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Pyridine N-oxide and derivatives

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Gold-catalyzed intermolecular oxidation enables an efficient conversion of various terminal alkynes into the corresponding α-acetoxy ketones in the presence of 8-methylquinoline 1-oxide as the oxidant. The reaction probably proceeds through an α-oxo gold carbene intermolecular O-H insertion.
C. Wu, Z. Liang, D. Yan, W. He, J. Xiang, Synthesis, 2013, 45, 2605-2611.

Flexible and chemoselective methods for the transition-metal-free oxidation of amides provide α-keto amides and α-hydroxy amides. These highly valuable motifs are accessed in good to excellent yields and stereoselectivities with high functional group tolerance.
A. de la Torre, D. Kaiser, N. Maulide, J. Am. Chem. Soc., 2017, 139, 6578-6581.

The use of pyridine-N-oxides as oxidants under molecular iodine catalysis enables a rapid, metal-free dioxygenation of ynamides. Furthermore, this protocol could be extended to nonactivated alkynes, such as diarylacetylenes, to provide various benzil derivatives.
S. W. Kim, T.-W. Um, S. Shin, J. Org. Chem., 2018, 83, 4703-4711.

An expedient and reliable method for accessing reactive α-oxo gold carbenes via gold-catalyzed intermolecular oxidation of terminal alkynes offers a safe and economical alternative to strategies based on diazo substrates. Its synthetic potential is demonstrated by expedient preparation of dihydrofuran-3-ones containing a broad range of functional groups.
L. Ye, L. Cui, G. Zhang, L. Zhang, J. Am. Chem. Soc., 2010, 132, 3258-3259.

A general synthesis of various oxetan-3-ones uses readily available propargylic alcohols as substrates and proceeds without the exclusion of moisture or air. The facile formation of the strained oxetane ring provides strong support for the intermediacy of α-oxo gold carbenes. This safe and efficient generation of gold carbenes offers an entry into α-oxo metal carbene chemistry without using hazardous diazo ketones.
L. Ye, W. He, L. Zhang, J. Am. Chem. Soc., 2010, 132, 8550-8551.

Catalytic generation of a ketene species directly from a terminal alkyne in the presence of a Rh(I) catalyst and 4-picoline N-oxide provides a novel and efficient entry to the Staudinger synthesis of β-lactams with high trans diastereoselectivity under mild conditions. Mechanistic studies suggest that the reaction proceeds through a metalloketene rather than free ketene intermediate.
I. Kim, S. W. Roh, D. G. Lee, C. Lee, Org. Lett., 2014, 16, 2482-2485.

A gold(I)-catalyzed oxidative rearrangement of propargyl alcohols provides an efficient and selective route to 1,3-diketones under mild conditions in the presence of pyridine-N-oxides as external oxidants.
A. S. K. Hashmi, T. Wang, S. Shi, M. Rudolph, J. Org. Chem., 2012, 77, 7761-7767.

A metal-free C≡C bond cleavage of terminal alkynes in the presence of tBuONO as a powerful nitrogenating agent provides a vast range of nitriles containing aryl, heteroaryl, and natural product derivatives.
U. Dutta, D. W. Lupton, D. Maiti, Org. Lett., 2016, 18, 860-863.

An MCM-41-immobilized phosphine-gold(I) complex as catalyst enables a [2 + 2 + 1] annulation of terminal alkynes, nitriles, and oxygen atoms using 8-methylquinoline N-oxide as oxidant under mild conditions to provide 2,5-disubstituted oxazoles in very good yields with broad substrate scope. The heterogeneous catalyst can easily be recovered by simple filtration and recycled for at least eight times without significant loss of activity.
W. Yang, R. Zhang, F. Yi, M. Cai, J. Org. Chem., 2017, 82, 5204-5211.

A facile gold-catalyzed heterocyclization provides 2-amino-1,3-oxazoles functionalized at the nitrogen atom as well as at the fifth position of the heterocyclic ring in good to moderate yields. The reaction is based on intermolecular trapping of intermediate α-oxo gold carbenes with various cyanamides.
V. A. Rassadin, V. P. Boyarskiy, V. Yu. Kukushkin, Org. Lett., 2015, 17, 3502-3505.