Cross-conjugated cyclic dienones

Matlin, A. R., Photocycloaddition/Trapping Reactions of Cross conjugated Cyclic Dienones Capture of Oxyallyl Intermediates. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, FL, 2004, Chapter 81, PP- 1 12. 

Photocycloaddition/ Trapping Reactions of Cross-Conjugated Cyclic Dienones Capture of Oxyallyl Intermediates... 

Cross-conjugated cyclic dienones with 6-, 7-, and 8-membered rings exhibit a rich and diverse photochemistry. The deep-seated photochemical rearrangements found in these systems have attracted numerous mechanistic studies. Cydohexadienones have received the most attention and provide the general framework for the mechanistic understanding of the photochemistry for this class of compounds. The key intermediate in these systems is an oxyallyl transient that is formed by ring closure across the P, P -carbons in the dienone s) tems A —> B. 

Ichikawa and co-workers have examined the Nazarov cyclization of difluorodienone 107 (Scheme 19.28) and have shown that it leads selectively to cross-conjugated dienone product 110. The Nazarov cyclization was initiated by exposure of 107 to trimethylsilyl triflate in a mixed solvent of dichloromethane and hexafluoroisopropanol (HFIP), leading to the resonance tautomers 108 and 109 of the intermediate cyclic allylic cation. Tautomer 109 is disfavored because of the electrostatic effect of the two fluorines on the adjacent carbon atom therefore, the process is terminated through proton loss from the methyl group rather than from the n-butyl group. Elimination of one of the (3 fluorine atoms leads to 110 as the observed final product. 

Fleming, M., NucleophiUc Trapping of Cross-Conjugated Dienone Derived Cyclic OxyaHyls and Their Potential Torquoselective Formation, Ph.D. dissertation, University of Utah, 2000. 

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