Synthesis of 3‐Oxatricyclo[7.2.0.0^1,5]undeca‐7,10‐dienes through Intramolecular Photocycloaddition Reactions of Alkenes to 1,8 a‐Position of Naphthalene Ring

Abstract

Benzene solutions of 1-(5-aryl-2-oxapent-4-enyl)-2-cyanonaphthalenes 6 were irradiated with a high-pressure mercury lamp for 20 h to give 3-oxatricyclo[7.2.0.0 1,5 ]undecadienes 7 in good yields. In all cases, the trans isomer was preferentially obtained. Sensitization and quenching experiments showed that the reaction proceeds via an excited singlet state. When a dichloromethane solution of 6 was irradiated for only 1 h, an eightmembered ring compound 8 was obtained as an intermediate. When the obtained 8 was irradiated, the final product 7 was produced. The long wavelength absorption of 6 originates from the 2-cyanonaphthalene chromophore, indicating that the reaction is initiated by photoexcitation of the naphthalene ring. In addition, the fluorescence of 6 was intramolecularly quenched, and a weak intramolecular exciplex emission was observed at 400-550 nm. In this reaction, after photoexcitation of 6, a [2 + 2] photocycloadduct is formed at the 1,8a-position of the naphthalene ring via an intramolecular singlet exciplex, followed by thermal ring-opening and 4π disrotatory ringclosure leads to 7.