Photooxygenation of 3-acetyl-5-aryl-2-methylfurans via endoperoxide intermediate and the following reactions

The photooxygenation of 3-acetyl-5-aryl-2-methylfurans 1a–e selectively produced 2,2-diacetyl-3-aroyloxiranes 2a–e, 3-acetyl-1-aryl-2-pentene-1,4-diones 3a–e, and 3-acetyl-1-aryl-2-hydroxy-2-pentene-1,4-diones 4a–d via the endoperoxide intermediate A depending on the reaction conditions and the work-up procedure. The oxiranes 2a–e were mainly obtained in 56–77% yields by allowing the reaction mixture to stand at ambient temperature after the irradiation, while the treatment of the reaction mixture with water mainly gave the 1,4-diones 3a–e (62–69%). Heating the reaction mixture at 80°C after the irradiation decreased the total yield of the products, however, the enols 4a–d were newly formed in 8–12% yields. Direct UV irradiation of the endoperoxide intermediate A led to the homolytic fission of the peroxide linkage to produce the same enols 4a–e (16–39%). The self-sensitized photooxygenation of 1a–d using a UV light also gave 4a–d in a similar yield. The reaction pathway is discussed based on these results.