School of Chemistry and Chemical Engineering, Key Lab of Analytical Chemistry for Life Science, Ministry of Education of China, Nanjing University, Nanjing, 210093, People's Republic of China.
J Org Chem. 2010 Nov 19;75(22):7757-68. doi: 10.1021/jo101764f. Epub 2010 Oct 28.
Photoinduced reactions of the 1,2-dicarbonyl compounds phenanthrenequinone (PQ), 1-acetylisatin (IS), and benzil (BZ) with the oxazoles 1a-j have been investigated. In photoreactions of PQ with the oxazoles, in addition to the 1,4-dioxins derived from [4 + 2] cycloaddition and the oxetanes from the Paternó-Büchi [2 + 2] reactions, [4 + 4] cycloaddition products are formed in the reactions with 1a, 1c, 1g, 1i, and 1j, with the quinone's dicarbonyl unit (O═C-C═O) and the oxazole's C═N-C═C moiety as two 4π addends. Photoreactions of IS with the oxazoles 1f and 1g give the [4 + 4] cycloaddition products exclusively, while in photoreactions of IS with 1a, 1c, 1e, 1h, and 1i, [4 + 4] products are formed together with the [2 + 2] products. Reaction pathway partitioning in these photocycloaddtions strongly depends on the substitution pattern on the oxazole ring. The presence of a substituent at the oxazole's C2 atom hampers the [4 + 4] pathway by causing steric hindrance to radical pair recombination in the corresponding 1,7-diradical intermediate to form the [4 + 4] cycloaddition products. A substituent at the C4 atom results in steric hindrance for ring closure of the 1,4-diradicals in the [2 + 2] cycloaddition pathway, therefore favoring the [4 + 4] and [4 + 2] cycloaddition pathways. Regio- and diastereoselectivity in the [2 + 2] and [4 + 4] cycloadditions have been discussed based on the thermodynamic stability of the relevant triplet diradical intermediates and the conformations of these diradicals suitable for the intersystem crossing process. Photoreactions of BZ with the oxazoles afford only [2 + 2] cycloaddition products.
苯并醌（PQ）、1-乙酰靛红（IS）和二苯甲酮（BZ）与噁唑 1a-j 的光诱导反应已经被研究过了。在 PQ 与噁唑的光反应中，除了源于[4+2]环加成的 1,4-二恶烷和源于 Paternó-Büchi [2+2]反应的氧杂环丁烷之外，与 1a、1c、1g、1i 和 1j 的反应还形成了[4+4]环加成产物，其中苯醌的二羰基单元（O═C-C═O）和噁唑的 C═N-C═C 部分作为两个 4π 给体。IS 与噁唑 1f 和 1g 的光反应仅生成[4+4]环加成产物，而在 IS 与 1a、1c、1e、1h 和 1i 的光反应中，[4+4]产物与[2+2]产物一起形成。这些光环加成反应中反应途径的划分强烈依赖于噁唑环上的取代模式。噁唑 C2 原子上的取代基会阻碍[4+4]途径，因为它会对相应的 1,7-二自由基中间体中自由基对的重排造成空间位阻，从而形成[4+4]环加成产物。C4 原子上的取代基会对[2+2]环加成途径中 1,4-二自由基的环闭造成空间位阻，因此有利于[4+4]和[4+2]环加成途径。基于相关三重态二自由基中间体的热力学稳定性和这些二自由基适合系间窜越过程的构象，讨论了[2+2]和[4+4]环加成的区域和立体选择性。BZ 与噁唑的光反应仅生成[2+2]环加成产物。
