Three-Component Tandem Annulation for the Synthesis of Pyrido[4,3- <i>d</i> ]pyrimidine-2,4-diones

An efficient catalyst- and base-free, three-component tandem annulation has been developed for the synthesis of pyrido[4,3-<i>d</i>]pyrimidine-2,4-diones. This strategy employs readily available 5-carbonyl-6-methyluracils, aldehydes, and ammonium iodide under an oxygen atmosphere, affording the target products in good to excellent yields with broad substrate compatibility. The transformation is proposed to proceed via a condensation/cyclization/oxidative dehydrogenation cascade. This method offers notable advantages, such as operational simplicity and environmental benignity, thereby presenting a practical alternative route for the construction of pyrido[4,3-<i>d</i>]pyrimidine-2,4-diones.