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

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