734 VII. VITAMINS A 



(a) Structure of Anhydrovitamin A. Although Heilbron, Morton, and 

 Webster ^^" referred to the product formed on selenium dehydrogenation as 

 "c^^clized" vitamin A, more recent studies of Shantz ei al. have shown that 

 it is an unsaturated hydrocarbon formed from vitamin A by the loss of one 

 molecule of water. ^''^ Analysis of a crystalline cyclized vitamin A prepara- 

 tion prepared by Robeson (cited by Hickman'*'*) would seem to indicate 

 that the composition corresponds to C20H2S rather than to one of C20H30O. 

 The analytical data are given in Table 13. 



TABLE 13 

 Analysis of Anhydrovitamin A" 



Analysis C. % H, % 



Experimental 88.57 10.43 



Anhydrovitamin A, CjoHos, theory 89.55 10.45 



"Cyehzed" vitamin A, CioHaiO, theory 83.92 10.49 



" K. Hiolcman, Ann. Rev. Biochem., IS, 353-396 (1943). 



Anhydrovitamin A is readily prepared by treating a dilute alcoholic 

 solution of the vitamin with anhydrous hydrochloi'ic acid'^*'^^* or by reflux- 

 ing an alcoholic solution of the natural esters. ^"'^^^ The change to anhy- 

 drovitamin A may be brought about simply by allowing & vitamin A solu- 

 tion to stand for a few minutes with the alcoholic hydrochloric acid ; on the 

 other hand, when the reaction is allowed to proceed for 12 to 16 hours, a 

 further change occurs. Meunier et al.^"^' have suggested that the formation 

 of anhydrovitamin A involves an ionization of vitamin A in the presence of 

 acid, followed by a shift of the positive charge to a carbon in the ionone 

 ring; the molecule eventually regains its electrical neutrality by losing a 

 proton from this carbon atom. 



Anhydrovitamin A has 6 double bonds instead of the 5 in vitamin A; 

 all appear to be conjugated. Shantz, Cawley, and Embree^^'^ have pro- 

 posed the formula shown here as the most probable for anhydrovitamin A. 



HjC CH3 



r C H 3 C H 3 



/ \ H H I H H H I 

 HzC C • C : C • C : C • C : C • C : C *• CHj 



I II 



HpC c 



\ / \ 



C CHj 



Hj 



.\nh.\drovitainin .\ 



2« E. M. Shantz, J. D. Cawlev, and H. D. Embree, ./. Am. Cheni. Soc, 65, 901-906 

 (1943). 



2« E. Le B. Gray and J. D. Cawlev, /. Nutrition, 23, 301-307 (1942). 



2" p. Meunier, R. Dulou, and A. Vinet, Bull. soc. chim. bioL, 25, 371-378 (1943). 



