Multicomponent Synthesis of Furo-Thiocoumarin Hybrids and Their Photocyclization into Polycyclic Fused Furans

A new scandium triflate-catalyzed multicomponent reaction has been developed for the one-pot synthesis of structurally diverse furo-thiocoumarins linked with indole or naphthol moiety. The reaction of aryl glyoxals, 4-hydroxy-1-thiocoumarin, and indoles in a mixture of 1,2-dichloroethane and ethanol, using Sc(OTf)3 as a Lewis acid catalyst, provided indole-linked substituted furo-thiocoumarins. When indole was replaced with either α- or β-naphthol, the reaction yielded the corresponding naphthol-linked furo-thiocoumarins. Interestingly, α-naphthol undergoes linkage with the furan ring through the C-4 position, whereas β-naphthol participates through the C-1 position. Additionally, we developed a visible-light-induced photocyclization strategy to transform these indole-linked furo-thiocoumarins into heptacyclic, highly conjugated furan systems fused with thiocoumarin and benzo[a]carbazole moieties. This photocyclization was achieved without the use of any oxidant or photocatalyst. The photocyclization was also extended to β-naphthol-linked furo-thiocoumarin. This method is significant as it involves a two-step double cyclization process-comprising an initial multicomponent cyclization followed by photocyclization, to afford a range of structurally diverse, novel polycyclic aromatic heterocycles.