Memory of Chirality of Tertiary Aromatic Amides: A Simple and Efficient Method for the Enantioselective Synthesis of Quaternary α-Amino Acids

A new methodology for the asymmetric synthesis of quaternary a-substituted amino acids using memory of chirality has been developed. The strategy utilizes the dynamic axial chirality of tertiary aromatic amides to memorize the initial chirality of an alpha-amino acid during an enolization step. Starting from five different L-amino acids, the corresponding oxazolidin-5-ones containing a tertiary aromatic amide group have been synthesized in one step and then alkylated with various electrophiles, with good yields and enantioselectivities (up to 96% and up to > 99% after recrystallization). One-step deprotection affords enantioenriched or enantiopure quaternary alpha-amino acids. We describe here the optimization process, the results obtained in each series and a plausible explanation, based on NMR studies, DFT calculations and crystallographic structures. The methodology presented herein constitutes an efficient synthesis of enantiopure quaternary alpha-amino acids (three steps only) starting from tertiary L-amino acids, without any external source of chirality.