The Photochemical Reaction of 2-Alkoxy-1,4-naphthoquinones with Olefins. III. The Re-examination of the Structures of the Photo-addition Compounds and the Reaction Mechanism

In the photochemical reaction of 2-alkoxy-l,4-naphthoquinones with a variety of olefins, two types of photoaddition compounds are found to be produced: the cyclobutane-ring compounds and the oxetanol-ring compounds. Although the present author has assigned the structure of a tetrahydropyran-ring compound to one of the photoaddition compounds primarily on the basis of the analyses of the spectral data, the previously proposed structures should be corrected to an oxetanol-ring compound on the basis of the chemical reactivities. Although some cyclobutane-ring compounds were stable enough to be isolated as final reaction products, the other cyclobutane-ring compounds, though once produced, were too unstable to undergo the subsequent photochemical reaction to give oxetanol-ring compounds on further irradiation. The stability of the cyclobutane-ring compounds was dependent on both the substituents of the olefins and the wavelength of the light. The origin of the stability and the reaction mechanism were investigated.