Although 1,3-dipolar cycloaddition is a useful method for the generation of five-membered heterocyclic compounds, few methods exist to synthesize five-membered carbocyclic rings in a single step via annulation. Most of these, like TMM cycloaddition, rely on the generation of a suitable three-atom component for combination with a stable two-atom partner such as an alkene or alkyne. When heated, cyclopropene acetals rearrange to vinylcarbenes, which can serve as the three-atom component in cycloadditions with highly electron-deficient alkenes. Zinc homoenolates can also serve as indirect three-atom components, and undergo cyclization to cyclopentenones in the presence of an unsaturated ester. Tandem intermolecular-intramolecular cyclization of homopropargylic radicals leads to methylenecyclopropanes.