The invention provides a compound of formula (I) wherein B is a nucleobase； U is O or S； Rx is -OC(=O)Ry, -OC(=O)CH(Ry)NH2, -OCH2OC(=O)Ry； Ry is optionally substituted alkyl or alkenyl or the side chain of a natural or unnatural amino acid R1 is H, or optionally substituted phenyl, benzyl, naphthyl, pyridyl or indolyl, or Rx and R1 together define a bond thus forming a cyclic phosphate； R2 and R2' together define the side chain of a natural or unnatural amino acid； R3 is optionally substituted alkyl, cycloalkyl, phenyl or benzyl； and pharmaceutically acceptable salts and compositions thereof which are useful in the treatment of cancer, especially leukemias.
