Geometrical Isomers of Retinene by Ruth Hubbard,

We have described experiments involving the use of five crystalline isomers of retinene: the ordinary crystalline substance known previously from the work of Ball, Goodwin, and Morton (1948); neoretinenes a and b, first iso. lated in our laboratory; and isoretinenes a and b, isolated in the Organic Re. search Laboratory of Distillation Products Industries (Hubbard and Wald 1952; 1952-53). By all indications these substances are cis-trans isomers of one another. Ordinary crystalline retinene apparently is the all-trans isomer; while neoretinenes a and b and isoretinene a appear to be mono-cis forms, and isoretinene b a di-cis isomer. This series of retinenes offers an unusual opportunity to examine interrelationships within a compact stereoisomeric "set." I t has, however, also a special interest. According to the present theory of cis-trans isomerism in such isoprenoid structures as the carotenoids, the cis configuration occurs readily only at the double bonds adjacent to methyl groups (Pauling, 1939; Zech. meister, 1944). Retinene has two such bonds, numbered 3 and 5 in Fig. 1. Consequently only four stereoisomers of retinene are expected: the all-trans, 3-cis, 5-cis, and 3,5-di-cis. We have, however, five isomers. Specifically, we have three apparently mono-cis isomers where only two were expected; but if all these are genuine, one must 10ok also for three di-cis forms and one tri-cis retinene--eight isomers in all where four were expected. These substances therefore pose a dilemma. Either they are not all geometrical isomers of one another; or the theory of cis-trans isomerization in this class of compounds must be expanded. In this paper we examine the properties and interconversions of the isomeric retinenes with an eye to this problem.