A series of 1N-substituted derivatives of (2R, 4R)-4-aminopyrrolidine-2, 4-dicarboxylate (2R, 4R-APDC) has been prepared as constrained analogs of γ-substituted glutamic acids and examined for their effects at recombinant metabotropic glutamate receptor (mGluR) subtypes in vitro. Appropriate substitution of the 1N position of 2R, 4R-APDC resulted in the identification of a number of selective group II mGluR antagonists.