3-Cyclopropyl-1-(2-fluoro-4-iodophenyl)-6-(methylamino)-2,4(1H,3H)-pyrimidinedione acts as a reagent in the preparation of 5-amino-2,4,7-trioxo-3,4,7,8-tetrahydro-2H-pyrido[2,3-d]pyrimidine derivatives and related compounds for the treatment of cancer and rheumatic diseases.