The ground-state conformer composition is shown to play a determining role in the orientation of the photochemical reactions of two hexatrienes. A pronounced wavelength effect is observed for Z-2, 6-dimethyl-1, 3, 5-heptatriene; photocycloaddition leads to a bicyclo [3.1. 0] hexene and [1, 5] sigmatropic hydrogen migration occurs from an s-cis-s- trans conformation preferentially excited by longer wavelength light, whereas the ZE ...
