The neo-b isomer of vitamin A and retinene.

Among the geometrical isomers of vitamin A and retinene, the neo-b isomer occupies a special position. As the precursor of rhodopsin and iodopsin, it plays a central r81e in rod and cone vision (1, 2). It is stored almost exclusively in the eyes of certain Crustacea (3) and is equally distinctive chemically. The four unhindered geometrical isomers of vitamin A have been identified (4, 5); neo-b proved to be a fifth form (Fig. 1). Necessarily, therefore, it possesses a hindered cis linkage, the first such configuration to be found in nature. Apparently, also, it is monocis; hence, either 7or 11-cis. The synthesis of an 11-cis isomer, which was not neo-b, led us to conclude that the latter must be 7-cis (6). In inferring this, however, we were led astray by a prevalent misconception regarding the configuration of one of the components used: the presumed 11-cis isomer synthesized earlier was in fact 11,13-dicis (neo-c). What is apparently the 11-cis isomer has since been synthesized and is identical with neo-b (6). The present paper describes the preparation and properties of neo-b vitamin A and retinene.