SYNTHESIS AND STRUCTURAL STUDY OF NOVEL 5-ARYL SUBSTITUTED 2-AMINO-4,7-DIOXOPYRIDO[2,3-D]PYRIMIDINES

The title compounds 4a-c have been prepared in a one-step procedure fr om 2,4-diamino-6-hydroxypyrimidine (1) and the corresponding arylidene substituted Meldrum's acids 2a-e in very good yields. Semiempirical t heoretical calculations (AM1) reveal two favoured conformations (A and B) for compounds 4a-e. The H-1-nmr determinations, by using Karplus a nd Altona equations, clearly indicate that conformation A, with the ar yl group on C5 in a pseudoaxial position, is that predominant in solut ion. The calculated charge density values for the olefinic carbons are in agreement with the experimental push-pull effect observed in the C -13-nmr spectra.