Enyne [4 + 4] Photocycloaddition: Bridged 1,2,5-Cyclooctatrienes

Enyne photocycloaddition with a 2-pyridone yields a mixture of products including amide-bridged 1,2,5-cyclooctatrienes, the first examples of enyne [4 + 4] adducts. Four regio- and stereochemical isomers of the [4 + 4] adduct are possible. All appear to be too strained to be isolated, but they have been identified as their [2 + 2] cyclobutane dimers. Cyclobutane and cyclobutene adducts have also been isolated, [2 + 2] addition products possibly related to the unstable [4 + 4] adducts via Cope rearrangement. Calculations suggest that [3,3] rearrangements have high energy barriers, however, making thermal interconversion unlikely.

烯炔（enyne）与2-吡啶酮（2-pyridone）的光环加成反应可得到包含酰胺桥联1,2,5-环辛三烯的产物混合物，这是首例烯炔类[4+4]环加成加合物。该[4+4]环加成加合物存在四种区域及立体化学异构体，均因环张力过大无法分离，但可通过其对应的[2+2]环丁烷二聚体得到鉴定。环丁烷与环丁烯加合物亦已被分离，这类[2+2]加成产物可通过科普重排（Cope rearrangement）与不稳定的[4+4]环加成加合物发生相互转化。计算结果显示，[3,3]重排具有较高的能垒，因此热致互变几乎不可能发生。