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3,4-Diaza-2,2,8,8-tetramethyl-1,6,10-trioxaspiro[4.5]dec-3-ene (6) was synthesized. The single crystal X-ray structure shows that the 6-membered ring is in the chair conformation with the azo group in the axial position. Comparison of the C NMR spectra of 6 in solution and in the solid state indicates that the preferred conformation in solution is also that with the azo group axial. Thus, although an alkoxy group normally occupies the axial position (the anomeric effect), the azo group outranks it insofar as the axial preference is concerned. Thermolysis of 6 in benzene afforded N2, acetone, the carbene dimer 2-(5',5'-dimethyl-1,3dioxan-2-ylidene)-5,5-dimethyl-1,3-dioxane, and 2-hydroxy-5,5-dimethyl-1,3-dioxane from reaction of the carbene intermediate with adventitious water. Those products were taken as evidence for the intermediacy of the carbene, 5,5-dimethyl-1,3-dioxan-2-ylidene.

Structure of a spiro-fused 2,2-dioxa-Δ3-1,3,4-oxadiazoline