Photooxidation of a-Terpinene and Cyclopentadiene

A [4 + 2]-cycloaddition of singlet oxygen to a-terpinene in methanol yields ascaridole in the presence of catalytic amounts of Rose Bengal as photosensitizer. The reaction 

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. 

A xenon lamp was used as light source that irradiated the thin falling films flowing through a quartz glass window in the reactor [317]. Thiourea in methanol was used to reduce the labile endoperoxide directly to the stable diol product. The yield of cis-2- 

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