Photochemistry of polyenes. Control by orbital symmetry and ground state conformations?

n is shown that ground state conformations of cyclohexadienes aild acyclic hexatrienes play an important role, in addition to orbital symmetry •factors, to govern the stereochemistry of the photoisomerization reactions of these compounds.Studying the influence of the wave-length of Irradiation allows to attribute to discrete conformations some specific photoreactions (H shift, formation ofbicyclo (3.1.0 }hexene).New examples are given, wlth kinetic data, showing a (4rr a + 2TT a} stereespecificity for the hexatriene Photo-DIEL8-ALDER reaction with a predominent orbital control.The concertedness of the reaction is discussed.Kinetics of Z, E-photoisomerizations around C-C double bonds of hexatrienes are shown, in relation with the ease of terminal compared to central double bonds rotations.• Singlet and triplet states have different behaviours in this respect, as weil as different wave-lengths of excitation, The photochemical behaviour-of conjugated polyenes is an important field of interest in chemistry, and a good part of t.he efforts have been concentrated to the understanding of the properlies of the8e remarkable compounds (1).This is probably due not only to the theoretical significance ofthat class of TT-electron systemsbot also to their practical importance (2).A very stimulating challenge was indeed initiated eleven years ago by the fruitful ideas of WOODWARD and HOFFMANN (3) but a good deal of understanding is still left to be gained, as far as the excited state chemistry is concerned (4), if lt is true that the ground state behaviour is now very widely interpreted.The difficulty with the light initiated processes is indeed that: 1 -.The exact nature of the excited state is not always clearly known.2-.The reaction may occur after loss of the electronic excitation.3-.There may be.several consecutive fast photoreactions and the primary process is sometimes difficult to individualize.This is probably why several groups found it interesting to synthesise some specific polyenes and to !!ltudy their photoisomerization reactions, particularly in i-elation wlth orbital symmetry conservation concepts.