The stereo- and regio-specific formation of 11-oxatetracyclo[6.2.1.0.0]undeca-5,9-dienes by the substituent-directed photocycloaddition of furan to the benzene ring

Abstract

Furan undergoes specific 2,5–2′,6′(meta)-photocycloaddition to benzonitrile, the toluonitriles, phenylacetylene, and 1-phenylpropyne. 2-Phenylpropene yields exclusively a [2_π+ 2_π] cycloadduct with furan whereas both types of addition occur with styrene. In contrast, 1,4–2′,5′(para)-photocycloaddition is the sole or principal reaction from 254 nm irradiation of isoprene, 2,3-dimethylbuta-1,3-diene, or 2,3-dimethoxybuta-1,3-diene with benzonitrile; only with the fixed cis-1,3-diene, 1,2- dimethylenecyclohexane, is the meta-cycloadduct formed in significant amounts. The control over the regio- and stereo-chemistries of the meta-cycloaddition is discussed in terms of substituent stabilization of the developing polarity in the electronically excited aromatic ring on approach of the addends and secondary orbital interactions.