Asymmetric Syntheses of the Flavonoid Diels–Alder Natural Products Sanggenons C and O

Metal-catalyzed, double Claisen rearrangement of a bis-allyl­oxy­flavone has been utilized to enable a concise synthesis of the hydro­benzo­furo­[3,2-b]­chrom­enone core structure of the natural products sanggenon A and sanggenol F. In addition, catalytic, enantio­selective [4+2] cyclo­additions of 2′-hydroxy­chalcones have been accomplished using B­(OPh)3/BINOL complexes. Asymmetric syntheses of the flavonoid Diels–Alder natural products sanggenons C and O have been achieved employing a stereo­divergent reaction of a racemic mixture (stereodivergent RRM) involving [4+2] cycloaddition.