Photooxygenation of Heterocycles

The photooxygenation of heterocycles represents a versatile and widely accepted tool for introducing oxygenated functions in a mild, simple and selective way. The review evidences the synthetic potential of the photooxygenation with particular attention to the reaction of Type II involving singlet oxygen in the first electronically excited state (1Δg), which has been applied to the most studied heterocycles as furans, thiophenes, pyrroles, oxazoles, imidazoles, indoles, nitrogen-containing six-membered systems. The singlet oxygenation of these systems occurs mainly via [4+2] cycloaddition leading to unstable endoperoxides which, in addition to the classical transformations of peroxides (reduction, hydrolysis, deoxygenation, generally performed at low temperature), afford characteristic rearranged products depending on the heteroatom, substitution pattern and experimental conditions. 1,2-Oxygen addition can sometimes compete with the Diels-Alder-type reaction, especially for pyrroles, imidazoles and indoles. The attention has been also focused to the oxygenation of some biomolecules as histidine, triptophan and guanine which play a significant role in biological processes as photodynamic effects or in the photoinduced deactivation of nucleic acids.