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Palladium(II)-Catalyzed Asymmetric Cyclization of (Z)-4‘-Acetoxy-2‘-butenyl 2-Alkynoates. Role of Nitrogen-Containing Ligands in Palladium(II)-Mediated Reactions
journal contribution
posted on 2001-10-25, 00:00 authored by Qinghai Zhang, Xiyan Lu, Xiuling HanPd(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.