Sigmatropic Hydrogen Migration and Electrocyclization Processes in Compounds in the Vitamin A Series. Photochemistry of Polyenes. 10

Reactions of dienes, trienes, and tetraenes in the vitamin A series brought about by direct irradiation were examined. In addition to the previously known sigmatropic 1,5-hydrogen migration from C-18 to C-8 and the hitherto unnoticed electrocyclization, geometric isomerization also appears to be an important reaction from S₁. Electrocyclization and geometric isomerization are reversible processes; hence in most cases the end products are retro-γ derivatives. Mechanistically it was shown that the hydrogen migration process can originate from either the 7-cis or 7-trans isomers of the conjugated systems. A case of 6e-electrocyclic ring opening process involving both the excited states of the product and the reactant is presented.