NEW MATERIAL:The compound of formula I (R1 is uracilyl, cytosinyl, hypoxanthinyl, adeninyl or thiazolyl which may be substituted with halogen or carbamoyl; R2 is N-containing unsaturated hetero-ring which may be substituted with oxo or lower alkyl; R3 is H or phosphono) and its salt.

EXAMPLE: 1-[(2R, 6R)-3,5-Dihydroxy-6-hydroxymethyl-4-(1, 3, 4-thiadiazol-2-yl)-morpholin-2-yl]uracil.

USE: An antitumor agent.

PREPARATION: The compound of formula I can be prepared by reacting the compound of formula II with the compound of formula H2N-R2 in an organic solvent, preferably in the presence of an acid (e.g. hydrochloric acid, trifluoroacetic acid, etc.) at normal temperature or under heating.