2-Pyrones, photochemistry

Loyola University of Chicago), found that Hammond s estimate is accurate, and the fluorescent lifetime of 2 and 40 ate 180 ps. Triplet-sensitized reactions of 2-pyridones have not been extensively studied. However Sharp and Hammond found no evidence for 2-pyri-done photoproducts when xanthone is used as a photosensitizer. Using tethered pyridones, Nakamura found [2+2] and [4+2] cycloaddition products with benzophenone sensitization (see Figure 14). This is consistent with 2-pyrone photochemistry,but the generality of these results await further experimentation. 

Here we will present some basic theoretical photochemistry of a set of furocoumarin compounds, starting from the basic building blocks furan and pyrone, as shown in Fig. 4.13. Comparison is also made with the photochemistry of flavin, the active component of vitamin B2. 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

Pavlik, J. W., Phototransposition, Photo ring Contraction Reactions of 4 Pyrones and 4 Hydroxypyrylium Cations. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 237 249. 

At first sight, the photochemistry of 4-hydroxy-pyryliums, i.e. of 4-pyrones in acid solution, seems extraordinary, in that they are converted into 2-pyrones, however a rationalisation involving, first, a bicyclic hydroxyallyl cation, second, a bicycUc epoxy-cyclopentenone, and then a second photoexcitation, makes the transformation clear the sequence is shown below. Irradiation at higher pH leads to a trapping of the first-formed photointermediate by solvent and thus the isolation of dihydroxy-cyclopentenones. ... 

A very good example of the enormous benefits that DFT computations of IR spectra have brought to matrix-isolation studies giving the technique a veritable new lease of life in the study of organic reactive intermediates - has been provided by a study of the photolysis of a-pyrone (32) and its 4,6-dimethyl derivative (Breda et al.. Chapter 6). The photochemistry of a-pyrone was the subject of some of the earliest matrix-isolation studies of organic species, but the use of DFT computations has now allowed a virtually complete identification of the individual rotamers of the ring opened aldehyde-ketene (33)-(36). 

The photochemistry of a-pyrone (57a) (Scheme 10) was the subject of some of the earliest matrix-isolation studies of organic species. This system still has some interest for further study, however, especially on account of the success that DFT computations have had in assisting with the assignments of matrix IR spectra. Recently, the matrix photolyses of both a-pyrone and its 4,6-di-methyl derivative (57b) have been investigated with the aid of DFT calculations. When (57a) is irradiated (A > 285 nm) in Ar or Kr matrices, rapid formation of rotamers, (58a) and (59a), of the Z isomer of the ring-opened... 

Rieke, R.D. and Copenhafer, R.A., SoHd state organic photochemistry 11. Photolysis of 4, 6-diphe-nyl-a-pyrone. Tetrahedron Lett., 879,1971. 

Tessmann, J.W., Isaacs, S.T., and Hearst, J.E., Photochemistry of the furan-side 8-ethoxypsoralen-thymidine monoadduct inside the DNA hehx. Conversion to diadduct and to pyrone-side monoadduct, Biochemistry, 24,1669,1985. 

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