New general asymmetric synthesis of versatile .gamma.-alkylated butenolides and its application to expeditious synthesis of the chiral Geissman-Waiss lactones useful for (+)-retronecine synthesis

A new strategy for general asymmetric synthesis of chiral butenolides 7a–h is established by utilizing excellent diastereocontrolled alkylation with chiral tin(II) enolate 10 at the γ-position of hydroxy butenolides 4a–h. An efficient utility of the chiral butenolides is exemplified with the expeditious synthesis of the optically pure Geissman-Waiss lactone derivatives 23a and 23b.