Photocatalytic Tricyclization of Enediynes for the Synthesis of Chromenoquinolines in Continuous Flow.

A photocatalyzed radical-initiated tricyclization of enediynes was developed to synthesize a range of hitherto unknown sulfonated chromenoquinolines with moderate to excellent yields under continuous operational conditions. The reaction mechanism involves a complex series of steps including alkyl radical capture by sulfur dioxide (SO2), radical addition, 6-exo-dig/6-exo-dig/6-endo-trig tricyclization, and single electron transfer (SET). This cascade of reactions results in the direct formation of a pentacyclic framework structure, along with the creation of up to three new rings and five chemical bonds in a single step. This photocatalytic protocol exhibits a diverse substrate scope, remarkable tolerance to functional groups, and scalable adaptability.