CH, 



CII3 



CII3 



C 1 



\ 



C3 



C 



c 



C 4 



c 



C 5 



CH.OII 



CHj 



C 



CH, 



CH3 



all -</ans 



3-c/s 



C=C 



CHs 



C=C 



C=C 



C=C CH3 



/ \ / 



c c 



\ / \ 



C— C CH3 



c=c 



/ \ 



CHs CH2OH 



CH, 



5-c/s 



C=C 



/ \ 



CH3 C=C CH2OH 



\ / 



c=c 



/ 



CH3 c=c 



\ / 



C=C CH3 



/ \ / 



C c 



\ / \ 



C— C CH3 



CH3 



C=C 



CH3 



C=C 



c=c 



c=c 



/ \ 



c— c 



CH3 



CH3 



c=c 



CH, 



CH20H 



3, 5-(ii -c/s 



Fig. 21. Structures of four geometrical isomers of vitamin A. According to present 

 theory, cis-trans isomerization occurs readily only about double bonds 3 and 5, yield- 

 ing only the four isomers shown. Ordinary crystalline vitamin A, as also the bulk of 

 the commercial synthetic product is the a\\-trans isomer. It is not yet possible to 

 assign the other structures with confidence to specific preparations of the vitamin. 

 (From R. Hubbard and G. Wald.^") 



77 



