The transfer of oxygen atoms plays an important role in organic chemistry. Carbonyl oxides 1 and the isomer dioxiranes 2 are discussed as short-lived intermediates in decomposition processes, oxidations of organic compounds (Baeyer-Villiger Reaction, Ozonolysis), and enzymatic processes ^{1, 2, 3}. During the last years 2 have have been used as selective oxygen transfer reagents ^{4, 5, 6, 7, 8, 9}. A successful technique for the stabilization of reactive molecules is the introduction of sterically demanding groups such as t-butyl or mesityl. While several derivatives of dioxiranes 2 could be synthesized and characterized in solution, carbonyl oxides 1 are in general transient spezies.

Here we describe the preparative scale synthesis of carbonyl O-oxides 5a and 5b via oxidation of carbenes 4. Further irradiation leads to dioxiranes 6a and 6b. Dimesityl ketone O-oxide 5a was characterized by NMR spectroscopy in solution at low temperature. Dimesityldioxirane 6a forms a colorless crystalline material that could be characterized by x-ray structure analysis ^{10, 11}