Abstract
The conjugate addition of cyclic 1,3-dicarbonyl compounds to β,γ-unsaturated α-keto-esters was studied using a series of chiral bifunctional organocatalysts. Takemoto’s catalyst was found to be most efficient for this transformation. Excellent yields and good enantioselectivities were achieved for a variety of β,γ-unsaturated α-keto-esters and cyclic 1,3-dicarbonyl compounds. A bifunctional catalytic mechanism is proposed. The method provides a new asymmetric synthetic route for chiral courmarin derivatives.
Highlights
Asymmetric conjugate addition of 1,3-dicarbonyl compounds to various Michael acceptors is an important method for the preparation of chiral compounds.[1]
Jørgensen et al found that chiral bisoxazoline-copper catalysts are efficient for the asymmetric conjugate addition of cyclic 1,3-dicarbonyl compounds to β,γ-unsaturated α-keto-esters.[4]
Liu and co-workers found that chiral N,N′-dioxide-yttrium(III) complexes catalyze the conjugate addition of malonates to β,γ-unsaturated α-keto-esters with excellent yields and enantioselectivities.[6]
Summary
Asymmetric conjugate addition of 1,3-dicarbonyl compounds to various Michael acceptors is an important method for the preparation of chiral compounds.[1]. Quinine-derived thiourea 4 provided the expected product 3a in excellent yield and with moderate enantioselectivity (Table 1, entry 1). The same phenomenon was observed by Jørgensen and his co-workers.[4] Cinchonine-derived thiourea 5 gave 3a in excellent yield, but with lower enantioselectivity (Table 1, entry 2).
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