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Synthesis of 2H-azirines

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The combination of a copper and a photoredox catalyst enables the selective imination of unactivated C-H bonds under mild conditions. Trapping of iminyl radicals with copper ions results in high-valent Cu(III) imine intermediates, that quickly form nitrenes and then furnish 2H-azirines. A ring-expansion rearrangement from cyclobutyl oxime derivatives gives α-acylamino cyclopentanones.
L. Feng, C. Yang, W. Xia, Org. Lett., 2019, 21, 8323-8327.

An iron-catalyzed C(sp3)-H acyloxylation of aryl-2H azirines with hypervalent iodine(III) reagents (HIRs) provides a wide range of monoacyloxylated 3-aryl-2H-azirines in good yields.
L. Wang, H. Li, L. Wang, Org. Lett., 2018, 20, 1663-1666.

A visible-light-promoted regioselective coupling of aryl-2H-azirines and (diacetoxy)iodobenzene provides C(sp3)-H acyloxylated azirines in the presence of Rose Bengal as an organophotoredox catalyst. The reaction proceeds under aerobic condition at room temperature via a radical pathway.
A. De, S. Santra, A. Hajra, G. V. Zyrayanov, A. Majee, J. Org. Chem., 2019, 84, 11735-11740.

Various substituted enamine derivatives can be conveniently converted to the corresponding 2H-azirines mediated by phenyliodine (III) diacetate (PIDA). The formed 2-aryl-2H-azirines allow the synthesis of indole-3-carbonitriles or isoxazoles via thermal rearrangements.
X. Li, Y. Du, Z. Liang, X. Li, Y. Pan, K. Zhao, Org. Lett., 2009, 11, 2643-2646.

A facile and practical oxidative cyclization reaction of enamines to 2H-azirines is mediated by molecular iodine under mild conditions. This transition metal-free approach provides 2H-azirine derivatives in an efficient and scalable fashion.
M. Wang, J. Hou, W. Yu, J. Chang, J. Org. Chem., 2018, 83, 14954-14961.

Rh2(Piv)4 catalyzes a convenient gram-scale method for the preparation of 2-halo-2H-azirine-2-carboxylic acid esters and amides from 5-heteroatom-substituted 4-haloisoxazoles. In addition, rhodium catalysis is successfully applied in the synthesis of azirine-2-carboxylates from non-halogenated 5-alkoxyisoxazoles.
N. V. Rostovskii, A. V. Agafonova, I. A. Smetanin, M. S. Novikov, A. F. Khlebnikov, J. O. Ruvinskaya, G. L. Starova, Synthesis, 2017, 49, 4478-4488.

An efficient rhodium-catalyzed rearrangement of α-oximino ketenes derived from α-diazo oxime ethers provides 2H-azirines bearing quaternary centers and allows for subsequent rearrangement to highly substituted pyrroles in excellent yields.
Y. Jiang, W. C. Chan, C.-M. Park, J. Am. Chem. Soc., 2012, 134, 4104-4107.

The ketone, enol, and enolate forms of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones can afford different intramolecular cyclization products, depending on the conditions. Reactions under basic conditions afford 2H-azirines via a neutral enol (DABCO) or isoxazoles and oxazoles via an anionic enolate (t-BuOK), while acidic conditions afford oxazoles (AcOH) and isoxales (TFA) via neutral ketones.
Y. Ning, Y. Otani, T. Ohwada, J. Org. Chem., 2018, 83, 203-219.