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

The photooxygenation of 3-acetyl-5-aryl-2-methylfurans la-e selectively pro duced 2,2-diacetyl-3-aroyloxiranes 2a-e, 3-acetyl-1-aryl-2-pentene-1,4-dion es 3a-e, and 3-acetyl-1-aryl-2-hydroxy-2-pentene-1,4-diones 4a-d via the en doperoxide intermediate A depending on the reaction conditions and the work -up procedure. The oxiranes 2a-e were mainly obtained in 56-77% yields by a llowing the reaction mixture to stand at ambient temperature after the irra diation, 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 degreesC after the irradiation decreased the total yield of the products, however, t he enols 4a-d were newly formed in 8-12% yields. Direct UV irradiation of t he endoperoxide intermediate A led to the homolytic fission of the peroxide linkage to produce the same enols 4a-e (16-39%). The self-sensitized photo oxygenation of 1a-d using a UV light also gave 4a-d in a similar yield. The reaction pathway is discussed based on these results. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.