the so-called "wet oxidation" method of Kuhn and L'Orsa,'" 

 which employs chromic acid,^^~'^^ six molecules of acetic acid are produced 

 per molecule of /S-carotene.*^-'^ The acetic acid which arises with per- 

 manganate treatment belongs only to the aliphatic chain, while the two 

 additional acetic acid molecules which result from the wet oxidation 

 method must originate from the two /3-ionone rings at each end of the mole- 

 cule. Since one acetic acid molecule is produced for each branched methyl 

 group present, it is evident that four such groups exist in the central ali- 

 phatic part of the molecule, and one on each /3-ionone ring at the two ends 

 of the molecule. 



The relative position of the methyl groups in the molecule was deter- 

 mined from the products of thermal decomposition. Thus, it was shown 

 that toluene and w-xylene could be identified among these compounds,^'* ~''^ 

 as well as 2,6-dimethylnaphthalene," but not toluic acid, which was formed 

 only by bixin and azafrin.'^'^''^ Finally, the position of the methyl groups 

 was conclusively established by the method of successive oxidative deg- 

 radation, and also by the synthesis of the completely hydrogenated carot- 

 enoids and their hydrogenated fission products. 



Karrer, Helfenstein, Wehrli, and Wettstein^^ in 1930, and Karrer and 

 Morf^® in 1931, postulated the structural formula for /3-carotene now known 

 to be correct. The later investigations of Pummerer et al.^^ and of Strain^^ 

 give added confirmation to the above structure, as do the subsequent data 

 of Kuhn and Winterstein," who stated that 2,6-dimethylnaphthalene is 

 formed on thermal decomposition of /3-carotene. 



A further rigorous proof of the structure of /S-carotene was carried out by 

 Kuhn and Brockmann,^^ who established the correctness of the Karrer 

 formula. The demonstration of the conversion of (8-carotene to j8-carot- 



^o R. Kuhn and F. L'Orsa, Z. angeiv. Chem., U, 847-853 (1931). 

 ^> R. Kuhn and H. Brockmann, Ber., 67, 885-888 (1933). 

 72 R. Kuhn and L. Ehmann, Helv. Chim. Acta, 12, 904-915 (1929). 

 " R. Kuhn and F. L'Orsa, Ber., 64, 1732-1736 (1931). 

 '^ L. Zechmeister and L. v. Chohioky, Ann., 478, 95-111 (1930). 

 " R. Kuhn and A. Winterstein, Ber., 66, 1733-1741 (1933). 



7«R. Kuhn, A. Winterstein, and W. v. Wiegand, Helv. Chim. Acta, 11, 716-724 

 (1928). 



" R. Kuhn and A. Winterstein, Ber., 66, 429-432 (1933). 



78 R. Kuhn and A. Winterstein, Ber., 65, 1873-1880 (1932). 



" R. Kuhn and A. Winterstein, Ber., 65, 646-651 (1932). 



«o R. Pummerer, L. Rebmann, and W. Reindel, Ber., 64, 492-502 (1931). 



81 H. H. Strain, J. Biol. Chem., 102, 137-150 (1933). 



" R; Kuhn and H. Brockmann, Ann., 516, 95-143 (1935). 



