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 13C nmr chemical shifts.