The present invention provides a process for the preparation of 5-[[(2R,3S)-2-[(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-3-(4-fluorophenyl)-4-morpholinyl]methyl]-1,2-dihydro-3H-1,2,4-triazole-3-one (Aprepitant) comprising condensation of 2-(R)-[(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-3-(S)-(4-fluorophenyl)morpholine hydrochloride salt with 2-(2-chloro-1-iminoethyl)hydrazinecarboxylic acid methyl ester to obtain the reaction mixture containing 2-[2-[(2R,3S)-2-[(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-3-(4-fluorophenyl)-4-morpholinyl]-1-iminoethyl]hydrazinecarboxylic acid methyl ester, which is in-situ cyclized in the presence of dimethylsulfoxide and a polar protic solvent at a low temperature to yield aprepitant having purity &gE99.5%.
