Mechanistic studies in the photochemical Fries rearrangement of enol esters

Abstract

Photolysis of several enol esters produces the corresponding 1,3-Fries products, i.e. the keto forms of the β-diketones, as the sole primary photoproducts. The quantum yields of the rearrangements under a variety of conditions were measured. They decrease when the number of fused aromatic rings in the acyl part of the molecules is increased. The photobehaviour of the 2-anthroyl derivative is exceptional, in being affected both by the exciting wavelength and the solvent. Its irradiation at 254 nm results in a photo-Fries rearrangement (p.f.r.) with a yield twice that in ethanol; irradiation at 366 nm causes only inefficient dimerization. Explanations are forwarded for the general photobehaviour of the enol esters and its relation to the 1,3-p.f.r. of aryl esters and to other photoinduced 1,3-shifts.