32 VITAMINS A AND CAROTENES 



A is formed which has very httle or no biological activity. This reaction 

 will be discussed more fully elsewhere. 



d. Chemical Constitution 



The structure of vitamin A was first elucidated by Karrer and his as- 

 sociates.^^ Notable contributions in this field were also made by Heilbron 

 and his associates. ^^ The empirical formida of vitamin A has been shown 

 to be C20H30O. It has already been mentioned that vitamin A is an alcohol. 

 That this alcohol is a primary one was shown by the careful oxidation of 

 vitamin A to give an aldehyde (axerophthal ).!'"'• ^"^ Upon catalytic hydro- 

 genation vitamin A yields perhydrovitamin A with an empirical formula 

 C20H40O. This indicates the presence in vitamin A of five double bonds 

 and a carbon ring. That this ring is a six-membered ring and identical with 

 that present in ,S-ionone or /3-carotene was shown by ozonolysis which 

 yielded 1 mole of geronic acid per mole of the vitamin. Careful oxidation 

 with potassium permanganate in basic solution yielded 2 moles of acetic 

 acid per mole of vitamin A, showing the presence of two methyl groups in 

 the side chain. Similarly oxidation with hot chromic acid yielded 3 moles 

 of acetic acid per mole of the vitamin, indicating the presence of three 



I 

 groups, CH3 — C^, in the molecule. Furthermore, selenium dehydrogena- 



tion yielded 2 ,6-dimethylnapthalene, which relates the structure of vitamin 

 A to that of /3-carotene (p. 9). Finally, Karrer and Morf^^^ synthesized 

 perhydrovitamin A and showed it to be identical with that obtained by 

 the complete hydrogenation of vitamin A. On the basis of these results 

 Karrer proposed structure XLVI for vitamin A. This structure is in ac- 

 cordance with all the physical and chemical properties of vitamin A and 



H3C CH3 



\ / 



C CH3 CH3 



/'\ I I 



HaC^ 1 C— CH=CH— C=CH— CH=CH— C=CH— CH2OH 



I II 123456789 



H2C4' 2'C CH3 



\3'/ 



c 



XLVI Vitamin A 



38 (a) P. Karrer, R. Morf, and K. Schopp, Helv. Chhn. Acta 14, 1036, 1431 (1931). 



(b) P. Karrer and R. Morf, Helv. Chim. Acta 16, 557, 625 (1933). 

 99 I. M. Heilbron, R. N. Heslop, R. A. Morton, E. T. Webster, J. L. Rea, and J. C. 



Drummond, Biochem. J. 26, 1178 (1932). 

 ">« R. F. Hunter and E. G. E. Hawkins, /. Chem. Sue. 1944, 411. 

 '" S. Ball, T. W. Goodwin, and R. A. Morton, Biochem. J. 42, 516 (1948). 



