Synthetic Applications of Hydroxamic Acids and Their Derivatives in Organic Chemistry
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Synthetic Applications of Hydroxamic Acids and Their Derivatives in Organic Chemistry
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ACS Organic & Inorganic Au

Cite this: ACS Org. Inorg. Au 2026, 6, 2, 179–201
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https://doi.org/10.1021/acsorginorgau.5c00120
Published February 2, 2026

Copyright © 2026 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Hydroxamic acids and their derivatives are a versatile class of organic compounds with broad utility in synthetic chemistry. This review highlights key synthetic transformations involving these molecules, covering important reactions such as their use in Weinreb amide chemistry, rearrangement processes, C–H activation directed by hydroxamate groups, their role as precursors to N-centered radicals and aza-oxyallyl cations, reduction reactions, and umpolung transformations initiated by the N–O bond cleavage.

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1. Introduction

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Hydroxamic acids, first identified by Heinrich Lossen in 1869, are a class of amide analogues characterized by a distinct N–O bond (Scheme 1A). Wilhelm Lossen later introduced the term hydroxamic acid to reflect both its acidic nature and its structural and synthetic origins, namely its derivation from the reaction of carbonyl compounds with hydroxylamine. (1) Functionalized derivatives of hydroxamic acids are commonly referred to as hydroxamates, N-hydroxamides, or N-acyl hydroxylamines. Their structural and physicochemical properties were comprehensively described by Frey et al. (2)

Scheme 1

Scheme 1. Hydroxamic Acid and Related Pharmaceuticals
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Hydroxamic acids display significantly greater reactivity compared to the highly stable amides. Their strong metal-chelating properties, especially toward Fe3+ and Zn2+, render them potent inhibitors (3) of metalloenzymes, as exemplified by the drug Marimastat (Scheme 1B). Moreover, the hydroxamate moiety constitutes a key pharmacophore in several approved histone deacetylase inhibitors, which are employed in the treatment of conditions such as T-cell lymphoma and multiple myeloma (4) (Scheme 1B). Research on the polymerization of hydroxamic acids has also seen notable progress. (5) Beyond these established applications in biomedicine and polymer science, hydroxamic acids and their derivatives serve as versatile building blocks in organic synthesis, which is the focus of this review.

2. Chemistry of the Weinreb Amides

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A prominent class of hydroxamates known as Weinreb amides, first reported in 1981, has been widely employed as versatile synthetic tools over the past few decades. (6) Their utility stems from the formation of a stable five-membered chelate upon reaction with organolithium or Grignard reagents, which allows for controlled nucleophilic addition to afford ketones in a single step without overaddition (Scheme 2). This selectivity is of considerable synthetic value. Furthermore, Weinreb amides can be selectively reduced by hydride reagents such as lithium aluminum hydride (LiAlH4) or diisobutylaluminum hydride (DIBAL-H) to yield aldehydes (Scheme 2).

Scheme 2

Scheme 2. Addition Reaction of Weinreb Amides with Organometallic Reagents
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3. Rearrangement

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3.1. Lossen Rearrangement

The Lossen rearrangement is a classical transformation that converts hydroxamates into isocyanates, typically initiated by activating reagents (Scheme 3A). The resulting isocyanates readily react with a range of nucleophiles─including water, alcohols, amines, organometallic reagents, and carboxylic acids─to afford diverse products such as amines, carbamates, ureas, and amides. (7)

Scheme 3

Scheme 3. Classical Lossen Rearrangement and the “Direct Lossen Rearrangement”
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The classical Lossen rearrangement typically requires activating reagents to convert the hydroxyl group into a suitable leaving group. In contrast, a “direct” variant can proceed without such additives (Scheme 3B). The earliest examples date to 1998, when Podlaha et al. obtained N,N′-diarylurea in 88% yield from neutral aryl hydroxamate salts, which were proposed to exist as hydrogen-bonded potassium salt dimers. (8) In 2009, Roithová et al. described a Zn(II)-promoted rearrangement under gas-phase conditions. (9) Subsequent computational studies suggest a mechanism involving: (1) deprotonation and formation of a metal complex, (2) rearrangement to a metal-carbamate, and (3) decarboxylation to yield the amine. (10) More recently, studies on the “self-propagative Lossen rearrangement” (11) and its promoted variants (12) have provided further mechanistic insight into the direct rearrangement of free hydroxamic acids. This direct approach addresses several limitations of the classical method, such as poor atom economy and laborious procedures. With continued development, it is expected to find wider application in the synthesis of bioactive molecules, (13) carbamates, (14) polyurethanes, polyureas, and related materials. (15)

