THE PHOTO-CYCLOADDITION OF 3-CYANO-2,4-PENTANEDIONE WITH CYCLOHEXENE AND ITS QUENCHING BY ENOL ETHER

The 1,3-dicarbonyl compound, with an electro-negative substituent, 3-cyano-2,4-pentanedione, gives the photoaddition product with cyclohexane. The adduct must be formed by the retro-aldol reaction of the “2+2” cycloaddition product and the hemiacetalization of the resulting 1,5-dicarbonyl compound. Enol ether, with a lower ionization potential, does not photoadd to 3-cyano-2,4-pentanedione but quenches the photoaddition between 3-cyano-2,4-pentanedione and cyclohexene. The solvent effect, the ionization potential, and quenching experiments show that the photoaddition follows a process involving a singlet excited complex.