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
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