Formation of bicyclo [3.2.1]oct-2-en-8-ones and 1-hydroxy-dihydrosemibullvalenes from the meta-photocycloaddition of cyclopentene to phenols

Abstract

Cyano or trifluoromethyl substituents in the 3-position of phenol markedly promote the 2,-photocycloaddition of cyclopentene. The cyanophenol 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-trifluoromethylphenol with cyclopentene gives solely stereoisomeric 1-hydroxy-2-trifluoromethyldihydrosemibullvalenes 23 and 24. The ketones are deduced to arise from a facile 1,5-hydrogen shift in the 1-hydroxydihydrosemibullvalenes. The specificity of the photoaddition to the trifluoromethylphenol and the stability of the resulting 1-hydroxydihydrosemibullvalenes are considered to originate from intramolecular hydrogen bonding between the hydroxy and trifluoromethyl groups.