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Oximes are valuable synthetic building blocks with reactivity modes that enable their use in diverse methodologies, from cycloadditions to bioconjugation. Their reactivity towards photocatalysis and transition metals makes them ideal starting materials for N-containing heterocycles, amino alcohols and amines. Developments in oxime reactivity since 2016 have enabled transformations such as the addition of iminyl radicals to alkenes to generate functionalized imines, and [2 + 2]-cycloadditions to access azetidines. The unique properties imparted by the oxime N-O bond have also been used to integrate dynamic chemistries into materials. In this Review, we discuss the innovative use of this powerful functional group, with a focus on N-O bond fragmentation and cycloadditions, along with applications including dynamic materials, energetic materials and biocatalytic oxime reductions. We conclude by highlighting methodologies based on oxime starting materials, along with the challenges of using oximes for diverse applications, and offer insight into future directions in these areas.

Azetines, four-membered unsaturated nitrogen-containing heterocycles, hold great potential for drug design and development but remain underexplored due to challenges associated with their synthesis. We report an efficient, visible light-mediated approach toward 1- and 2-azetines relying on alkynes and the unique triplet state reactivity of oximes, specifically 2-isoxazolines. While 2-azetine products are accessible upon intermolecular [2 + 2]-cycloaddition via triplet energy transfer from a commercially available iridium photocatalyst, the selective formation of 1-azetines proceeds upon a second, consecutive, energy transfer process. Mechanistic studies are consistent with a stepwise reaction mechanism via N–O bond homolysis following the second energy transfer event to result in the formation of 1-azetine products. Characteristic for this method is its operational simplicity, mild conditions, and modular approach that allow for the synthesis of functionalized azetines and tetrahydrofurans (via in situ hydrolysis) from readily available precursors.

Azetines are valuable 4-membered nitrogen-containing heterocycles with unique reactivity and useful synthetic applications. Recent methods to access these compounds and a comprehensive review of their application as intermediates is presented.

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<p>Intermolecular [2+2] photocycloadditions represent a powerful method for the synthesis of highly strained, four-membered rings. While this approach is commonly employed for the synthesis of oxetanes and cyclobutanes, the synthesis of azetidines via intermolecular aza Paternò-Büchi reactions remains highly underdeveloped. Herein we report a visible light-mediated intermolecular aza Paternò-Büchi reaction that utilizes glyoxylate oximes as reactive intermediates activated via triplet energy transfer. This approach is characterized by its operational simplicity, mild conditions and broad scope, and allows for the synthesis of highly functionalized azetidines from readily available precursors.<b></b></p>

Four-membered nitrogen-containing heterocycles are highly desirable functional groups with synthetic and biological applications. Unsaturated 4-membered N-heterocycles, 1- and 2-azetines, are historically underexplored, but have recently been gaining increased interest due to the development of new synthetic methods to access these compounds, and to their potential as reactive intermediates. This review covers new strategies for the synthesis of 1- and 2-azetines with a particular focus on advances made since 2018. Additionally, the use of these compounds as intermediates to access other heterocycles (3- to 6-membered) and complex products is comprehensively discussed.

Azetines, four-membered unsaturated nitrogen-containing heterocycles, hold great potential for drug design and development, but remain underexplored due to challenges associated with their synthesis. We report an efficient, visible light-mediated approach towards 1-and 2-azetines relying on alkynes and the unique triplet state reactivity of oximes, specifically 2-isoxazolines. While 2-azetine products are accessible upon intermolecular [2+2]-cycloaddition via triplet energy transfer from a commercially available iridium photocatalyst, the selective formation of 1-azetines proceeds upon a second, consecutive, energy transfer process. Mechanistic studies are consistent with a stepwise reaction mechanism via N-O bond homolysis following the second energy transfer event to result in the formation of 1-azetine products. Characteristic for this method is its operational simplicity, mild conditions and modular approach that allows for the synthesis of functionalized azetines and tetrahydrofurans via in situ hydrolysis from readily available precursors.