Bulky methyl ketones show significantly decreased reactivities toward the Corey-Chaykovsky methylenation reagent dimethylsulfoxonium methylide (DMSM). The excess of base and temperature increase opens an alternative reaction channel that instead leads to the corresponding cyclopropyl ketones. Computations suggest that the initial reaction step involves the methylene group transfer from DMSM on the ketone enolate followed by the intramolecular cyclization. The key step is associated with a barrier of 22 ± 3 kcal mol(-1) and is driven by exothermic elimination of DMSO.
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