Chiral resolution of protected dl-thiopropionic acid derivative (I) is performed by first condensation with (1R,2S)-(-)-2-amino-1,2-diphenyl ethanol in acetonitrile followed by diastereomer separation and finally hydrolysis of the optically active salt to afford the desired stereoisomer (S)-(II). Activation of (S)-(II) with oxalyl chloride in dichloromethane and subsequent coupling to L-isoglutamine derivative (III) in CH2Cl2 in the presence of pyridine provides amide (S)-(IV), which is finally converted into the target product by alkaline hydrolysis with LiOH in THF.