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Diels-Alder cycloaddition utilizing singlet oxygen as the dienophile with dimethyl cis-3,8-dihydroheptalene-3,8-dicarboxylate (5) has been investigated, and monoaddition product 7 has been isolated. The addition of a second singlet oxygen to the cycloheptatriene unit in 7 gave syn-bis(norcaradiene) bis(endoperoxide) 4. (1)H NMR spectral studies and theoretical calculations indicate the increased pyramidalization in syn-4 compared with carbon analogue. The increased pyramidalization results from hyperconjugation between the central pi-bond and the four adjacent C-O bonds and by rehybridization at C3, C4, C5, and C6. Furthermore, the increased reactivity for syn-4, which is probably arising from further folding of the central double bond, is also in agreement with theoretical calculations.

Pyramidalized Double Bonds Containing Endoperoxide Linkages: Photooxygenation of Dimethyl cis-3,8-Dihydroheptalene-3,8-dicarboxylate