NEW MATERIAL: A compound of formula I (R1 is formula II, formula III) and its pharmaceutically acceptable salt.

EXAMPLE: 2-[2-amino1,6-dihydro-6oxo-9H(purine9-yl)methoxy]ethyl L-valinate.

USE: A pharmaceutical. The compound has anti-viral property and is used for preventing or treating herpes simplex (I) infection, etc.

PROCESS: A compound of formula I is obtained, for example, by esterifying the compound of formula IV (X is optionally protected hydroxyl; Y is optionally protected amino) with optionally protected valine or isoleucine or the compound functionally equivalent to the amino acids in a solvent such as dimethylformamide in the presence of a coupling reagent such as N,N'- dicyclohexylcarbodiimide and a base catalyst such as 4dimethylaminopyridine, and then eliminating the protective group of the compound thus obtained by catalytic hydrogenation using Pd-catalyst or by other methods.