Determination of the Source of the N-Methyl Impurity in the Synthesis of Pemetrexed Disodium Heptahydrate

The synthesis of Pemetrexed Disodium Heptahydrate has consistently resulted in a very low level (ca. 0.02%) unknown impurity. To ensure long-term control, the identity and source of the impurity were desired. Isolation and characterization identified the impurity as the N-methyl derivative. The source was identified as the methyl groups on the peptide coupling agent, 2,6-Dimethoxy-1,3,5-triazine (CDMT). Further work assured the current conditions provide adequate control.