Thiourea-Catalyzed Synthesis of Flavanones and Chromanones

Significance

Asymmetric intramolecular conjugate additions for the synthesis of flavanones (3, R = Ar) and chromanones (3, R = Alk) are reported. Chiral bifunctional thiourea 2 mediates the reaction of chalcones 1 to heterocycles 3 in good yields and enantioselectivities. The tert-butyl carboxylate at the α-position of 1 presumably enhances the reactivity of the conjugate acceptor, favors the cyclization, and provides a second Lewis basic site for the coordination of catalyst 2. In addition, a one-pot Knoevenagel condensation-conjugate addition-decarboxylation sequence starting from β-keto ester 4 and aldehyde 5 to give natural product flindersiachromanone (7) is presented.