Diazo compounds 3a and 3b were irradiated with l > 515 and 495 nm in oxygen saturated CCl₃F or in 1 : 1 mixtures of CCl₃F/(CBrF₂)₂ at low temperatures. The absorption maxima of the intense yellow solutions were at 398 and 394 nm. The yield of 5a is estimated to 20 - 25 %. As expected, 5a is very photolabile and irradiation with l > 475 nm rapidly results in the decolorization of the solution and formation of dioxirane 6a as the major product (Scheme 1) ^{10, 12}. Alternatively, carbonyl oxide 5a was synthesized by the reaction of diazo compound 3a with ¹O₂ in a variety of solvents with small amounts of photosensitizers. Good results were obtained in CCl₃F / octafluoroporphyrin, THF / rose bengal, or pentane / C₆₀ at temperatures below -80° C and l > 570 nm excitation. Carbonyl oxide 5a is the first carbonyl oxide that could be characterized by ¹H and - after ¹³C isotopic labeling - ¹³C NMR spectroscopy. The chemical shift of the "carbonyl" carbon atom of 5a was found at 211.0 ppm ¹⁰.

Figure 1: ¹³C-NMR spectra showing the thermal reaction of 5a to produce 7a and 8a.
Bottom: Spectrum produced by irradiation (l > 515 nm) of ¹³C-3a in O₂ saturated CFCl₃/(CBrF₂)₂ at 77K and subsequent warming to -70° C.
Top: Same as bottom, but additional warming to room temperature and re-cooling to -70° C.