Synthesis and theoretical calculations of novel 5‐aryl substituted 2,4,7‐trioxo and 4,7‐dioxo‐2‐thioxopyrido[2,3‐ d ]pyrimidines

New substituted pyrido[2,3‐ d ]pyrimidines 5 and 6 have been prepared in one‐step from the readily available 6‐amino‐2,4‐dioxotetrahydropyrimidine ( 1 ) or 6‐amino‐4‐oxo‐2‐thioxotetrahydropyrimidine ( 2 ) and the arylidene substituted Meldrum's acid. The substitution pattern of the ethylene moiety in compounds 5 and 6 results in a strong push‐pull electronic effect. The semiempirical calculations using the AM1 method reveal two equally favoured conformations showing a distorted geometry. The calculated charge density values confirm the observed 13 C nmr chemical shifts.