Diastereoselective and Enantioselective Synthesis of α- p -Methoxyphenoxy-β-Lactones: Dependence on the Stereoelectronic Properties of the β-Hydroxy-α- p -Methoxyphenoxycarboxylic Acid Precursors.

Treatment of β-hydroxy-α- p -methoxyphenoxy carboxylic acids derived from the asymmetric glycolate aldol addition reaction with p -nitrobenzenesulfonyl chloride yielded divergent results depending on the nature of the β-substituent of the carboxylic acid. Substrates bearing either alkyl substituents (R = - n -butyl, - n -octyl, -benzyl, isopropyl, - tert -butyl) or aryl systems bearing electron-withdrawing substituents (R = - p -C 6 H 4 Cl, - p -C 6 H 4 Br, - p -C 6 H 4 NO 2 ) yielded β-lactones. In contrast, α- p -methoxyphenoxy-β-hydroxycarboxylic acids bearing electron-donating aryl groups or the sterically demanding 2-naphthyl group formed ( Z )-alkenes.