3.2. HERON Rearrangement

The HERON rearrangement occurs in bis-heteroatom-substituted amides (anomeric amides), driven by the inherent instability arising from their anomeric effect (Scheme 4A). In this transformation, the more electronegative substituent migrates from nitrogen to the amide carbonyl carbon, leading to cleavage of the original C–N bond and generating an acyl derivative along with a stabilized nitrene species. (16) In the case of hydroxamates, dimerization in the presence of heavy-metal oxidants such as NiO2·H2O, ceric ammonium nitrate (CAN), Ag2O, or Pb(OAc)4 can be followed by a HERON rearrangement to furnish the corresponding carboxylic esters (17,18) (Scheme 4B).

Scheme 4

Scheme 4. HERON Rearrangement
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In 2002, Glover et al. reported that treating hydroxamates with tert-butyl hypochlorite generates N-alkoxy-N-chloroamides, which subsequently undergo an SN2-type azidation followed by a HERON rearrangement to yield esters (19) (Scheme 5A). In 2013, Zhao et al. developed a tandem oxidative coupling–HERON rearrangement of hydroxamates mediated by N-bromosuccinimide (NBS), providing a mild and general route to carboxylic esters from readily accessible starting materials (20) (Scheme 5B). This method allows access to sterically hindered esters, such as tert-butyl esters, which are difficult to prepare via conventional esterification. Expanding further on hypervalent iodine reagents, Duan et al. demonstrated in 2015 that iodobenzene diacetate (PIDA) in combination with iodine can mediate an analogous oxidative coupling–HERON rearrangement of hydroxamic acid esters (21) to afford carboxylic esters (Scheme 5C). More recently, in 2020, Chattopadhyay et al. reported a similar oxidative HERON rearrangement using molecular iodine (I2). (22)

Scheme 5

Scheme 5. Oxidative HERON Rearrangement
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4. C–H Bond Activation

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Selective functionalization via C–H bond activation has emerged as a powerful strategy for streamlining synthetic routes. However, the presence of multiple similar C–H bonds often requires directing groups to guide the metal catalyst to specific sites, enabling precise and site-selective transformations. O-Methyl hydroxamic acid (commonly referred to as N-methoxyamide) serves as a key directing group in this context. It acts as a weakly coordinating auxiliary that forms kinetically labile metallacyclic intermediates, which are more readily functionalized compared to those derived from strongly coordinating directing groups. (23) A significant advantage of this motif is its ligand compatibility, allowing reaction activity to be modulated through ligand design rather than by modifying the directing group itself. (24,25) Additionally, the O-methyl hydroxamate unit is easily installed and removed, exhibits versatile functional group interconversion chemistry, and offers distinct advantages for downstream synthetic manipulations. (17,26,27)
In 2008, Yu et al. reported a palladium-catalyzed cross-coupling of β-C(sp3)–H bond in O-methyl hydroxamic acid with sp3- or sp2-boronic acids, using air as the terminal oxidant to avoid stoichiometric metal-based oxidants like Ag2O or Ag2CO3 (28) (Scheme 6). Compared to carboxylic acids, the O-methyl hydroxamic acid directing group exhibits stronger coordination to palladium, effectively suppressing competitive self-coupling and β-H elimination.

Scheme 6

Scheme 6. Pd-Catalyzed Cross-Coupling Reaction of Hydroxamates with Boronic Acid
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In 2009, Wang et al. reported a palladium-catalyzed ortho-alkoxylation of N-methoxybenzamides (29) (Scheme 7). The reaction proceeds in good yields across a range of electronically diverse arenes, regardless of the presence of electron-withdrawing or electron-donating substituents. The use of K2S2O8 as a stoichiometric oxidant suggests that the transformation likely operates via a Pd2+/Pd4+ catalytic cycle.

