Ethyl 2, 3, 4, 8-tetrahydro-3-methyl-2, 4-dioxopyrido [2, 3-d] pyrimidine-6-carboxylates (IV) and their bis-compounds (VI) were synthesized by the condensation of 6-alkyl-or 6-aryl-amino-3-methyluracils (II) and appropriate ethyl 3-chloro-2-formylprop-2-enoates (III) in dimethylformamide. Hydrolysis of the esters IV and VI with base resulted in a novel rearrangement of a substituent (R²) at the 7-position onto the 6-substituent to give the corresponding 6-acyl-1, 2, 3, 4, 7, 8-hexahydro-3-methyl-2, 4, 7-trioxopyrido [2, 3-d] pyrimidines (VII) and their bis-compounds (X). The mechanism of the rearrangement is discussed.