The syntheses of five homoserine phosphate analogs (1-4, Z-5) are described. A brief summary of their potential to inhibit threonine synthase from Escherichia coli is given. Bacterial threonine synthase (TS) [EC 4.2.99.2], a pyridoxal phosphate (PLP) dependent enzyme, catalyzes the final step in the biosynthesis of L-threonine from L-aspartic acid, i.e. the conversion of L-homoserine phosphate into L-threonine. The reaction involves the non-hydrolytic elimination of phosphate from the PLP bound substrate and subsequent addition of water to a PLP bound vinylglycine intermediate1). We were interested in homoserine phosphate analogs both as potential inhibitors of TS and also in order to study intermediates of this enzymic reaction by UV spectroscopy. The sulfur containing phosphonic acid 1 has been reported to be a slow binding inhibitor of TS^2, and the nitrogen analog L-2 to possess N-methyl-D-aspartic acid (NMDA) antagonistic activity3). Since no experimental details for the synthesis of these two compounds have been published yet, we developed syntheses for racemic and enantiomerically pure phosphonic acid 1, as well as for the racemic analogs 2 and 3. The pentynoic acid 4 not only resembles homoserine phosphate, it is also a simple derivative of propargylglycine, which is a well known inhibitor of various PLP dependent enzymes4). Amino acid Z-6 represents the C-terminus of two classes of naturally occuring di- and tripeptides, the rhizocticins and the plumbemycins. Z-6^{5,6} has the L-configuration in the fungicidal rhizocticins7, whereas the D-enantiomer is in the plumbemycins8,9), which are active against bacteria. Moreover, Natchev has also reported that the L-enantiomer of Z-6 displays herbicidal and fungicidal activity6). Inhibition studies with racemic Z-6, performed in our laboratory10, clearly showed that this compound is an inhibitor of bacterial TS (Table 1). The amino acids E-5^{(1)} and E-6^{(2)} have already been described in the literature as NMDA antagonists. Therefore, we undertook the synthesis of the unsaturated amino acid Z-5 to complete the series of pentenoic acids 5 and 6. In this letter we describe the facile syntheses of phosphonic acids 1-4, and Z-5, and give a brief summary of their action on bacterial TS.