ABSTRACT
By contrast, the X-ray study of 11-cis-retinal established a single conformation in the crystalline state, 12-s-cis, with a 39° twist from a planar s-cis conformation. Ebrey, and co-workers synthesized the sterically hindered 11-cis isomer of 14-methylretinal in which the 12-s-cis conformation accessible to the parent 11-cis-retinal is precluded by severe steric repulsion between the 14-methyl and the C10-hydrogen. The absorption maxima of the several isomers maintaining the 11-cis geometry reveal a dramatic reversal of the intensity of the retinal absorption bands. In fact, the 12-s-cis-locked aldehydes exhibit their main maximum to slightly lower wavelengths than the corresponding alcohols. The selection rules for the thermally-allowed processes predict the [l,5]-sigmatropic hydrogen shift to be suprafacially allowed and antarafacially forbidden. The vinylcuprate approach proved to be less than satisfactory, not only because of the side products obtained in the coupling step, but also because the method was found to be restrictive.
