Koser's reagent-promoted synthesis of substituted isoxazoles and their photophysical properties

A practical and transition-metal-free strategy for the synthesis of structurally diverse isoxazole architectures has been developed. The protocol employs Koser's reagent, PhI(OH)OTs (hydroxy(tosyloxy)iodobenzene) to promote the oxidative cyclization of readily accessible arylaldoximes with a range of dipolarophiles under mild ambient conditions. Notably, the reaction of arylaldoximes with 1,4-naphthoquinone and 1,4-anthraquinone provides an efficient method for accessing isoxazole-fused polycyclic quinone frameworks in moderate to excellent yields. In addition, the methodology is broadly applicable to intermolecular (3 + 2) cycloadditions of aldoximes with arylacetylenes and dialkyl acetylenedicarboxylates, furnishing functionalized isoxazoles with high functional-group tolerance. The synthetic utility of the protocol is further demonstrated through gram-scale reactions and diverse post-synthetic transformations. Photophysical investigations reveal solvent-dependent absorption and emission behaviour, with pronounced intramolecular charge-transfer characteristics in selected derivatives. Electrochemical studies, supported by DFT calculations, establish a clear correlation between experimentally determined and computed frontier molecular orbital energies.