SYNTHESIS OF 4 DIASTEREOMERIC L-2-(CARBOXYCYCLOPROPYL)GLYCINES - CONFORMATIONALLY CONSTRAINED L-GLUTAMATE ANALOGS

To determine what conformations of L-glutamate (L-Glu) activate that compound's different receptors in the mammalian central nervous system, four diastereomeric L-2-(carboxycyclopropyl)glycines, 1-4, which are conformationally constrained analogues of the extended and folded conformers of L-Glu, were synthesized and subjected to neutrophysiological assay. Compounds 1-4 were efficiently synthesized from chiral amino acids. Cyclopropanation of the (2S)-2-amino-3-butenol derivative 5b gave intermediates for the synthesis of all four diastereomers. Stereoselective cyclopropanation of both the alpha,beta-unsaturated gamma-lactam 16 and the delta-lactone 19 gave precursors of (2S,1'S,2'R)-3 and (2S,1'R,2'S)-4, respectively. Neurophysiological assays of 1-4 performed with the newborn rat spinal cord demonstrated that the compounds induced a variety of depolarizing effects. The results of the assays strongly suggested that the N-methyl-D-aspartic acid (NMDA) receptor is activated by the folded conformer of L-Glu and that the extended conformer of L-Glu activates the metabotropic L-Glu receptor. The four analogous D-2-(carboxycyclopropyl)glycines (D-1-D-4), which were synthesized from (2R)-5b, proved to be NMDA agonists.