Diastereoselectivity in the synthesis of unnatural α-amino acid esters by phase transfer catalysis

Two unnatural alpha-amino acid esters were prepared in good yields via phase transfer catalyzed Michael addition of ethyl N-acetylaminocyanoacetate to chalcone and benzalketone. For both alpha-enones, a progressive increase in product diastereomeric excess (d.e.) was observed during the course of reaction, even in the absence of quaternary ammonium salt. However, for a fixed reaction time, higher d.e. values were obtained under phase transfer catalytic condition. Analogous reactions were performed using S-aryl thiocinnamates as Michael acceptors, affording a 2-pyrrolidinone in good yield but low d.e. These results were interpreted on the basis of the reversibility of the Michael reaction.