II. CHEMISTRY AND INDUSTRIAL PREPARATION 



43 



in pottMuy from 500,000 to 1,()00,000 U.S. P. units per gram and are sold 

 at this writing at a price of 12 cents per million units. 



3. Stereoisomers of Vitamin A 

 a. Stercoisomeric Considerations 



It has already been mentioned elsewhere that \'itamin A (structure 

 XLVI) may exist in several stereoisomeric forms. Of the five double bonds 

 present, only four in the side chain can contribute toward stereoisomerism, 

 and owing to the existence of steric hindrance""'*^ only two (3 and 5) may 

 assume the cis configuration. On this basis there should be four stereoiso- 



H,C CH, CHs CH, 



H I H I 

 ,Q c 3 c* JC5 ^CHjOH 

 >Q ^C ^C "^C 

 H H H H 



CH, 



Lxx?:viii 



2,3,4,5-Telra- irons vitamin A or vitamin Ai 



CH, 



H 



^C=C^ ,C, C 

 C C" C^ TH,OH 



HjC CHj 



CHj 



LXXXIX 



2,4,0-TTi-trans 3-cis vitamin A 



H H 



^C^ 

 H 



CH, 



H,C CH, CH, CH, 



y H I H l_H 



r J ^c ^ ^c 



[^ H H H 

 CH, 



XC 

 2,3A-Tn-trans o-cis vitamin A 



CH,OH 



H,C CH, CH, 



\/ H I H H 



CH, 



XCI 



2,3-Di-trans 3,o-di-cis vitamin A 



^C" ^C=C" 



H I H 

 CH, 



CHjOH 



or neovitamin A 



Fig. 9. Stereoisomers of vitamin A. 



mers of vitamin A as shown in Fig. 9. Of these only two are definitely 

 known; the all-/ra?i6' form, commonly known as vitamin Ai, and neovitamin 

 A which has probably the 5-cis configuration. It is (juite possible that under 

 certain conditions all stereoisomers ma\' exist in nature at the same time, 

 and, like their precursors, they can be interconverted on exposure to light 

 in the presence of traces of iodine."^' 



b. Neovitamin A or 2 ,3 ,4-Tri-trans 5-cis Vitamin A 



This vitamin was recognized by Hcjbeson and Baxter,'^- who isolated it 

 from the "non-crystallizable portion" of natural vitamin A samples. It was 

 found to constitute about 35% of the total vitamin A present in fish liver 



'«' K. Ilublnird and G. Wald, Science 116, 60 (1952). 



1" C. D. Robeson and J. G. Baxter, J. Am. Chcm. Soc. 69, 136 (1947). 



