Asymmetric oxa‐Michael‐aza‐Henry Cascade Reaction of 2‐Hydroxyaryl‐Substituted α‐Amido Sulfones and Nitroolefins Mediated by Chiral Squaramides

A series of chiral squaramides were employed in the oxa‐Michael‐aza‐Henry cascade reaction of 2‐hydroxyaryl‐substituted α‐amido sulfones and nitroolefins. In the presence of 10 mol % of a squaramide catalyst and K 2 CO 3 (aq) in CHCl 3 at 0 °C, this reaction proceeded smoothly to afford chiral multisubstituted 4‐amino‐3‐nitrobenzopyrans, bearing a readily removable N‐Boc protecting group (Boc= tert ‐butoxycarbonyl), in excellent yields (up to 98 %) and enantioselectivity [up to 98:2 enantiomeric ratio ( er ) and 93:7 diastereomeric ratio ( dr )]. The 4‐amino‐3‐nitrobenzopyrans were converted into chiral 3,4‐diamino chromanes, which were further exploited as chiral ligands in the asymmetric transfer hydrogenation of acetophenone, providing 1‐phenylethanol in up to 84 % yield and 93.5:6.5 er .