Photochemical Decarbonylation of Oxindoles as a Gateway to ortho -Quinone Methide Imines: Divergent Access to Nitrogen Heterocycles

We report a UV-C-induced (254 nm) photochemical transformation of oxindoles that provides rapid access to structurally diverse nitrogen-containing scaffolds under operationally simple conditions. Mechanistic studies suggest that photoexcited oxindoles undergo decarbonylation to generate highly reactive ortho -quinone methide imine intermediates in situ, enabling intermolecular [4 + 2] cycloaddition to furnish 2-spirotetrahydroquinolines in excellent yields. Strikingly, subtle modification of the nitrogen substituent dramatically alters the reaction outcome: N -vinyl oxindoles undergo cascade processes leading to highly decorated pyrrole and tetrahydroacridine frameworks, whereas related lactam substrates display rearrangement or oligomerization pathways. The practicality of the methodology was demonstrated through continuous-flow implementation, enabling efficient reaction scale-up and in-line trapping of carbon monoxide generated during photodecarbonylation. Furthermore, the spirocyclic products could be readily transformed to valuable diamine derivatives. This work reveals an underexplored photochemical reactivity mode of oxindoles and establishes them as convenient synthetic surrogates for elusive o -quinone methide imines.