Formal Addition of Acetone to Unactivated Michael Acceptors via Ring-Opening and Retro-Claisen Fragmentation of Dihydropyranones
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We present a general protocol for the formal Michael addition of acetone to a,b-unsaturated esters and amides, a transformation difficult to perform using current methods. The protocol comprises of an amidine catalyzed relay ring-opening and fragmentation of 3,4-dihydropyranones. The reaction proceeds under mild conditions, has a broad substrate scope and the products can be isolated in good to excellent yields (30 examples, with up to 97 % yield). The method can be applied to homochiral substrates with total preservation of chiral information, generating products in high optical purity. Kinetic experiments upported by quantum chemical modeling indicates that the reaction is of second order with respect to the catalyst. The kinetic isotope effect has been determined to be 2.3, which supports a mechanism in which the catalyst takes a bifunctional role, acting both as a Brønsted base and as a hydrogen bond donor. The findings presented here enables a rapid entry to compounds previously considered difficult to access and highlights the dual functionality of amidine superbases as catalysts.