An underexplored type of heterocyclic α-diazocarbonyl compounds, α-diazo glutaconimides was involved in the generation of oxonium ylide species by Rh(II) catalysis in the presence of tetrahydrofuran and 1,4-dioxane. The formation of a spirocycle through a formal Stevens-type O→C [1,2]-shift within the oxonium ylide species in the case of THF was low-yielding and was accompanied by the known formal O→O [1,4]-shift to give fused bicyclic products. Reactions with 1,4-dioxane proved noticeably higher-yielding with respect to the target spirocycle. In this case, however, the observed by-products resulted from hitherto not described alternative formal O→C [1,4]-shift accompanied by the transposition of the double bond. The latter fate of the oxonium ylide species, although minor, is likely attributable to the unique nature of α-diazo glutaconimides as precursors to Rh(II) carbenes and subsequently formed oxonium ylide.