德国
一个多世纪以来，光化学[2+2]环加成反应一直被合成化学家用来合成环丁烷，即四元碳基环。在该反应中，通常两个烯烃亚基（每个烯烃两个π电子）环化形成两个新的C–Cσ键。尽管光化学[2+2]环加成反应的发展在上个世纪取得了重大进展，但研究一直集中在这种[2π+2π]体系上，其中两个π-键转化为两个新的σ-键。
该文中，研究人员报道了一种分子间[2+2]光环加成反应，它使用双环[1.1.0]丁烷（BCB）作为2σ-电子反应物。这种由应变释放驱动的[2π+2σ]-光环加成反应是通过可见光介导的三重态能量转移催化实现的。该文公开了一种从杂环烯烃偶联剂，即香豆素、黄酮和吲哚组成的，简单、模块化且非对映选择性地合成双环[2.1.1]己烷（BCH）的方法。鉴于BCH作为生物同构体在药物研究中的重要性日益增加，并考虑到其可获得性有限，仍然需要新的合成方法。应用该策略，研究人员可以将分子间[2+2]光环加成扩展到σ-键，并提供以前无法获得的结构基序。
附：英文原文
Title: Intermolecular [2π+2σ]-photocycloaddition enabled by triplet energy transfer
Author: Kleinmans, Roman, Pinkert, Tobias, Dutta, Subhabrata, Paulisch, Tiffany O., Keum, Hyeyun, Daniliuc, Constantin G., Glorius, Frank
Issue&Volume: 2022-03-21
Abstract: For more than one century, photochemical [2+2]-cycloadditions have been used by synthetic chemists to make cyclobutanes, four-membered carbon-based rings. In this reaction, typically two olefin subunits (two π-electrons per olefin) cyclize to form two new C–C σ-bonds. Although the development of photochemical [2+2]-cycloadditions has made significant progress within the last century, research has been focused on such [2π+2π]-systems, where two π-bonds are converted into two new σ-bonds1,2. Here, we report an intermolecular [2+2]-photocycloaddition which uses bicyclo[1.1.0]butanes (BCBs) as 2σ-electron reactants3–7. This strain-release-driven [2π+2σ]-photocycloaddition reaction was realized by visible light-mediated triplet energy transfer catalysis8,9. A simple, modular, and diastereoselective synthesis of bicyclo[2.1.1]hexanes (BCHs) from heterocyclic olefin coupling partners, namely coumarins, flavones and indoles, is disclosed. Given the increasing importance of BCHs as bioisosteres – groups which convey similar biological properties to those they replace – in pharmaceutical research and considering their limited access10,11, there remains a need for new synthetic methodologies. Applying this strategy allowed us to extend the intermolecular [2+2]-photocycloadditions to σ-bonds and provides previously inaccessible structural motifs.
DOI: 10.1038/s41586-022-04636-x
Source: https://www.nature.com/articles/s41586-022-04636-x