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

• Published in: Asian journal of organic chemistry 2017-06, Vol.6 (6), p.775-779
• Main Authors: Chen, Shudan, Huang, Binlei, Sun, Shiyuan, Ding, Yun, Hu, Aiguo
• Format: Article
• Language: English
• Subjects: Bergman cyclization; Crystals; Cycloaddition; Dilution; enediyne; Enediynes; Irradiation; Molecular structure; NMR; Nuclear magnetic resonance; Organic chemistry; photo-cycloaddition; Solid state; solid state reaction
• ISSN: 2193-5807; 2193-5815
• DOI: 10.1002/ajoc.201700095

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. Interesting selectivity! In contrast to the Bergman cyclization under photo‐irradiation conditions in dilute solution, we observed an unexpected [2+2] photocycloaddition dimerization between maleimide‐based enediyne compounds in the solid state or when the reaction was conducted under concentrated conditions in solution. Computational investigations revealed the initial formation of a [4+4] dimer structure, which was subsequently converted to the [2+2] product. Interestingly, the [2+2] reaction was found to be reversible, affording the enediyne monomer, which could undergo a photo‐Bergman cyclization under dilute conditions in solution to afford “masked” enediynes for further applications.