FORMATION OF BICYCLO[3.2.1]OCT-2-EN-8-ONES AND 1-HYDROXYDIHYDROSEMIBULLVALENES FROM THE META-PHOTOCYCLOADDITION OF CYCLOPENTENE TO PHENOLS

Cyano or trifluoromethyl substituents in the 3-position of phenol mark edly promote the 2,6-photocycloaddition of cyclopentene. The cyanophen ol yields both 2- and 4-cyanobicyclo[3.2.1] oct-2-en-8-ones 13 and 15 with little selectivity. In contrast, the photoreaction of 3-trifluoro methylphenol with cyclopentene gives solely stereoisomeric 1-hydroxy-2 -trifluoromethyldihydrosemibullvalenes 23 and 24. The ketones are dedu ced to arise from a facile 1,5-hydrogen shift in the 1-hydroxydihydros emibullvalenes. The specificity of the photoaddition to the trifluorom ethylphenol and the stability of the resulting 1-hydroxydihydrosemibul lvalenes are considered to originate from intramolecular hydrogen bond ing between the hydroxy and trifluoromethyl groups.