Exposure of spiro β-lactones and 3-substituted 4,4-dialkyl oxetan-2-ones to magnesium bromide in diethyl ether solvent resulted in the smooth generation of β,γ-unsaturated acid derivatives in high yield and isomeric purity. It is believed that this reaction occurs via the formation of a stable tertiary carbocation at the β-carbon resulting from cleavage of the carbon-oxygen sigma bond due to the complexation of magnesium cation with the ring oxygen atom. Rapid loss of an adjacent proton then furnishes the unsaturated acids. The β-lactone precursors were prepared by the dehydration of β-hydroxy acid derivatives (obtained from the condensation of ketones with acetic acid dianions) employing benzenesulfonyl chloride in pyridine solvent. Since this synthetic method offers ready access to a wide range of β,γ-unsaturated acids which were previously difficult to obtain, a detailed synthetic methodology and suggestions pertaining to the reaction mechanism are of prime interest and are discussed in detail
