lewis acid catalysis has been used in the asymmetry-setting step for the syntheses of many natural products. the first reaction shown below, from the synthesis of taxane skeleton, uses a copper-based catalyst supported by a chiral phosphoramidite ligand for a conjugate carbonyl addition reaction. the second reaction, from the synthesis of "ent"-hyperforin, uses an iron-pybox catalyst for an asymmetric diels-alder reaction.