A series of new 3-methylene-1-oxa-8-heteraspiro[4.5]decan-2-ones have been prepared as the first members of the family. Reaction of 1-oxa-6-heteraspiro[2.5]octanes with diethyl malonate, followed by hydrolysis, gave 3-carboxy-1-oxa-8-heteraspiro[4.5]decan-2-ones. The carboxy lactone, upon treatment with diethylamine and formaldehyde followed by sodium acetate and acetic acid, produced α-methyl-eneheteraspiro lactones in good yields and in high purity. Moreover, the carboxy lactones could be decarboxylated smoothly to give l-oxa-8-heteraspiro[4.5]decan-2-ones in good yields. The present method appears to be superior to the widely used Reformatsky reaction since only low yields of the required lactones could be obtained when applied in the work reported herein.