Bimetallic Catalytic Asymmetric Tandem Reaction of β-Alkynyl Ketones to Synthesize 6,6-Spiroketals.

The enantioselective tandem reaction of β,γ-unsaturated α-ketoesters with β-alkynyl ketones was realized by a bimetallic catalytic system of achiral AuΙΙΙ salt and chiral N,N'-dioxide-MgΙΙ complex. The cycloisomerization of β-alkynyl ketone and asymmetric intermolecular [4+2] cycloaddition with β,γ-unsaturated α-ketoesters subsequently occurred, providing an efficient and straightforward access to chiral multifunctional 6,6-spiroketals in up to 97 % yield, 94 % ee and >19/1 d.r. Besides, a catalytic cycle was proposed based on the results of control experiments.