Scheme 7

Scheme 7. Pd-Catalyzed Alkoxylation of N-Methoxybenzamides
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In 2011, Booker-Milburn et al. developed a PdII-catalyzed synthesis of alkylidene isoindolinones from N-methoxybenzamides via an efficient E-selective C(sp2)–H alkenylation/Wacker oxidation sequence (30) (Scheme 8). Notably, by adjusting the reaction conditions, the method could be diverted toward carbonylation with CO to afford substituted phthalimide derivatives.

Scheme 8

Scheme 8. PdII-Catalyzed Synthesis of Alkylidene Isoindolinones and Phthalimide
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The selective C–H functionalization of medicinally relevant heterocycles remains challenging because heteroatoms (e.g., N, S, P) often coordinate more strongly to metals than the directing group, resulting in catalyst poisoning or nonselective functionalization. To address this limitation, in 2014, Yu et al. reported a Pd0-catalyzed cyclization of N-methoxyamide-tethered heterocycles with isocyanides under aerobic conditions (31) (Scheme 9). In this system, the acidic N-methoxyamide group facilitates oxidation of the Pd0 precatalyst to PdII, which also anchors the metal at the ortho-position and enables directed C–H activation.

Scheme 9

Scheme 9. Pd0-Catalyzed Directed C–H Functionalization of Heterocycles
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Subsequently, they extended the use of the N-methoxyamide directing group to alkene substrates (32) (Scheme 10). This strategy enables activation of the α-C–H bonds in alkenes to afford a series of 4-imino-β-lactams. Notably, when the α-position is substituted, activation could be shifted to the β-C–H bonds, allowing access to 5-imino-γ-lactams.

Scheme 10

Scheme 10. Pd0-Catalyzed Directed C–H Functionalization of Alkenes
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While directing groups enable efficient C–H activation, achieving ligand-controlled selectivity is a more attractive objective. Toward this goal, Yu et al. developed a ligand-enabled, tunable mono- or diarylation of β-C(sp3)–H bonds in alanine derivatives. (25) To improve practicality and facilitate downstream functionalization, they further screened both ligands and conditions by incorporating an N-methoxyamide directing group, which also provides a versatile handle for subsequent transformations (26) (Scheme 11).

Scheme 11

Scheme 11. Ligand-Enabled β-C(sp3)-H Arylation of α-Amino Acids
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Besides palladium, the N-methoxyamide also plays important roles in C–H bond activations mediated by other metal catalysts, including ruthenium and rhodium. In 2010, Guimond et al. reported the rhodium-catalyzed C–H activation using an N-methoxyamide as a directing group (33,34) (Scheme 12). The cyclization of N-methoxybenzamide with alkynes provides access to isoquinolone scaffolds. Notably, the choice of additive strongly influences the outcome: replacing copper(II) acetate monohydrate with cesium acetate shifts the nitrogen- versus oxygen-cyclization ratio from 1.1:1 to 20:1. It was proposed that Cu(OAc)2·H2O oxidizes Rh+ to Rh3+ during C–O reductive elimination, whereas in the absence of an external oxidant, the N–O bond of the substrate itself serves as an internal oxidant, promoting the conversion of Rh+ to Rh3+ and thereby enhancing reaction selectivity.

Scheme 12

Scheme 12. Rh-Catalyzed Isoquinolone Synthesis from N-methoxybenzamide and Alkyne
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In 2013, Glorius et al. reported a rhodium-catalyzed synthesis of azepinones via cyclization of α, β-unsaturated aldehydes or ketones with N–methoxybenzamide (35) (Scheme 13). Mechanistic studies suggest that subsequent to alkene insertion, protonolysis of the RhIII species generates a Rh–N intermediate. Intramolecular 1,2-addition to the carbonyl followed by hydrogenolysis produces a seven-membered hemiaminal, which dehydrates to afford the azepinone product.

Scheme 13

Scheme 13. Rh-Catalyzed Azepinone Synthesis from N-methoxybenzamide and α, β-Unsaturated Aldehydes or Ketones
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In RhIII-catalyzed C(sp2)–H activations using the N-methoxyamide directing group, allenes can also serve as versatile coupling partners alongside alkenes and alkynes. Ma et al. reported reactions with 1,1-disubstituted and trisubstituted allenes (36) (Scheme 14). In contrast to the reductive-elimination pathway observed by Glorius et al., (37) protonolysis occurs following allene insertion in this system, enabling a redox-neutral catalytic cycle. Notably, stepwise allylation of the substrate could be achieved simply by modulating the reaction temperature.

