The additions of singlet oxygen to four 1,3-dienes are reported. Spectral studies of the reaction products corroborate their suggested structures. Hydrolysis of these products resulted in the formation of several novel new peroxides. In the past five years the use of siloxybutadienes in 4+2 cycloadditions has attracted considerable attention.' The appeal of these diene substrates is in part due to the presence of the latent carbonyl functionalities in the cycloadducts which can be utilized for additional transformations. The Danishefsky group in particular have utilized these dienes in a number of elegant total syntheses. 2 In addition to the commonly utilized hydrocarbon dieneophiles, heterodieneophiles such as aldehydes,3 ketones,4 and imines have also been shown to react smoothly with these substrates. In conjunction with our program to synthesize and explore the properties of novel new peroxides we report the successful addition of singlet oxygen to dienes l-4. The additions of singlet oxygen to dienes 1,6 2,7 and 3* proceeds smoothly to give the 4+2 cycloadducts with only trace amounts of by-products. In contrast, the reaction of 4' produced a 25% yield of a nonperoxidic material in addition to the endoperoxide. This material was identified as phenoxytrimethylsilane 5 by comparison of its spectral properties to that of an authentic sample. We envisioned that formation of this product occurred by one of two ene reactions followed by elimination of hydrogen peroxide as shown in Scheme I. Monitoring the reaction progress at-76Β°C by low temperature l3 C NMR demonstrated that a symmetrical intermediate, (13C NMR 6 130.81 (d, J =