46 VITAMINS A AND CAROTENES 



dehydrogenase and cozymase. It is surprising that when the natural vita- 

 min A was replaced with crystalline vitamin A and neovitamin A the sys- 

 tem failed to synthesize rhodopsin. However, when solutions of the crystal- 

 line vitamins were exposed to light in the presence of traces of iodine, the 

 products formed became effective precursors of rhodopsin. Shice enzymes 

 are known to be stereospecific, it is quite possible that other stereoisomers 

 than the all-^mws vitamin A and neovitamin A are responsible for the pro- 

 duction of rhodopsin. But both the a\\-trans vitamin A and neovitamin A 

 are effective in all body processes including vision; therefore it is quite 

 likely that they are isomerized in vivo to the stereospecific isomer which is 

 responsible for the production of rhodopsin. 



4. Vitamin A Aldehyde (Retinene) 



Vitamin A aldehyde (LIX, Fig. 6) may be considered as an oxidation 

 product of vitamin A. It was first isolated from retinas by Wald,^^^ who 

 named it retinene, and it was later recognized as the vitamin A aldehyde 

 by Morton. ^^^ Its importance in the visual processes warrants here a brief 

 description of its preparation and physical and chemical properties. 



Vitamin A aldehyde was first prepared in small yields by the direct 

 oxidation in dilute acid solution of vitamin A with potassium permangan- 

 ate^^" and by the application of the Oppenauer reaction to vitamin A.^"" 

 Better yields were obtained by the oxidation of vitamin A with a specially 

 prepared manganese dioxide. ^"^ It has also been obtained by the oxidation 

 of /3-carotene.'*-^ A direct synthesis from Cis-ketone (LIV — > LVII — > 

 LVIII — > LIX, Fig. 6) was published by Arens and van Dorp.^^^ 



Vitamin A aldehyde, like vitamin A, should exist in four stereoisomeric 

 forms. 1^^ The one described in the previous paragraph is presumably the 

 aW-trans form. The 3-cis isomer was synthesized via the 3-cis Cis-ketone 

 according to the same sequence of reactions used for the synthesis of the 

 a]\-trans form (Fig. 6).^^'' Neovitamin A aldehyde or neoretinene (probably 

 the 5-cis form) was prepared by the oxidation of neovitamin A with man- 

 ganese dioxide. '^^' ^^^ The physical properties and some of the derivatives 

 of these aldehydes or retinenes are given in Table XII. 



The 2,4-dinitrophenylhydrazones of retinenci and neoretinene were also 

 prepared!"^ • ^^^ and were found to have essentially the same m.p. 207 to 208° 

 (214 to 215°'^^). With antimony trichloride in chloroform retinenci gives a 



168 G. Wald, /. Gen Phi/siol. 19, 351 (1935). 

 16' R. A. Morton, Nature 153, 69 (1944). 



'^0 R. A. Morton and T. W. Goodwin, Nature 153, 405 (1944). 

 '^' J. F. Arens and D. A. van Dorp, Hec. trav. chim. 68, 604 (1949). 

 ''2 P. Meunier and J. Jouanneteau, Bull. soc. chim. hiol. 30, 260 (1948). 

 '" K. R. Farrar, J. C. Hamlet, H. B. Ilenbest, and E. R. H. Jones, /. Chem. Soc. 

 1952, 2657. 



