Photocycloaddition reactions are of special interest from the point of molecular orbital symmetry considerations. 293 Recently we have found that irradiation of tropone in acidic media provided a trans (6+6)X type cycloadd;tion product, dimer I, 4 whereas in neutral media such as in ether it afforded two different dimers, one of which was confirmed to be a (6+4)n type cycloaddition product, dimer II. The recent publication of Xende, 596 describing the formation of dimer II in addition to two other dimers on irradiation of tropone in acetonitrile, prompted us to report an outline of our study on dimer II. Tropone, on external irradiation of an ethereal solution in a Pyrex vessel with mercury'lamp (Toshiba H-400p), afforded dimer II, m.p. 137" (decomp), 7 (5% yield) and colorless needles, m.p. 117-118" (9 %) 8 accompanied by a large amount of oily and polymeric materials. The structure of II was elucidated from the following physical and chemical evidence. The mass spectrum included peaks with m/e 212 (molecular ion), 149, 106'(corresponding to tropone) and 78 (base peak, benzene).' Upon heating at melting point for 10 min II decomposed to tropone, which was identified by IR spectrum and by formation of picrate, m.p. 99".
The UV and IR spectra of II supported the presence of cis diene and saturated and a,8-unsaturated keto groups: