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.