Photooxygenation cyclopentadiene

Photooxygenation of a-terpinene 155 in the presence of eosin (equation 87) produces ascaridole 156, a constituent of the essential oil Chenopodium ambrosioides L 1. The ewrfo-peroxide 157 derived from cyclopentadiene is a crystalline solid, stable at —100 °C78 above this temperature it rearranges to a mixture of the bis-epoxide 158 and the epoxyaldehyde 159 (equation 88)79,80. 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

The photooxygenation of cyclopentadiene with Rose Bengal as photosensitizer was performed using a falling-film microreactor in methanol [317]. The endoperoxide is first generated and then reduced to 2-cyclopenten-l,4-diol, which is used as an intermediate in pharmaceutical drug synthesis. This route is not easily possible by batch processing because the explosive endoperoxide intermediate is formed in substantial amounts. 

Endoperoxidation is the preferred process for cyclic dienes, and simple five, six, seven, and eight membered ring aliphatic dienes lead, by photooxygenation, only to the corresponding endoperoxides which, except for that of cyclopentadiene, are quite stable (Sch. 17) [28]. 

Except for the endoperoxide from 1,3-cyclopentadiene, the higher homologues are thermally stable compounds, which have been isolated in moderate to good yields under various photooxygenation conditions. " Cyclic 1,3,5-trienes, which can undergo valence tautomerization, give the respective cycloaddition products (Table 25.2). 

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