A facile synthesis of the individual Sp and Rp 3,5-cyclic thymidine phosphorothioates is reported. The key intermediate is a methyl phosphite triester which is readily converted under free-radical conditions to the diastereomeric thiophosphates which are separated on SiO 2 and then cleanly dealkylated by t -BuNH 2, to the ami ne salts of title compounds.