Efficient Synthesis of Uracil-Derived Hexa- and Tetrahydropyrido[2,3-d]pyrimidines

A reaction of 6-amino-1,3-dimethyluracil with 3-(hetero)aroylacrylic acids and their methyl esters leads to hexahydropyrido[2,3-d]pyrimidine-5-carboxylic acids or the corresponding methyl esters in high to excellent yields. One-pot oxidation of the acid derivatives with CAN is accompanied by decarboxylation to give tetrahydropyrido[2,3-d]pyrimidines, while oxidation with bromine resulted in the formation of tetrahydropyrido[2,3-d]pyrimidine-5-carboxylic acids. The aromatization of methyl hexahydropyrido[2,3-d]pyrimidine-5-carboxylates was achieved by K2CO3-mediated air oxidation under ambient conditions.