Abstract: Tai-Ding-An(3-phthalimido-2-oxo-n-butyraldehyde bisthiosemicarbazone,TDA)is an antiviral drug first synthesized in this institute,In order to clarifv the difference between the twoenantiomeric isomero of TDA,(R)-and (S)-TDA were synthesized from (R)- and(S)-alanine,respectively,via the following steps: fusing with phthalic anhydride gave 2-phthalimido alanine(2a or2b).The resulting acid was reacted with thionyl chloride to offer the corresponding acid chloride(3a or3b), which was treated with diazomethane to give the diazoketone(4a or 4b).Bromination of theketone with hydrobromic acid gave the key intennediate 3-phthalimidca2-oxo-1-bromobutanone (5a or5b).Compound 5a or 5b was oxidized with DMSO to give 6a or 6b, which was directly condensedwith thiosemicarbazide to afford the desired(R)or(S)-TDA.(R)-TDA,(S)-TDA and(RS)-TDA have been tested in cell culture for antiHerpes simplexvirus I (HSV-1)and HSV-2 activities by plaque reducing method.All of them showed inhibitryeffects on HSV-1 and HSV-2 replication with IC59 of 0. 0296 mmol·L^-1,0.0359 mmol·L^-1 and0.0418 mmol·L^-1 for HSV-1 and 0.88 mmol·L^-1,1.04 mmol·L^-1 and 1.06 mmol·L^-1 for HSV-2.Not much difference was found among these compounds either on IC₅₀ or on therapeutic indexes.