A novel, efficient and sustainable methodology involving visible-light-mediated construction of pyrazolo[4,3-]pyrimidin-7(6)-ones from easily available 4-amino-1-methyl-3-propyl-1-pyrazole-5-carboxamide and aldehydes in the presence of 4CzIPN under an oxygen atmosphere has been developed. This photocatalytic system worked under mild conditions and avoided the use of any excess amount of toxic oxidants and transition metals. A variety of functionalized products were obtained in good to excellent yields. Notably, the marketed drug sildenafil was successfully synthesized. In addition, a plausible reaction mechanism involving a radical process has been proposed and further applications of this protocol are under way in our laboratory.
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