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Dehydrogenative TEMPO-Mediated Formation of Unstable Nitrones: Easy Access to N-Carbamoyl Isoxazolines

机译:脱氢TEMPO介导的不稳定硝基的形成:易于获得N-氨基甲酰基异恶唑啉

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摘要

N-carbamoyl nitrones represent an important class of reagents for the synthesis of a variety of natural and biologically active compounds. These compounds are generally converted into valuable 4-isoxazolines upon cyclization reaction with dipolarophiles. However, these types of N-protected nitrones are highly unstable, which limits their synthesis, storage and practical use, enforcing alternative lengthy or elaborated synthetic routes. In this work, a 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated formal dehydrogenation of N-protected benzyl-, allyl- and alkyl-substituted hydroxylamines followed by in situ trapping of the generated unstable nitrones into N-carbamoyl 4-isoxazolines is presented. A plausible mechanism is also proposed, in which the dipolarophile shows an important assistant role in the generation of the active nitrone intermediate. This simple protocol avoids the problematic isolation of N-carbamoyl protected nitrones, providing new synthetic possibilities in isoxazoline chemistry.
机译:N-氨基甲酰基硝酮代表用于合成各种天然和生物活性化合物的重要试剂类别。这些化合物通常在与双亲性试剂环化反应后转化为有价值的4-异恶唑啉。然而,这些类型的N-保护的硝酮高度不稳定,这限制了它们的合成,储存和实际使用,从而迫使选择冗长或复杂的合成路线。在这项工作中,N-保护的苄基,烯丙基和烷基取代的羟胺经2,2,6,6-四甲基哌啶-1-氧基(TEMPO)介导的正式脱氢,然后将所生成的不稳定硝酮原位捕获介绍了N-氨基甲酰基4-异恶唑啉。还提出了一种可能的机制,其中亲双极性体在活性硝酮中间体的产生中显示出重要的辅助作用。该简单规程避免了N-氨基甲酰基保护的硝酮的分离问题,为异恶唑啉化学提供了新的合成可能性。

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