62 SYNTHESIS VIII 



The most characteristic property of the epoxides is their extreme sensitivity 

 to dilute mineral acids. Even traces of hydrogen chloride such as are present 

 in chloroform after standing, are sufficient to rupture the epoxide ring^^. The 

 isomeric furanoid oxide is obtained, together with the original carotenoid 

 which is formed as by-product by the loss of oxygen*. 



The fact that the oxygen atom of these epoxides is readily lost, suggests 

 that it is bound in an unusual way not well represented by the epoxide formu- 

 lation. Karrer^^ has suggested a polar structure which explains the facile 

 conversion into the furanoid oxide as well as the ready loss of oxygen. 



C CH3 



' / \( + )2 3 I 



CH, ^C-CH = CH-C=CH- 



X-CH C— 0' ^ 



CH, CH, 



CH2I CH2I 



CH3 III CH3 IV 



According to this formulation, hydrogen chloride adds to the polar oxide I 

 with the formation of II. This is transformed into III, converted in turn to the 

 furanoid oxide IV by the loss of hydrogen chloride. 



The conversion of the polar oxide I into the furanoid oxide IV can also be 

 interpreted in the following way: an electron pair shared by carbon atoms 2 

 and 3 (formula I) is displaced towards carbon atom i under the influence of the 

 positive charge, and simultaneously the oxygen atom and its electron pair 

 adds to carbon atom 3. 



The investigations of Karrer and Jucker^^ showed that some natural 

 carotenoids, the constitution of which had not previously been established, 

 were the furanoid oxides of known carotenoids. Thus fiavoxanthin proved to 

 be identical with furanoid xanthophyll oxide, and auroxanthin with furanoid 

 zeaxanthin dioxide. 



The action of alkyl magnesium salts on carotenoid epoxides gives rise to 

 the same products as the reaction with hydrogen chloride, namely the furanoid 



* This applies to all partially synthetic epoxides so far prepared, cf. table 14. 

 References p. 64-65. 



