New compounds of the formula ##STR1## (wherein R.sub.1 represents a lower alkyl group: and ##STR2## represents an alkylidene radical in which at least one of R.sub.2 and R.sub.3 is a lower alkyl which may optionally be substituted by phenyl, phenyl or a 5- or 6-membered aromatic ring including S, N or O, and the remainder, if any, is hydrogen; or a cycloalkylidene radical of up to six carbon atoms, to which a benzene ring may optionally be fused). These compounds are produced by reacting the corresponding 6-acyloxy compounds with a compound of the formula ##STR3## The new compounds have antiviral activity and are useful to prolong the life spans of tumor-bearing animals.