Scheme 14

Scheme 14. Rh-Catalyzed C(sp2)-H Activation Reactions Involving 1,1-Disubstituted Allenes and Trisubstituted Allenes
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In 2015, Wang et al. identified a tandem catalytic system (RhIII/PdII) that provides rapid access to vinyl-substituted 5,6-dihydropyridin-2(1H)-ones and 3,4-dihydroisoquinolin-1(2H)-ones (38) (Scheme 15A). The reaction proceeds under mild reaction conditions with the generation of water and CO2 as the only byproducts. A variety of aromatic amides and alkenyl amides bearing diverse substituents are compatible with the reaction conditions, delivering the cyclization product with high regio- and stereoselectivities. The vinyl group has been demonstrated to be a reliable handle for functional group interconversions. The alkene is found to be the key factor for the success of the reaction.

Scheme 15

Scheme 15. Rh-Catalyzed C(sp2)-H Activation Reactions
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By taking advantage of the unique reactivity of 2,2-difluorovinyl tosylate, Wang and Li et al. accomplished facile synthesis of 4,4-difluoro-3,4-dihydroisoquinolin-1(2H)-ones, 4-fluoroisoquinolin-1(2H)-ones, and 5-fluoropyridin-2(1H)-ones by using Rh(III)-catalyzed C–H activation (39) (Scheme 15B). The reactions proceed under mild and redox-neutral reaction conditions. The robustness of the protocol is fully investigated, which turns out to be quite decent.
While palladium-catalyzed C–H bond activation and coupling with organometallic reagents have been extensively studied, (40) the development of analogous rhodium-catalyzed transformations has progressed slowly. In 2012, Cheng et al. reported a rhodium-catalyzed C(sp2)–H activation using N-methoxyamide as a directing group, achieving the synthesis of phenanthridinone derivatives from arylboronic acids (41) (Scheme 16, top). The addition of Ag2O as an external oxidant enables a RhIII/RhI dual catalytic cycle. Within the same catalytic framework, they later showed that arylsilanes could also serve as effective coupling partners for this transformation (42) (Scheme 16, middle). Furthermore, organotin reagents were successfully applied to accomplish ortho-vinylation of arylamides under related conditions (43) (Scheme 16, bottom).

Scheme 16

Scheme 16. Rh-Catalyzed C(sp2)-H Activation Reactions Involving N-methoxyamides and Organometallic Reagents
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The redox mechanism for Ru-catalyzed C(sp2)–H activation parallels those of Pd0/PdII and RhI/RhIII systems. In 2011, Wang et al. described a ruthenium-catalyzed cyclization of N-methoxybenzamides with alkynes to give quinolinones (44) (Scheme 17A). The substrate scope was comparable to that of the related Rh-catalyzed reaction. (33) Notably, when electronegative substituents (e.g., −OMe) are at the meta-position, the more sterically hindered isomer predominates, a result likely governed by C–H acidity or Ru–C bond stability. In the same year, Ackermann et al. employed the same catalytic system using bulky carboxylate additives and water as the solvent, enabling free hydroxamic acids to serve as directing groups under additive-mediated conditions (45) (Scheme 17B). This environmentally benign approach provides a more straightforward route to quinolinone derivatives.

Scheme 17

Scheme 17. Ru-Catalyzed C(sp2)-H Activation Reactions Involving N-Methoxyamide and Alkynes
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These examples in this section collectively suggest that hydroxamate derivatives, particularly N-methoxyamides, have emerged as pivotal directing groups in C–H activation. Their simple structure, stability, and ease of synthesis and functionalization make them highly valuable in synthetic chemistry.

5. N-Centered Radical Precursor Reagents

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Free radicals constitute a highly reactive class of intermediates with broad utility in chemical synthesis. While carbon-centered radicals have been extensively studied, nitrogen-centered radicals remain relatively underdeveloped, largely due to limited synthetic access. Over recent decades, N-centered radicals have gained recognition as valuable intermediates, particularly for their ability to forge C–N bonds. (46,47) Current methods for generating N-centered radicals primarily rely on the cleavage of weak nitrogen–heteroatom bonds. These include the reductive cleavage of N–X, N–O, N–S, and N–N bonds (Scheme 18), as well as the oxidative cleavage of N–H bonds. In this context, hydroxamates, derived from hydroxylamine, are particularly useful, as their N–O bonds undergo facile reductive cleavage to deliver N-centered radicals.

