The synthesis of lactones. Part III: δ-Enollactones and lactones with exocyclic double bonds

A series of 4-alken-5-olides has been prepared by dehydration of the corresponding 5-oxoalkanoic acids. These enollactones are sensitive to both oxygen and moisture. For the synthesis of 9cis-dodecen-5-olide, 4-heptynylmagnesium bromide was coupled with cyclopentanone to form 1-(4-heptynyl)cyclopentanol, which was oxidized to 5-oxo-9-dodecynoic acid. Two-step reduction and dehydration yielded the lactone. Similarly, 5-oxo-6-dodecynoic acid, prepared from 1-heptynylmagnesium chloride and glutaric anhydride, was converted into 6cis-dodecen-5-olide. 6cis-Dodecen-4-olide was synthesized by coupling 2-octynylmagnesium bromide with methyl 4-oxobutanoate to give methyl 4-hydroxy-6-dodecynoate; this was converted into 6-dodecyn-4-olide, which was partially hydrogenated. The product was separated gas-chromatographically from the accompanying 5-vinyl-4-decanolide.