title: Regio- and endo-selective
abstract: Irradiation of various meta- and para-substituted homobenzoquinones with ethyl vinyl ether gave the [2 + 2] photoadducts, tricyclic diones, regio- and endo-selectively and in good yields. The tricyclic skeleton has an anti-form built by the addition of ethyl vinyl ether from the less hindered side of homoquinones. All of the CH(3), Cl, Br, and CH(3)O substituents at the reacting C=C double bond afforded head-to-head (HH) addition predominantly. In the case of CH(3), Cl, and Br, the ethoxy group was oriented in the endo-position, while the CH(3)O substituent led to a 1/5 mixture with the exo-isomer. It was also found that the Br-substituted [2 + 2] adducts undergo a facile skeletal rearrangement, being converted into dihydro-o-benzoquinone monomethide derivatives for para-substitution and dihydrobenzofuran derivatives for meta-substitution, probably under the influence of the in situ generated HBr. Intramolecular [2 + 2] photocycloaddition of an alkenylhomobenzoquinone afforded a tetracyclic dione.
authors.affiliation: Department of Applied Chemistry, Faculty of Engineering, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.initials: K
authors.lastname: Kokubo
authors.initials: Yi
authors.lastname: Nakajima
authors.initials: K
authors.lastname: Iijima
authors.initials: H
authors.lastname: Yamaguchi
authors.initials: T
authors.lastname: Kawamoto
authors.initials: T
authors.lastname: Oshima