Asymmetric Synthesis in the Methoxy-mercuration of α, β-Unsaturated Esters

The methoxy-mercuration of (−)-menthyl crotonate, cinnamate, and β-methylcinnamate resulted in partial asymmetric syntheses of the corresponding β-methoxylated derivatives, (−)-R-β-methoxybutyric acid, (−)-S-β-methoxy-β-phenylpropyl alcohol, and (−)-S-β-methoxy-β-phenylbutyl alcohol. The results suggested an ionic mechanism involving a cyclic mercurinium ion as an intermediate, formed by the electrophilic attack of mercuric acetate from the least-hindered side of the double bond in the Cram-Prelog model of the systems, followed by the attack of the methoxy anion from the opposite side. The reaction mechanism leading to the predominant enantiomers was discussed.