CH3 



I 



H 



CH3 



H2C4 6c V V 



H 



I 



CH3 



H 



H 



\J?' 



H 



13 



I 

 H 



CH2OH 



15 



H2C 3 2 1 C 



H, 



CH3 



CH3 



I 



H 



I 



H2C 

 H2C 



/^\ /^\ 



c 



C— CH3 



CH3 



I 



H 



H2C 

 H2C 



H2 

 CH3 



li 



C— CH3 



CH3 



H 



H2C 

 H2C 



H2 



CH3 

 /C 



H 



I 



V 



C— CH 



CH3 



i 



H 



H2C 

 FI2C 



H2 



CII3 



I 



\CV 



II 



CH3 



H 



II 



\, 



I 



H 



H 



CH3 



II 



I 



16 



N) 



u 



I 



CH20H 



CH3 

 a 



Xc/'Vc/- X 



H 



1 



c 



H 





H 



H 



CH2OH 



CH, 



I 



li 



H 



Ar 



H 



CII, 



CH3 Y 

 II 



H 



CH2OH 



\c^^\c^^r/ 



II 



C— CII3 



n 



H 



, CII3 



H2 



H 



CH3 



I 



CHaOII 



SiW-trans Vitamin A 



Retinene 



I'i-cis (neo-a) 



9-ris (iso-a.) 



9,13-dici.s (iso-b) 



1 1-m (neo-h) 



Fig. 2. Geomctrif isomers of \iianiin A and retinenc. A similar scries of strnc- 

 Uircs, (lifTcring onlv in |)ossc'ssing an extra (l()nl)le l>on(i in tlic 3.4 jjosilion. repre- 

 sents vitamin A. antl retinene:.,. The upper four structnres represent the un- 

 hindered, and hence most prohable configurations. The lowermost structure is 

 the liindered cis isomer from which all known \isnal pigments are derived. 



