Base-promoted reaction between dialkyl ketones having y-hydrogens and dialkyl (diazomethy1) phosphonates leads to formation of cyclopentenes in modest to high yields. The relative reactivity of various types of carbon-hydrogen bonds for the process, which involves insertion by an alkylidenecarbene, has been assessed, and the result has been compared to those reported when the carbene is generated by a elimination and by 1, 2- ...