All 16 2-(2‘-carboxy-3‘-phenylcyclopropyl)glycine (PCCGs) stereoisomers 32−47 have been prepared from the corresponding racemic aldehydes 12−15 following an enantiodivergent synthetic protocol. Compounds 32−47 were evaluated by a number of binding and functional experiments as potential ligands for several classes of excitatory amino acid receptors, including metabotropic glutamate receptors (mGluR1a, mGluR2, mGluR4) and ionotropic glutamate receptors (NMDA, KA, AMPA) as well as sodium-dependent and calcium/chloride-dependent glutamate transport systems. The stereolibrary of compounds 32−47 appears to be endowed with a peculiar pharmacological profile. PCCG-2 (33) and PCCG-3 (34) displaced labeled kainate at low micromolar concentration; PCCG-9 (40) and PCCG-11 (42) weakly interacted with the NMDA site; PCCG-5 (36), PCCG-10 (41), and PCCG-12 (43) showed to be potent inhibitors of Ca2+/Cl--dependent glutamate transport system. Most interestingly, PCCG-4 (35) has been shown to be able to antagonize (IC50 = 8 μM) the effects of glutamate on forskolin-stimulated cAMP formation in BHK cells expressing mGluR2. Uneffective at mGluR1, 35 is a weak mGluR4 agonist (EC50 = 156 μM) and has no effect on either ionotropic receptors or glutamate transport systems, thus demonstrating to be a novel selective mGluR2 antagonist with a 6-fold increase in potency over previously reported antagonists.
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