Cyclopropyl building blocks in organic synthesis. Part 81: Striving for unusually strained oxiranes: epoxidation of spirocyclopropanated methylenecyclopropanes

1-Oxa[3]triangulane , the epoxide of methylenespiropentane, is thermally stable up to 300°C, but immediately rearranges to spiro[2.3]hexan-4-one () in the presence of lithium iodide at ambient temperature. The permethylated bicyclopropylidene is simply less reactive than the parent bicyclopropylidene () towards dimethyldioxirane, but yields the isolable epoxide (94%) with mCPBA. In contrast, the partially or fully spirocyclopropanated bicyclopropylidenes , , and , upon treatment with mCPBA or dimethyldioxirane, did not furnish the corresponding epoxides, but underwent oxidation with rearrangement to the corresponding cyclobutanones , and in yields of 59, 100 and 97%, respectively.