II. CHEMISTKY AXD INDUSTRIAL PREPARATION 



11 



The thermal decomposition of these caroteuoids leads to the production 

 of w-xylene^'- '"* and 2 jG-dimethylnaphthalene.'" w-Xylene is assumed to be 

 derived from an unsaturated fragment; 



CH, CH, 



I I 

 =CH— C=CH— CH=CH— C=--- 



CH 



J 



H 



H 



whereas 2,6-dimethylnaphthalene is derived from the central portion of 

 the conjugated chain. 



CH 



CH 



CH 



/ \ 

 CH C— CH3 



CH,— C 



\ 



CH 



/ 



CH 



H3C 



CH HC 



CH3 



On the basis of these conclusions the structure of lycopene and that of 

 the naturally occuring pro\'itamin A carotenoids is given in Table IV. 



The structure of 5-carotene mentioned in Table I is not definitely known, 

 although a tentative structure has been proposed recently by Porter and 

 Murphey.^^ The structure of echinenone is also in doubt, although Lederer^® 

 suggested that it may be identical with myxoxanthin. Aphanicin may be 

 considered as two molecules of aphanin joined by means of an oxygen 

 bridge. 



6. Stereochemical Configuration of Provitamin A Carotenoids 



It has ah'eady been shown that provitamin A carotenoids contain several 

 conjugated double l)onds in an open chain. Since the terminal double bonds 

 in the cyclohexenyl rings are fixed, only the double bonds in the open chain 

 are subject to geometrical isomerism. Theoretically /8-carotene with nine 

 double bonds in an open chain should exist in 512 cis-trans isomers. How- 

 ever, on certain theoretical grounds Pauling" and later Zechmeister^^ have 

 shown that some of the double bonds are sterically hindered and can exist 

 only in the trans form. Furthermore, the symmetry of the structure of a 



" R. Kuhn and A. Wintorstein, Heh. Chim. Acta 11, 427 (1928). 



" L. Zechmeister and L. Cholnoky, Ann. 478, 95 (1930). 



35 J. W. Porter and M. M. Murphey, Arch. Biochcm. and Biophys. 32, 21 (1951). 



'8 P. Karrer and E. Juckcr, Carotenoids (trans, and revised by E. A. Brande), p. 



325. Elsevier Publishing Co., New York, 1950. 

 " L. Pauling, Fortschr. Chem. org. Naturstoffe 3, 203 (1939). 

 38 L. Zechmeister, Chem. Revs. 34, 267 (1944). 



