Photoassisted Synthesis of Complex Polyheterocycles via ESIPT-Driven Dearomative Intramolecular Cycloadditions

This research is focused on the development of novel photoassisted synthetic methodologies that provide straightforward access to complex and diverse libraries of polyheterocycles from modularly assembled precursors. All methods are based on the dearomative cycloadditions between two components: (i) o-azaxylylenes, generated by the excited-state intramolecular proton transfer, and (ii) tethered arene groups. We have demonstrated the synthetic utility of four new photoinduced processes that yield complex products amenable for post-photochemical modifications: [4+4] Cycloaddition of o-azaxylylenes to 1,3,4-oxadiazoles with subsequent dinitrogen extrusion. This reaction furnishes compounds outfitted with epoxide fragments, which undergo ring-openings with various nucleophiles; furthermore, the oxidized products rearrange into triazocanoindolinones under Schmidt reaction conditions. [4+2] Reaction between amino-o-azaxylylenes and tethered pyrroles. The primary pyrroline photoproducts are converted into stable polyheterocycles by 1,3-dipolar cycloaddition with tosyl azide and an intramolecular Friedel-Crafts reaction. Photodearomatization of benzenoid arenes via [2+4] reactions with o-azaxylylenes. This process transforms readily available benzene units into 1,3-cyclohexadienes, which are reactive in acid-catalyzed modifications and thermal [4+2] cycloadditions. Photocascade reaction involving [2+4] photocycloaddition of tethered anilines to o-azaxylylenes followed by secondary cyclization enabled by the oxalyl linker. This transformation permits strategic installation of specific functionalities into the photoproducts, with the purpose of further ring-forming modifications