Ultrasound-Promoted Enantioselective Decarboxylative Protonation of [alpha]-Aminomalonate Hemiesters by Chiral Squaramides: A Practical Approach to Both Enantiomers of [alpha]-Amino Esters

Herein, we report an ultrasound-promoted enantioselective decarboxylative protonation reaction of [alpha]-aminomalonate hemiesters 1 in the presence of chiral cinchona-derived squaramide Brønsted bases under mild conditions, which afforded both the (S)- and (R)-enantiomers of [alpha]-amino acid derivatives 2 in excellent yields (> 90%) and excellent enantioselectivities (up to 98% ee).