Photoinduced Tandem C-O Bond Reduction/Ketyl Radical Addition Reactions of α-Keto-N,O-Acetals Enabled by Proton-Coupled Electron Transfer

The C10a-acetoxylated tetrahydroazepino[1,2-a]indole-6,11-diones are a class of tricyclic oxindoles that feature an alpha-keto-N,O-acetal substructure, rendering them highly susceptible to SET reduction followed by fragmentation. In protic medium, they undergo a PCET-assisted two-step reduction including an interposed C-O bond cleavage that can be initiated photocatalytically as well as by cathodic reduction, and which generates nucleophilic ketyl radicals. In the presence of acrylonitrile and DIPEA as additional reactants, the photoinduced reaction unfolds as a tandem C-O bond reduction/ketyl radical conjugate addition, to furnish C10a-reduced, C11-cyanoethyl-substituted tricyclic azepino-[1,2-a]indole derivatives with high stereoselectivity.