A general, stereospecific, synthesis of S,Y-unsaturated a-amino acids using the S-anion derived from aspartic acid is described . S,Y-Unsaturated amino acids have been found to be reversible or irreversible inhibitors of a number of enzymes,' and this has prompted a number of racemic syntheses of this class of amino acids .'' 2 Although a few members of this group of amino acids have been synthesised stereospecifically, 3 only two reports dealing with the general synthesis of optically active 3,Y-unsaturated a-amino acids have so far been published 4,5 and both make use of the Schollkopf bis-lactim ether . In this paper we report the synthesis of S,Y-unsaturated amino acids using the S-ester enolate derived from aspartic acid . This approach has also been used for the synthesis of other non-proteinogenic amino acids 6-e Diester (1) was reacted with two equivalents of lithium diisopropylamide (LDA), or two equivalents of lithium hexamethyldisilazide (LHMDS) under the conditions previously described, 6 followed by benzaldehyde or propanal, to give hydroxydiesters (2a,b) in CO2Me 1) 2 LHMDS , OH COZMe (1) CO2-t-Bu a, R=Ph, R'=H b, R=Et, R'=H Scheme 1 2) RCOR' C0 2-t-Bu 0040-4020/89 $3 .00+ .00 'c`1989 Pergamon Press plc in 50-60% yield . Only two of the four possible diastereomers were obtained (as a 1 :1 mixture), and these were identified as the stereoisomers with the hydroxyl and 6-ester groups in a syn configuration as described later . Hydrogenolysis of hydroxydiester (2a) to amine (3) and subsequent conversion to the Mosher amide (4) allowed determination of the optical purity of the products . The corresponding amide (5), derived from racemic aspartic acid, was prepared similarly . The 19 F n .m .r spectrum of amide (5) showed 6319