Stereospecific photoisomerisations of 6-acetoxy-2,3,4,5,6-pentamethylcyclohexa-2,4-dienone

Abstract

U.v. irradiation of 6-acetoxy-2,3,4,5,6-pentamethylcyclohexa-2,4-dienone in ether or methanol gives exo-6-acetoxy-1,3,4,5,6-pentamethylbicyclo[3,1,0]hex-3-en-2-one, whose structure is established by degradation to tetramethylcyclopent-2-enone derivatives. Low-temperature spectroscopy and trapping experiments show the dienone to suffer stereospecific photochemical ring opening to a keten, which can be trapped by cyclohexylamine, and may isomerise to the bicyclohexenone in a stereospecific thermal bond-crossing reaction, or to the dienone by a thermal and probably also a photochemical reversion. The latter processes are strongly solvent-dependent.