Palladium(II)-Catalyzed Asymmetric Cyclization of (Z)-4‘-Acetoxy-2‘-butenyl 2-Alkynoates. Role of Nitrogen-Containing Ligands in Palladium(II)-Mediated Reactions

Pd(OAc)₂ combined with nitrogen-containing ligands (e.g., 2,2‘-bipyridine) catalyzed the cyclization of (Z)-4‘-acetoxy-2‘-butenyl 2-alkynoates (1) in acetic acid to afford the α-(Z)-acetoxyalkylidene-β-vinyl-γ-butyrolactones (2) with high efficiency and high stereoselectivity. The nitrogen-containing ligands, like halides, served to favor β-heteroatom elimination over β-hydride elimination in Pd(II)-mediated reactions. The generality of this ligand effect was probed in both stoichiometric and catalytic reactions. With these results in hand, the catalytic asymmetric protocol was achieved with high enantioselectivity (up to 92% ee) when pymox (pyridyl monooxazoline) or bisoxazoline was used. The absolute configuration of the products and the synthetic utility of this asymmetric transformation were established through the convenient synthesis of (3S)-(+)-A-factor.