The enantioselective tandem reaction of b,g-unsaturated a-ketoesters with b-alkynyl ketones was realized by ab imetallic catalytic system of achiral Au III salt and chiral N,N'-dioxide-Mg II complex. The cycloisomerization of balkynyl ketone and asymmetric intermolecular [4+ +2] cycloaddition with b,g-unsaturated a-ketoesters subsequently occurred, providing an efficient and straightforwarda ccess to chiral multifunctional 6,6-spiroketals in up to 97 %yield, 94 % ee and > 19/1 d.r.Besides,acatalytic cycle was proposed based on the results of control experiments. Scheme 2. Substrate scope for b-alkynyl ketones. Unless otherwise noted, all reactions were performed with Mg(OTf) 2 (2.5 mol %), AuCl 3 (2.5 mol %), l-RaPr 3 (5 mol %), 1a (0.10 mmol), 2b-2o (0.15 mmol), 5 M. S. (30 mg) in DCM (1.0 mL) at 40 8 8Cfor 2d ays. [a] In DCM (0.5 mL) at 40 8 8Cf or 4d ays. Scheme 3. The transformations of the chiral product 3a.Scheme 4. Control experiments and plausible mechanism.
A highly efficient enantioselective [2+2] cycloaddition between alkynones and cyclic enol silyl ethers was developed by using a chiral N,N'-dioxide-zinc(II) complex as a catalyst. This method functions well for a variety of terminal alkynes as well as cyclic enol silyl ethers, with good to excellent enantioselectivity (up to 97 % ee). This is also the first successful example for the catalytic enantioselective [2+2] cycloaddition of internal alkynes with cyclic enol silyl ethers to give fully substituted cyclobutenes. Meanwhile, the desired cyclobutene product can easily be transformed into fused cyclobutane derivatives.
A highly efficient enantioselective [2+2] cycloaddition between alkynones and cyclic enol silyl ethers was developed by using a chiral N,N′‐dioxide‐zinc(II) complex as a catalyst. This method functions well for a variety of terminal alkynes as well as cyclic enol silyl ethers, with good to excellent enantioselectivity (up to 97 % ee). This is also the first successful example for the catalytic enantioselective [2+2] cycloaddition of internal alkynes with cyclic enol silyl ethers to give fully substituted cyclobutenes. Meanwhile, the desired cyclobutene product can easily be transformed into fused cyclobutane derivatives.
An efficient asymmetric vinylogous Michael−aldol domino reaction between α-arylidene pyrazolinones and β,γunsaturated-α-ketoesters catalyzed by a chiral N,N′-dioxide-Sc III complex in aqueous media has been established. A variety of spirocyclohexene pyrazolones with three stereocenters including vicinal tetrasubstituted stereocenters were obtained in excellent yields with good diastereoselectivities and enantioselectivities. A retro-aldol process was observed, which led to epimerization at the spirocyclic quaternary carbon center.
The direct and sequential Friedel–Crafts alkylation/hemiketalization procedures for the synthesis of chiral chromans resulted in excellent yields, high enantioselectivities and high diastereoselectivities for a broad range of substrates.
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