Oxyallyl intermediates, capture

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... 

More recently, the synthetic potential of the oxyallyl intermediates has been explored in intra- and intermolecular trapping studies. This chapter focuses on studies directed at capturing oxyallyl intermediates generated by the irradiation of cychc dienones. 

Samuel provided the first example that the oxyallyl intermediate in the Type A rearrangement could be trapped intermolecularly with a diene in a (3-t4]-cycloaddition. Samuel found that the apparent lifetime of simple 4,4-dialkyl substituted cyclohexadienones was too short to permit intermolecular capture. However, irradiation of Schuster s 4-methyl-4-trichloromethyl-2.5-cyclohexadienone 1 (Rj = CHj, Rj = CCI3) in the presence of either furan or cyclopentadiene produced 1 1 adducts 7 (X = O or CHj). This type of [4-1-3]-cycloaddition reaction has long been considered as evidence for an oxyallyl transient and is compatible with either a biradical or zwitterionic species. ... 

In contrast to Schultz s studies with 2,5-cyclohexadienones, West demonstrated that the Type A oxyallyl intermediate in pyran-4-one systems could be captured by alkenes and fiuans. Irradiation of pyran-4-ones with alkenes tethered to the 2-position 25 gave diquinanes 26,27, and 28 via the endocycHc addition of the oxyaUyl to the alkene (Scheme 4, Table 81.2). The mode of cychzation could be switched to exocyclic by placement of electron-donating substituents on the interior of the alkene (29) to give hydrindanes 30. 

West also demonstrated that oxyallyls derived from pyran-4-ones can participate in electrophilic aromatic substitution reactions. Irradiation of pyran-4-ones 34 with aryl groups appended in the 2-position produced tricyclic tetrahydrobenz[e]indenone 35 in fair to moderate yield (Table 81.4). Capture of the oxyallyl intermediate by solvent and Type A rearrangement to pyran-2-ones competed with the desired intramolecular EAS reaction. Interestingly, the yield of 35 was concentration dependent (higher yields with dilution). It is not clear how concentration affects the bifurcation of the oxyaUyl intermediate between intramolecular trapping and rearrangement. 

In an effort to provide experimental evidence for the formation of a second oxyallyl cationic intermediate, a gem-dichlorocyclopropane substrate was envisaged with two internal nucleophiles one to participate in the initial interrupted Nazarov reaction, and the other to capture the second cationic species. Surprisingly, when the carefully designed substrate 80 was subjected to the optimized reaction conditions, an alternate mode of trapping occurred to generate the intriguing bridged bicyclic product 81 (Scheme 4.25). 

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