Enantioselective Synthesis of Chromanones via a Tryptophan‐ Derived Bifunctional Thiourea‐Catalyzed Oxa‐Michael–Michael Cascade Reaction

Tryptophan‐derived tertiary amine‐thiourea catalysts were shown to promote a cascade oxa‐Michael–Michael reaction between ethylene β‐keto esters and nitroolefins, resulting in the formation of 3,3‐disubstituted 4‐chromanones bearing a quaternary center in high diastereoselectivity and enantioselectivity. The chromanone products can be readily converted to biologically important fused and spiro tricyclic chromans.