Pemetrexed Impurity 18

N-[4-[2-(2-Amino-4,7-dihydro-4-oxo-3H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl]benzoyl]-L-glutamic Acid 5-Methyl Ester Sodium Salt may be useful in the preparation of salts, esters and derivatives of glutamate moiety in pemetrexed.