Scheme 18

Scheme 18. Methods for the Generation of N-Centered Radicals
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As early as 1995, Zard et al. reported an AIBN and n-Bu3SnH initiated radical cascade cyclization reaction of hydroxamates to generate amidyl radicals. (48) In 2001, Weinreb et al. developed a method for generating amidyl radicals from N-sulfinyl ester hydroxamates, which are in situ formed from hydroxamic acid derivatives under weakly basic conditions. This reaction leverages olefin-containing hydroxamic acids as substrates, sulfinyl chloride (49) or diethyl phosphonochloridite (50) as activating reagents, and diphenyl diselenide, diphenyl disulfide or TEMPO as radical scavengers to achieve the intramolecular radical cascade cyclization between amidyl radicals and olefins.
In 2016, Leonori et al. reported a photocatalytic system employing eosin Y as the photoredox catalyst and 1,4-cyclohexadiene as both a hydrogen-atom donor and a single-electron transfer agent. (51) This system enables the 5-exo-trig hydroamination cyclization of O-aryl hydroxamates with alkenes, as well as the N-arylation of O-aryl hydroxamates with arenes (Scheme 19). Serving as amidyl radical precursors, the O-aryl hydroxamates readily undergo single-electron reduction under the action of eosin Y.

Scheme 19

Scheme 19. Hydroamination of Amidyl Radicals Derived from Hydroxamates
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Subsequently, the authors extended this strategy to achieve the intramolecular 5-exo-dig hydroamination of O-aryl hydroxamates with alkynes (52) (Scheme 20). Notably, this transformation proceeds under visible-light irradiation without an external photocatalyst, requiring only potassium carbonate and 1,4-cyclohexadiene. The 5-exo-dig cyclization, however, proceeds with lower efficiency than the 5-exo-trig pathway due to its slower radical cyclization rate, which allows competing hydrogen-atom transfer to the amidyl radical. Despite this limitation, introducing α-substitution─through the Thorpe–Ingold effect (53) could raise the yield of the cyclized product to nearly quantitative levels.

Scheme 20

Scheme 20. Intramolecular 5-exo-dig Radical Cyclization of Amidyl Radicals and Alkynes
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In 2017, Wang et al. reported a visible-light-promoted synthesis of pyrroloindolines through amidyl-radical cyclization followed by carbon-radical addition (54) (Scheme 21). Using indole-based O-aryl hydroxamates as amidyl-radical precursors, the method exhibits broad tolerance toward both electron-withdrawing and electron-donating substituents on the indole ring, and accommodates various functionalized alkenes as radical acceptors. To demonstrate its synthetic utility, the authors achieved a concise synthesis of (±)-Flustramide B. Moreover, by employing tert-butyl thiol or oxygen as radical-trapping reagents, they extended the protocol to access functionalized pyrroloindolines (55) (Scheme 21).

Scheme 21

Scheme 21. Synthesis of Pyrroloindoline via an Amidyl Radical Cyclization/Carbon Radical Addition Cascade
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Pyridines are prevalent in various drug molecules and natural products. (56) The activation and functionalization of pyridines provide access to diverse pyridine derivatives. (57) Drawing on advances in imino radical chemistry, (47,58) amide activation (59) and pyridine activation, Hong et al. recently reported a mild, catalyst-free remote C(sp3)–H pyridylation of hydroxamate derivatives (60) (Scheme 22). This method employs an amide-activation strategy to generate oxime-containing pyridinium salts in situ. Under visible-light irradiation and in the absence of an external photocatalyst, these salts undergo homolytic N–O bond cleavage to form imino radicals. A subsequent 1,5-hydrogen migration and endo-type radical addition to the C2-position of the pyridine ring are followed by deprotonation, single-electron transfer, and C–N bond cleavage to furnish the final product.

Scheme 22

Scheme 22. γ-Selective Pyridylation of Hydroxamate
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Thanks to the elegant research of Murphy on the superelectron-donors (SEDs), (61) remarkable progress has been made in the realm of C–C bond construction. (62,63) Drawing upon prior work associated with SEDs, (62) Yang et al. developed a method for synthesizing 1,2-diamines via the coupling of 2-azaallyl anions (as SEDs) with hydroxamates (as electron acceptors) (64) (Scheme 23). This transformation is notable for involving a thermodynamically disfavored 1,2-hydrogen migration step. EPR and DFT studies support a net 1,2-hydrogen atom transfer mechanism.

