704 VII. VITAMINS A 



from the imsaponifiable fraction of ling cod liver oil/®^ as well as from pike 

 liver concentrates. Morton ei al.^^^ demonstrated the relationship of reti- 

 nene2 to vitamin A2, basing their conclusions upon its conversion to the 

 latter substance when fed to rats, or when exposed to aluminum isopro- 

 poxide (Ponndorf reaction). The absorption maxima and the extinction 

 coefficients of crystalline retinene-j were found to be the following: cyclo- 

 hexane, 385 m/x, 1-100; light petroleum, 388 m^, 1455; chloroform, 40G m/x, 

 1390; ethanol, 395 m/ji, 1320. Retinene2 also reacts with antimony tri- 

 chloride to show a band with a maximum color at 735 m/x, E (1%, 1 cm.) = 

 3885, and an inflection at 705 m/x.'^^ 



(2) Vitamin A Acid 



Vitamin A acid is the product obtained when the terminal alcohol group 

 on the side chain of vitamin A is replaced by a carboxyl group. The struc- 

 ture of vitamin A acid is given here. 



HjC CH3 



C CH3 C Hj 



/\ hhIhhhIh /O 

 HjC C • c :c- c: c- c: C* c: C • C-OH 



I II 



HzC C 



\ X \ 

 C CHj 



Hz 



Vitamin Ai acid 



Vitamin A acid was originally synthesized from /3-ionylidene acetalde- 

 hyde by Heilbron et al. ^^^ in 1936. Subsequently, a number of workers ^^^' ^^^ 

 were unable to prepare the j8-ionylidene acetaldehyde as described by Heil- 

 bron et al.,^^^ and therefore doubt was cast on the usefulness of this method. 

 Arens and van Dorp^^^-"" later synthesized vitamin A acid by employing (3- 

 ionylidenecrotonic acid as the starting material. Other methods have also 

 been employed for the synthesis of this acid (see Section 5). 



Vitamin A acid has a strong absorption band in the ultraviolet with 

 ^(max.) = 347 mjtx. The value for the molecular extinction coefficient (log 

 E) equals 4.64. According to Karrer, Jucker, and Schick, ^^' vitamin A acid 

 produces a red color with a purplish luster when antimony trichloride is 

 added to it ; the maximum absorption of the resulting compound is at 572 

 m/x. 



18^ M. K. Salah and R. A. Morton, Biochem. J., 43, Ivi-Ivii (1948). 

 i«« I. M. Heilbron, W. E. Jones, A. Lowe, and H. R. Wright, /. Chem. Soc, 1936, 

 561-563. 



16^ H. Sobotka, E. Bloch, and D. Glick, J. Am. Chem. Soc, 65, 1961-1963 (1943). 



i«8 P. Karrer and A. Riiegger, Helv. Chim. Acta, 28, 319 (1945). 



i«9 J. F. Arens and D. A. van Dorp, Nature, 157, 190-191 (1946). 



"0 D. A. van Dorp and J. F. Arens, Rec. trav. chim., 65, 338-345 (1946). 



"1 P. Karrer, E. Jucker, and E. Schick, Helv. Chim. Acta, 29, 704-711 (1946). 



