674 VII. VITAMINS A 



TABLE 1 

 Properties of Several Crystallixe Vitamin A Esters 



« J. G. Baxter and C. D. Robeson, /. Am. Chem. Soc, 64, 2411-2416 (1942). 

 '' J. G. Baxter and C. D. Robeson, /. Ajh. Chem. Soc, 64, 2407-2410 (1942). 

 <= S. Hamano, Sci. Papers Inst. Phys. Chem. Research Tokyo, 28, 69-73 (1935); 32, 

 44-49 (1937). 



<* T. H. Mead, Biochcm. ./., 33, 589-600 (1939). 



Other crystalline derivatives include the dimaleic adduct of acetylated 

 vitamin A (m.p., 261-262°C.).^^ and the corresponding benzoylated com- 

 pound (m.p., 263-264°C.). Hamano^'^'^^ prepared the dimaleic adduct of 

 vitamin A palmitate (m.p., 220°C.) by using the liver oils themselves. It 

 was later demonstrated by Tischer^" that this same compound \vas produced 

 in cod liver oil after treatment with maleic anhydride ; it was separated from 

 the mixture by molecular distillation. 



The structure of vitamin A was first correctly postulated by Karrer, 

 Morf, and Schopp.'*^ This structure was confirmed by the synthesis of 

 perhydrovitamin A/^ which was shown to be identical with the saturated 

 alcohol prepared from the natural product. The many syntheses for vita- 

 min A now in current use are all based upon the Karrer structure.*^ 



The confusion regarding the chemical nature of vitamin A has been 

 largely cleared up by the recognition that several types of isomers are pos- 

 sible. In addition to ordinary vitamin A (Ai), which is formed in marine 

 forms as well as in most mammals, a second type, probably including an 

 additional double bond, occurs in the storage depots of fresh-water fishes. 

 This is called vitamin Ao. Isomers of another type have also recently been 

 demonstrated. These are the stereoisomers. Ordinary vitamin A pre- 

 sumably represents the all-/rans form ; only 2 of the 5 double bonds in the 

 molecule are able to assume a cis configuration, while the others are pre- 

 vented from doing so because of steric hindrance. '*'* Zechmeister^^ has in- 



3^ K. Kawakami, Sci. Papers Inst. Phys. Chem. Research Tokyo, 26, 77-81 (1935). 



39 S. Hamano, Sci. Papers Inst. Phys. Chem. Research Tokyo, 26, 87-90 (1935). 



« A. O. Tischer, J. Biol. Chem., 125, 475-477 (1938). 



"> P. Karrer, R. Morf, and K. Schopp, Helv. Chim. Ada, 14, 1431-1436 (1931). 



« P. Karrer and R. Morf, Helv. Chim. Acta, 16, 625-641 (1933). 



■*' N. A. Milas, Vitamins and Hormones, 5, 1-38 (1947). 



'' L. Pauling, Fortschr. Chem. organ. Naturstoffe, 3, 203-235 (1939). 



"^ L. Zechmeister, Vitamins and Hormones, 7, 57-81 (1949). 



