The photo-cycloaddition of enol ethers to benzene

Abstract

The photo-reactions of benzene with ethyl vinyl ether, n-butyl vinyl ether, 1,1 -dimethoxyethylene, 2,3-dihydropyran, 2,3-dihydrofuran, 2,3-dihydro-1,4-dioxin, and 1,3-dioxole are described. The meta-photo-cycloaddition of ethyl vinyl ether and n-butyl vinyl ether to benzene shows little selectivity, and regio- and stereo-isomers are isolated. ortho-Cycloaddition is observed from all systems but only with 2,3-dihydrofuran as addend are both stereoisomers of the adduct formed, and although the exo-isomer is very photolabile, the endo-product is essentially stable under its conditions of formation. The ortho-cycloaddition of 1,1 -dimethoxyethylene to benzene provides the first step in a convenient synthesis of cyclo-octatrienone. Contrary to previous proposals there is little correlation between the stereochemistry of the ortho-cycloaddition and preferred orientation of the addends in the ground state. The relationship between the relative efficiency of the two addition modes and ionisation potential of the ethylene is discussed.