The mechanism of intramolecular cycloadditions of azaxylylenes photogenerated via excited-state intramolecular proton transfer (ESIPT) in aromatic o-amido ketones and aldehydes bearing unsaturated functionalities was studied experimentally and computationally. In time-correlated single-photon counting experiments, no relation was found between lifetimes of singlet species and the nature of the amide pendant, either unsaturated furanpropanamide, capable of photocyclization, or the acetamide control. Steady-state emission for amido-tetralone derivatives showed comparable dual emission bands, but bromo substitution decreased the intensity of the ESIPT band. The most reactive derivatives of amidobenzaldehydes were virtually lacking the ESIPT band. The quantum yield of cycloaddition is decreased in the presence of triplet quenchers, O2 or trans-piperylene, and improved with heavy atom substitution in the aromatic ring, providing further evidence for the initial mechanistic hypothesis in which the fast singlet-state ESIPT is accompanied by the ISC in the tautomer (azaxylylene), which undergoes stepwise addition to the tethered unsaturated pendants.

Modular preassembly of azaxylylene photoprecursors, halogen-substituted in the aromatic ring, their intramolecular [4 + 4] or [4 + 2] cycloadditions to tethered unsaturated pendants, and subsequent postphotochemical capstone modification of the primary photoproducts via Suzuki coupling provides rapid access to diverse biaryls of unprecedented topology. This synthetic sequence allows for rapid growth of molecular complexity and is well aligned with methodology of Diversity-Oriented Synthesis.

The cascade photoassisted synthetic strategy for accessing complex N,O-polyheterocycles of unprecedented molecular architecture is developed. It is based on intramolecular cycloadditions of aza-o-xylylenes formed in situ via excited-state intramolecular proton transfer (ESIPT) of o-acylanilides. The photoprecursors are synthesized via a two-step one-pot method from readily available starting materials. The photochemical cascade (one experimentally simple step) results in the formation of four new heterocyclic rings with high control of diastereoselectivity.

Photoassisted synthesis of complex polyheterocyclic molecular architectures via excited state intramolecular proton transfer (ESIPT) is for the first time implemented for the reactions of o-keto phenols. This adds the 2,5-epoxybenzoxacine core to the previously obtained 2,5-epoxybenzazocine cores and offers rapid access to primary photoproducts which lend themselves to diverse yet simple postphotochemical modifications to further grow the complexity of the target structures, specifically – access to polyheterocycle-carbohydrate chimeras containing up to five contiguous stereogenic centers and benzazocine or benzoxacine heterocyclic cores.

No abstract