Compounds wherein at least one of R2 or R3 is CH2 CH2 X, wherein X is a leaving group such as Cl, Br, I, p-toluenesulfonyl (OTs) or a similar leaving group, can function as irreversible inhibitors of TS, while compounds wherein R2 and R3 taken together with the N atom form a 5-6 membered ring can be hydrolyzed intracellularly to release the bioactive nucleotide NucOPO3 -2 wherein Nuc is the corres