Photo Fries Rearrangement of 7-acyloxypiridoindoles and 2-acyloxycarbazole. A comparative study

PONCE. M. A; BONESI, S.M; CABRERIZO F.M; ERRA-BALSELLS, R

Salvador de Bahía, Brasil

Congreso; 17th I-APS Conference on Photochemistry; 2006

The photo Fries rearrangement has received considerable attention since its discovery in 1960 by Anderson and Reese and this photoreaction have been studied from a synthetic and mechanistic points of view. It is known that the photo Fries rearrangement takes place in mild conditions, at room temperature and no hazardous catalysts are needed. Also, this photoreaction gives o-hydroxyketone or o-aminoketone derivatives in moderate to good chemical yields, which are interesting synthons for the synthesis of a wide variety of natural products. Taking into account that the photo-Fries rearrangement have never been applied to piridoindole (b-carboline) derivatives, in this communication we report the photochemical studies of several 7-acyloxyharmoles and the results are compared with those obtained with 2-acyloxycarbazoles. The chemical structures of these compounds are shown in Scheme I. All the compounds have been synthesized for the first time and have been characterized by means of physical and spectroscopic methods (mp, MS, IR, NMR). Scheme I In order to prepare o-hydroxy ketone derivatives that show a b-carboline moiety in their structures the photo-Fries rearrangement have been carried out systematically in different experimental conditions. Thus, the effects of the solvent, the atmosphere and the excitation wavelength on the photochemical reaction have been analyzed. Also, the effect of the length of the hydrocarbon chain on the photochemical reaction has been analyzed. The photoproducts have been isolated, identified and fully characterized by means of chromatographic and spectroscopic methods. The photochemical reaction of 7-acyloxyharmol is shown in Scheme II. Scheme II               It is interesting to mention that as the R group of the 7-acyloxyharmol changes from methyl or phenyl group to caproyl group, the photoreaction turns quite inefficient and the conversion of the starting material is lesser than 5% and harmol is the sole photoproduct detected. In the case of 2-caproyloxycarbazole the photo-Fries rearrangement takes place efficiently and the 1,3-migrated photoproducts are formed in moderate to good chemical yield. Therefore, the distinct photochemical behavior of the b-carboline derivative could be ascribed to the presence of the pyridine moiety in the b-carboline structure because an intramolecular electron transfer process between the pyridine moiety and the ester group competes efficiently with the photo-Fries rearrangement. Likewise, quenching of the photochemical reaction of 2-caproyloxycarbazole with increasing concentration of pyridine was also observed and the photochemical reaction turns inefficient.