Novel intermediate compounds are disclosed. These compounds are useful in the synthesis of 4-aminopyrimidine-5-one derivatives that inhibit cyclin-dependent kinases, in particular cyclin-dependent kinase 4 (Cdk4). The 4-aminopyrimidine-5-one derivatives and their pharmaceutically acceptable salts have antiproliferative activity and are useful in the treatment or control of cancer, in particular solid tumors.