Stereoselective Chemoenzymatic Synthesis of the Four Stereoisomers of l-2-(2-Carboxycyclobutyl)glycine and Pharmacological Characterization at Human Excitatory Amino Acid Transporter Subtypes 1, 2, and 3

The four stereoisomers of l-2-(2-carboxycyclobutyl)glycine, l-CBG-I, l-CBG-II, l-CBG-III, and l-CBG-IV, were synthesized in good yield and high enantiomeric excess, from the corresponding cis and trans-2-oxalylcyclobutanecarboxylic acids 5 and 6 using the enzymes aspartate aminotransferase (AAT) and
branched chain aminotransferase (BCAT) from Escherichia coli. The four stereoisomeric compounds were
evaluated as potential ligands for the human excitatory amino acid transporters, subtypes 1, 2, and 3 (EAAT1,
EAAT2, and EAAT3) in the FLIPR membrane potential assay. While the one trans-stereoisomer, l-CBG-I,
displayed weak substrate activity at all three transporters, EAAT1−3, we found a particular pharmacological
profile for the other trans-stereoisomer, l-CBG-II, which displayed EAAT1 substrate activity and inhibitory
activity at EAAT2 and EAAT3. Whereas l-CBG-III was found to be a weak inhibitor at all three EAAT
subtypes, the other cis-stereoisomer l-CBG-IV was a moderately potent inhibitor with 20−30-fold preference
for EAAT2/3 over EAAT1.