Hetero Diels–Alder reaction: a novel strategy to regioselective synthesis of pyrimido[4,5-d]pyrimidine analogues from Biginelli derivative

A number of potent pyrimido[4,5-d]pyrimidine analogues have been efficiently synthesized by hetero Diels–Alder cycloaddition of 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid methyl ester, a Biginelli compound with N-arylidine-N′-methylformamidines and N-arylidine guanidine in dry toluene. Structures of the newly obtained cycloadducts were established on the basis of elemental and spectral (IR, NMR and Mass) data. The molecular mechanism of the observed cycloaddition reaction has been investigated theoretically by means of PM3 semiempirical method. Transition state structure determinations and activation energy calculations have shown the preference for the endo approach over the exo approach of dienophile towards the diene fragments used, which is consistent with the experimental results. The studied cycloadditions proceed via an asynchronous concerted mechanism. It was demonstrated that FMO theory could reasonably predict the relative reactivities between dienes as well as indicating that these reactions belong to normal Diels–Alder type cycloadditions.