Catalytic Asymmetric Domino Michael/Annulation Reaction of Bifunctional Chromone Synthons with β,γ-Unsaturated α-Keto Esters: Rapid Access to Polysubstituted Spirocyclic Hexahydroxanthones

Abstract

A thiourea-catalyzed asymmetric domino Michael/annulation process was devised employing bifunctional oxindole-chromones as C4 synthons and β,γ-unsaturated α-keto esters as C2 synthons. This reaction enables the highly diastereo- and enantioselective synthesis of a range of biologically relevant spirocyclic hexahydroxanthones with one quaternary and four tertiary stereogenic centers, also featuring an intriguing combination of two privileged motifs, including hexahydroxanthone and oxindole substructures, in good yields (up to 76%) and excellent stereoselectivities (up to >99% ee and >20:1 dr). Moreover, using β,γ-unsaturated α-keto esters as the C2 building blocks, which are different from enone substrates such as chalcone and benzalacetone, with the reversible Michael reaction further expanded the scope of the method. In addition, scale-up also demonstrated the applicability of this protocol.