Iterative synthesis of selectively substituted .alpha.,.beta.-unsaturated and saturated medium-ring lactones

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-[n.1.0]oxanes. Thermal rearrangement of these adducts led to the α,β-ethylenic lactones, corresponding to a one-carbon ring expansion of the starting lactones. After hydrogenation, with the same iterative sequence a new one-carbon ring expansion could be performed. This method allowed the preparation in good yields of hitherto unknown medium-ring lactones