Abstract
Seven-step syntheses of the antitumor cyclic peroxy ketals la, 2a, chondrillin (lb), and plakorin (2b) from (me- thoxymethoxy)benzene (8) have been achieved in 26-28% overall yields. The key step is the photooxygenation of enone 4 with a sun lamp using rose bengal lactone or CuS04 as a sensitizer which gives a mixture of peroxy hemiketals 15 and 16 in 75-85% yields. Acetal formation in acidic methanol completes the syntheses of 1 and 2. The mechanism of photooxygenation was ascertained using 3-nonen-2-one (22) as a model for 4. Irradiation converts 22 to the cw-enone 23 which undergoes photoenolization to give 24. Dienol 24 undergoes a sensitized reaction with oxygen to give 29 and 30. The detailed mechanism of this last step is not known, although singlet oxygen is probably not involved. This reaction is general for any enone or enal
which can undergo photoenolization to give a dienol. Peroxy hemiketals 33a, 41, and 43-46 were prepared in 30-80% yields. Peroxy ketals can be used for the synthesis of furans, diones, and pyridazines.