1-Phenyl-3-pyrrolidinone-d5 is the isotope labelled analog of 1-Phenyl-3-pyrrolidinone (P336730); a reagent used in the synthesis of a series of non-classical antifolates which were evaluated for their in vitro cytotoxicity. Also used in the preparation of azetidine- and pyrrolidine-containing alkynes as mGluR5 regulators.