Cyclopentenone photochemistry

The photo-[4+2] cycloaddition of furan with Pummerer s ketone 102 [70,71] gives evidence for the intermediacy of the highly twisted enone intermediate 103, and a biradical cycloaddition pathway (Sch. 23). The structures of the endo and exo products 104 were confirmed by X-ray crystallography [72,73]. In a related comparison of cyclohexenone and cyclopentenone photochemistry, conditions that gave [4+2] adducts for the cyclohexenone produced only [2+2] adducts from cyclopentenone [74]. 

With other ring sizes, the photochemistry of cyclic enones may take different courses. For cyclopentenones, the principal products result from hydrogen abstraction processes. Irradiation of cyclopentenone in eyclohexane gives a mixture of 2- and 3-cyclohexyleyclopentanone. These products can be formed by intermolecular hydrogen abstraction, followed by recombination of the resulting radicals ... 

The photochemistry of o< -unsaturated ketones has attracted much attention and is still a field fiettd of current interest. /Numerous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition reactions and rearrangements. For example, cyclopentenones 1 and 2 readily rearrange to... 

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 full report of the photochemistry of the aryl-substituted cyclopentenones... 

Weedon, A. C, Photocycloaddition reactions of cyclopentenones with alkenes, in CRC Handbook of Organic Photochemistry and Photobiology, Horspool, W. M. and Song P-S., Eds., CRC Press, Boca Raton, FL, 1995, 634-651. 

DiUing, W. L., Tabor, T. E., Boer, F. R, and North, P. R, Organic photochemistry. IX. The photocycloaddition of 2-cyclopentenone to cis- and trans-dichloroethylene evidence for initial attack at carbon-3 and rotational equilibration of biradical intermediates,/.Am. Chem. Soc., 92,1399,1970. 

Li, Z.-H., Mori, A., Kato, N., and Xakeshita, H., Synthetic photochemistry. LVII. Facile photochemical construction of hexahydro-as- indacene skeleton, a carbon framework of ikarugamycin, from high pressure Diels-Alder adducts of tropones-cyclopentenone. Bull. Chem. Soc. Jpn., 64, 2778,1991. 

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