Reaction of Diketene with Grignard Reagents in the Presence of Cobalt Catalyst. A Convenient Method for the Synthesis of 3-Methylenealkanoic Acids Leading to Terpenoids

Abstract
Primary alkyl Grignard reagents react regioselectively with diketene in the presence of cobalt(II) iodide to afford 3-methylenealkanoic acids in good yields. The synthetic utility of this reaction is demonstrated in the syntheses of terpenoids by two methods. Utilizing the isomerization of double bond of 3-methylenealkanoic acids, geranic acid and farnesic acid were obtained in two steps. Another method, the tandem [3,3] sigmatropic rearrangement of the corresponding allylic esters was used for the synthesis of C18-Cecropia juvenile hormone.