Asymmetric Roadmap to Diverse Polycyclic Benzopyrans via Phosphine-Catalyzed Enantioselective [4 + 2]-Annulation Reaction

The
catalytic addition of the amino acid derived bifunctional N-acylamino­phosphine to an α-substituted allene
ester generated a zwitterionic dipole that engaged the vinylogous
ester function of 3-cyano-chromones in a [4 + 2] annulation reaction
to deliver tetrahydroxanthones embodying three consecutive chiral
centers in high yields and with excellent enantioselectivities. The
established asymmetric synthesis further paves the way to two different
classes of complex, sp3-rich tetracyclic benzopyrans via
efficient cascade reactions.