Abstract TPP-sensitized photooxygenation of tetramethylallene (4) in carbon tetrachloride yields acetone (5), 2, 4-dimethyl-4-hydroxy-1-penten-3-one (8) and 2, 4-dimethyl-1, 4- pentadien-3-one (9) in a ratio of 35: 20: 45, besides minor amounts of resinous products and carbon dioxide. Isomerization of 4 to 2, 4-dimethyl-1, 3-pentadiene (6) does not occur under the reaction conditions. DABCO quenches the photooxygenation, whereas 2, 4, 6-tri-t- ...