Stereocontrolled Construction of 3,4-Dihydrocoumarin Scaffolds with a Quaternary Amino Acid Moiety via Chiral Squaramide-Catalyzed Cascade Michael Addition/Lactonization Reaction

An asymmetric tandem Michael addition-lactonization between ortho-nitrovinylphenols and azalactones was investigated for constructing 3,4-dihydrocoumarin backbones with a quaternary amino acid moiety. Under the catalysis of the chiral squaramide derived from l-tert-leucine, a wide range of substituted (E)-2-(2-nitrovinyl)phenols and azalactones were well tolerated in this tandem reaction to provide the corresponding biologically significant 3,4-dihydrocoumarin derivatives in excellent yields with high levels of diastereo- and enantioselectivity.