Highly stereoselective oxy-Michael additions to β,γ-unsaturated α-keto esters:: Rapid enantioselective synthesis of 3-hydroxybutenolides

The highly diastereoselective oxy-Michael addition of the naked anion of (6S)-methyl delta-lactol to gamma-substituted beta,gamma-unsaturated alpha-keto esters leading to the direct formation of THP*-protected gamma-hydroxy alpha-keto ester derivatives is described. Subsequent acid-mediated deprotection affords the 3-hydroxybutenolides in high yields.