Chloro, chloromethyl, and chlorofluoro carbenoids, generated by reaction of a base on the corresponding halides, were added to trimethylsilyl enol ethers derived from lactones, to give 1-[(trimethylsilyl) oxy]-2-chlorobicyclo-[nlO] oxanes. Thermal rearrangement of these adducts led to the a, o-ethylenic lactones, corresponding to a one-carbon ring expansion of the starting lactones. After hydrogenation, with the same iterative sequence a new one- ...