Regioselective reaction of Grignard reagents in the presence of a copper (I) catalyst or of organocuprates with the terminal vinyl carbon of β-isopropenyl-β-propiolactone through the SN 2′ pathway affords 4-methyl-3-alkenoic acids in which the (E)-isomers predominate in good yields. Synthetic utility of this reaction is demonstrated in one-step synthesis of homoterpenoic carboxylic acids.
