The Photo-Fries rearrangement. A mild and convenient photochemical method for the synthesis of 4-pyranones and 4-quinolones

BONESI SERGIO M; ERRA-BALSELLS ROSA

Salvador do Bahia, Brasil

Congreso; XIV Inter-American Photochemistry Society Conference; 2006

Inter-American Photochemical Society.

The photo Fries rearrangement has received considerable attention since its discovery in 1960 by Anderson and Reese, and the studies have been devoted mainly to elucidating the mechanism of the reaction rather than its use in synthesis. However, this photochemical reaction has been successfully used in the synthesis of natural products such as griseofulvin, daunomycine and flavonoids. It is known that the photo Fries rearrangement takes place in mild conditions giving o-hydroxyketone or o-aminoketone derivatives in good chemical yields. These intermediates are interesting synthons for the synthesis of a variety of natural products that contains in their structure a 4-pyranone or 4-quinolone moiety.In this communication we report the photochemical studies of aromatic ester and amide derivatives. Several compounds have been synthesized for the first time and have been characterized by means of physical and spectroscopic methods (mp, MS, IR, NMR).   In order to prepare the 4-pyranone or the 4-quinolone moiety in high yield the photochemical reactions have been carried out systematically in different experimental conditions: (a) homogeneous neutral conditions (organic solvents), (b) heterogeneous basic conditions (cyclohexane – NaOH 10%) for ester derivatives and (c) heterogeneous acidic conditions (cyclohexane – AcOH/AcONa buffer solution). The irradiations have been carried out at two different wavelengths (254 and 310 nm), at room temperature and under argon atmosphere. The photoproducts have been isolated, identified and fully characterized by means of chromatographic and spectroscopic methods. In order to determine the photo reactive excited state (singlet or triplet) of the substrates, photosensitization and chemical quenching of the photochemical reaction were carried out.It is worthy to mention that in neutral conditions the 4-pyranone derivative is formed in low chemical yield (> 20 %) while in heterogeneous basic conditions this compound is formed in 90 % yield. Gratifyingly, an intramolecular Michael addition in basic conditions (thermal reaction) favored the cyclization of the fenoxyketone synthon giving the desiderate 4-pyranone derivative in one-pot photochemical step.