Convenient One-Pot Four-Component Synthesis of 6,8-Disubstituted-5,6,7,8-tetrahydropyrimido[4,5-d]pyrimidin-4(3H)-ones via a Triple Mannich Reaction

An efficient and simple one-pot four-component protocol has been developed and performed for the synthesis of 6,8-disubstituted-5,6,7,8-tetrahydropyrimido[4,5-d]pyrimidin-4(3H)-ones, involving a triple Mannich reaction of 6-amino-2-(ethylthio)pyrimidin-4(3H)-one, formaldehyde, primary amines, and alcohols. Secondary amines were also utilised instead of alcohols as Mannich nucleophiles, and a variety of functional groups and electronically varied reaction partners were tolerated. This one-pot reaction facilitated the generation of a library of pyrimido[4,5-d]pyrimidin-4(3H)-ones in very good to excellent yields. The regioselectivity of this reaction was investigated using atomic charge calculations, and spectroscopic data confirmed that the triple Mannich products were 6,8-disubstituted-5,6,7,8-tetrahydropyrimido[4,5-d]pyrimidin-4(3H)-ones rather than the isomeric 3,6-disubstituted-5,6,7,8-tetrahydropyrimido[4,5-d]pyrimidin-4(3H)-ones. The structures of all compounds synthesised using the triple Mannich reaction were confirmed via spectroscopic and elemental analyses. The reaction mechanism was studied and confirmed by isolation of the intermediate.