Autocatalytic photoinduced oxidative dehydrogenation of pyrido[2,3-d]pyrimidin-7(8H)-ones: synthesis of C5-C6 unsaturated systems with concomitant formation of a long-lived radical

5,6-Dihydropyrido[2,3-d]pyrimidin-7(8H)-ones are readily accessed by a variety of methods and are a good scaffold for the development of biologically active compounds. However, they are very reluctant to dehydrogenate to give C5-C6 unsaturated compounds, usually with higher activity. A serendipitous discovery has allowed us to develop an autocatalytic photochemical dehydrogenation process by irradiating at 450 or 365 nm in DMSO, in the presence of air, and at room temperature the corresponding 5,6-dihydro derivative (with a variety of substituents at C2, C4, C5, C6, and N8) without adding any external photosensitizer. A complete study including reactions in DMSO-d₆ followed by NMR spectroscopy, EPR experiments, the use of radical quenchers, spin-trapping techniques, and reaction with methyl viologen, complemented with ab initio calculations has allowed us to propose a mechanistic rationalization for such a process.