The invention discloses a process for preparation of highly pure dialkyl pemetrexed by reacting a 2-(4-hydroxy-6-aminopyrrolo(2,3-d)pyrimidin-3-y])ethylbenzoic acid with a glutamatic acid diester or its salt in presence of a safe, mild, inexpensive, non-oxidative and easy to handle reagent such as substituted triphe...