574 Vr. CAROTENOIDS AND RELATED COMPOUNDS 



pure pigment from 30 kg. of the ripe fruit of the yew {Taxus baccata). It 

 was present in the seed coverings. 



a. Structure. Rhodoxanthin has been show^n to have an empirical 

 formula of C40H50O2. The absorption of the longer wave lengths by rho- 

 doxanthin solutions differs from that of the other carotenoids; this fact 

 must be ascribed to variations in structure. It is impossible to form an 

 ester of rhodoxanthin ; on the other hand, it does form a well-crystallized 

 dioxime with hydroxjdamine, with the empirical formula C40H52O2N2. 

 Since such polyene aldehydes as lycopenal do not react with hydroxylamine, 

 it must be assumed that rhodoxanthin has a ketone group. Both car- 

 bonyl groups must be in conjugation wdth the double bond system, since a 

 considerable spectroscopic difference obtains between the pigment and its 

 dioxime. 



On catalytic hydrogenation, rhodoxanthin adds 12 molecules of hydro- 

 gen, indicating that it has a paired number of conjugated double bonds. 

 In fact, if rhodoxanthin is further hydrogenated, it takes up two additional 

 molecules of hydrogen, which are involved in the reduction of the ketones to 

 secondarj' alcohols. It is therefore evident that rhodoxanthin is the most 

 highly unsaturated of the carotenoids, since it will take up a total of 14 

 molecules of hydrogen. The structure postulated by Kuhn and Brock- 

 mann^"^ is supported by the fact that, on reduction to dihydrorhodoxanthin, 

 the resulting compound is optically similar to j3-carotene and zeaxanthin. 

 This argues in favor of the identity of their conjugated double bond sys- 

 tems. 



H3C CHj H3C CH3 



r CH3 CH3 CH3 CHi p 



/\ hhIhhhIhhhhIhhhIhh /\ 

 HjC c:c-c:c-c:c-c:c-c: C-C:c-c:c-c:c-C-. c-c:c CH? 



II II 



c c c c 



" \ -^ \ / ^ / "^0 



C CH3 H3C c 



H H 



Rhodoxanthin 



HjC CH3 H3C CH3 



^(^ -CHi CH, CHj CH3 ^c^ 



/\ hhIhhhIhhhhIhhhIhh /\ 

 H?C C'C:c-c:C-c:C-c:c-C:C-C:C-C:C-C:C'C:c-C CH? 



I II II I 



c c c c 



o^'' \ / \ / \ / -- 



C CH3 H3C c 



H? Hi 



Dihydrorhodoxanthin 



b. Occurrence. As noted earlier, one of the chief sources of rhodoxan- 

 thin is in the seed hulls of the yew ( Taxus baccata) . It has also been found 

 in the reddish brown leaves of Potamogeton natans (floating-leaf pond- 

 weed), ^^ as well as in leaf extracts of the moss-like herb Selaginella, and the 



