Unexpected [2+2] Photocycloaddition Between Enediyne Compounds in Solid State

Enediyne compounds generally undergo a Bergman cyclization reaction under photo‐irradiation conditions. In this study, we observed an unexpected [2+2] photo‐cycloaddition between enediyne compounds under photo‐irradiation conditions. The structure of the cycloaddition product was confirmed through NMR, HR‐MS and single crystal XRD analyses. The cycloaddition was only preferred when the reaction was conducted in the solid state or under highly concentrated conditions in the solution state. In contrast, the use of dilute conditions favored the Bergman cyclization. The concentration dependence of the enediyne substrate was further confirmed by EPR and MALDI‐TOF analyses of the products. Computational investigations showed that the initial step would involve the formation of a [4+4] dimer structure, which would be converted to the [2+2] product in the second step. Interestingly, the [2+2] reaction was found to be reversible back to the enediyne monomer, where it could undergo a photo‐Bergman cyclization under diluted conditions, thereby providing a new method for the formation of “masked” enediynes for further applications.