NEW MATERIAL: A compound of formula I [wherein B is a (substituted) pyrimidine or a (substituted) purine wherein the substituents are selected from amino, hydroxyl, halogen, thio or alkylthiol wherein the alkyl moiety of the alkylthiol has 1-6 carbon atoms, and the (substituted) purine is not guanine; X is O, S or methylene; and Z is formula II or formula III (wherein R is a cation)].

EXAMPLE: 1-[(1,3-Dihydroxy-2-propoxy)methyl]cytosine cyclic monophosphate.

USE: Anti-viral pharmaceuticals against herpes simplex virus and so on.

PREPARATION: A purine or pyrimidine acyclic nucleoside is reacted with an excess of a phosphorylating agent such as phosphorus oxychloride or pyrophosphoryl chloride in an inert solvent such as triethyl phosphate in the presence of a complexing agent such as SnCl4 to give a cyclic monophosphate, which is purified by separating by gradient elution from a column of an anionic exchange resin.