Diastereoselective synthesis of all four isomers of 3-(4-chlorophenyl) glutamic acid: identification of the isomers responsible for the potentiation of L-homocysteic acid-evoked depolarizations in neonatal rat motoneurons.

All four isomers of 3-(4-chlorophenyl)glutamic acid (5-8) were prepared by diastereoselective synthesis. Addition of (6S)-(+)-bis-lactim ether 15 to cis-4-chlorocinnamate 12 gave a mixture comprising mainly the (2R,3S)- and (2R,3R)-isomers 5 and 6, respectively (in a ratio of 56:40), while addition of (6R)-(-)-bis-lactim ether 16 to 4-chlorocinnamate 12 gave a mixture comprising mainly the (2S,3R)- and (2S,3S)-isomers 8 and 7, respectively (in a ratio of 56:42). The four stereoisomers (5-8) were therefore conveniently prepared by addition of either 3-lithio-(6S)- or -(6R)-bis-lactim ether (15 or 16, respectively) to 4-chlorocinnamate 12 and separation of the resultant mixtures of diastereoisomers (23-26) by flash silica gel chromatography. The absolute configurations of 6 and 7 were confirmed by X-ray crystallography. Both the (2S,3S)- and (2S,3R)-isomers (7 and 8, respectively) at a concentration of 100 microM significantly potentiated depolarizations induced by 10 microM L-homocysteic acid (L-HCA) (% control +/- sem: 130.4 +/- 3.6, n = 20 and 114.5 +/- 2.4, n = 11, respectively) while the (2R,3S)-isomer 5 significantly reduced L-HCA response amplitude (94.2 +/- 1.4, n = 9) and the (2R,3R)-isomer 6 was inactive. Experiments designed to compare the agonist-potentiating actions of 7 and 8 in the neonatal rat spinal cord with L-trans-pyrrolidine-2,4-dicarboxylic acid, the well-known L-Glu uptake inhibitor, provided additional evidence for the selective enhancement of depolarizations due to L-HCA and not those due to L-Glu. This selective action supports the existence of multiple excitatory amino acid uptake sites.