268 CAROTENOID CARBOXYLIC ACIDS XIII 



On treating labile bixin with methanolic potassium hydroxide, the pot- 

 assium salt^** is first obtained, but this is converted by prolonged shaking, or 

 by boiling for a short time, into labile norbixin. By boiling with aqueous 

 ethanolic potassium hydroxide, stable norbixin can be obtained besides the 

 labile form®^. 



Investigations by B. von Euler, H. von Euler and Karrer^^ j^^ve shown 

 that labile bixin has no vitamin A activity. 



The pigment dissolves in concentrated sulphuric acid with a cornflower-blue 

 colour. For further colour reactions see the communication by Kuhn and 

 co-workers®'. 



The sodium salt of labile bixin crystallises from 70% ethanol in dark, 

 copper-red crystals®*. The potassium salt forms deep violet needles which are 

 readily soluble in ethanol and methanol, but insoluble in water. 



Dihydrobixin C25H32O4: 



Karrer and co-workers®^ prepared this compound by a method analogous 

 to that employed for the preparation of dihydronorbixin (p. 265). Kuhn and 

 Winterstein®® prepared dihydrobixin by brief shaking of labile or stable 

 bixin in pyridine with zinc dust and acetic acid. M.p. 207-208° (uncorr.)®^ 

 (For the constitution of dihydrobixin, see p. 265.) 



Solvent Absorption maxima 



Carbon disulphide 454 428 m^ 



Chloroform 435 410 m// 



(of. Fig. 20, p. 356) 



Perhydrobixin, perhydronorbixin mono-methyl ester CjsH^gOj: 

 This compound is formed by the catalytic hydrogenation of labile bixin in 

 acetic acid in the presence of a palladium-barium sulfate catalyst®'. It is a 

 colourless oil, b.p. 2i3-2i7°/o.3 mm.®^ D^° 0.9368; n^" 1.4615. 



Labile methylbixin, dimethylester of labile norbixin C26H32O4: 

 Labile methylbixin can be obtained by the esterification of labile bixin or 

 labile norbixin with dimethyl sulphate®^. Labile methylbixin is also formed by 

 the action of diazomethane on labile bixin dissolved in chloroform'", or on labile 

 norbixin'^. It crystallises from ethyl acetate in red, pleochroic rhombs, m.p. 

 163° (uncorr.)'°. It is fairly readily soluble in chloroform, acetic acid and ethyl 

 acetate, sparingly in ethanol and very sparingly in methanol. For quantitative 

 extinction measurements, see the communication by Hausser and Smakula'^. 

 Labile methylbixin can be converted into the stable form by the action of 

 iodine in the same way as labile bixin''. Numerous investigations have been 

 made regarding its oxidative degradation. For details, the original literature 

 should be consulted'*. 

 References p. 2go-2g4. 