Scheme 23

Scheme 23. Amidyl Radicals α-C(sp3)–H Coupling via Net-1,2-HAT
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In 2022, Hyster et al. reported a radical hydroamination of alkenes by combining photocatalysis with biocatalysis (65) (Scheme 24). The system integrates a flavin-dependent “ene” reductase with a photoredox catalyst to selectively generate amidyl radicals from hydroxamates within the enzyme’s active site. This strategy enables not only intramolecular 6-endo-trig hydroamination but also─through directed enzyme evolution─5-exo-trig, 7-endo-trig, and 8-endo-trig reactions. Notably, the challenging intermolecular radical hydroamination is achieved using a triple GluER-T36A mutant.

Scheme 24

Scheme 24. Photo-Enzyme Co-Catalyzed Hydroamination of Alkenes and Hydroxamates
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6. Aza-Oxyallyl Cationic Precursor Reagents

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6.1. Aza-Oxyallyl Cations as 1,3-Dipoles in Cycloaddition

N-Heterocyclic compounds hold significant importance in medicinal chemistry, (66) organic synthesis, (67) and material science. (68) Diverse strategies for their synthesis have been developed, (69) among which cycloaddition reactions are particularly notable for their ability to construct multiple bonds in a concerted or stepwise manner with high regio- and stereoselectivity. (70) The aza-oxyallyl cation is a highly reactive, positively charged intermediate featuring an N–O bond within an allylic scaffold. Its utility in accessing bioactive N-containing frameworks has attracted considerable attention in organic synthesis. Due to its inherent reactivity, this intermediate cannot be isolated but is typically generated in situ under basic conditions. α-Halo-hydroxamates serve as the most common precursors for its formation and have played a pivotal role in advancing this chemistry.
The transient aza-oxyallyl cation was first observed in 1968 by Sheehan and Lengyel during studies on α-lactam intermediates. (71) Definitive evidence for its existence was provided by Kikugawa et al., who also highlighted the crucial stabilizing effect of the N-alkoxy group. (72) In 2011, Jeffrey et al. achieved a [3 + 4] cycloaddition between α-halo-hydroxamates─aza-oxyallyl cation precursors─and cyclic dienes (73) (Scheme 25). Notably, this intermediate reacts not only with cyclopentadiene but also with furan. Computational and experimental studies confirm that the N-benzyloxy group is essential for intermediate formation and stability. The same group later extended the [3 + 4] cycloaddition to intramolecular settings, showing that pyrrole, in addition to furan, serves as an effective diene component. (74)

Scheme 25

Scheme 25. [3 + 4] Cycloaddition of Aza-Oxyallyl Cationic Intermediates
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Subsequently, research on the aza-oxyallyl cation has progressed significantly. Various [3 + m] cycloaddition reactions have been developed with diverse partners such as dipoles, C═C double bonds, C═O double bonds, and dienes. (70,75)

6.2. Aza-Oxyallyl Cations as Alkylating Agents

Beyond as the dipoles in cycloadditions, aza-oxyallyl cations can also act as alkylating agents. In 2020, Singh et al. reported that under basic conditions in hexafluoroisopropanol (HFIP), aza-oxyallyl cations react with anilines to afford sterically hindered secondary arylamines (76) (Scheme 26A). The reaction proves highly solvent-dependent, failing in nonfluorinated media and underscoring the critical role of HFIP in stabilizing the cationic intermediate. The scope encompasses anilines bearing cyano, nitro, ester, halogen, and heterocyclic substituents, all of which furnish the corresponding secondary amine products.

Scheme 26

Scheme 26. Aza-Oxyallyl Cations as Alkylating Reagents for Amines
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Simultaneously, Kim et al. described an alkylation, in which the aza-oxyallyl cation acts as an alkylating agent toward amines, with the scope extending beyond arylamines to include aliphatic amines (77) (Scheme 26B). In 2021, using a similar approach, they employed TMSN3 as a nitrogen nucleophile to achieve α-azidation of amides (78) (Scheme 26C). Catalyzed by TBAF, TMSN3 reacts with a broad range of α-mono- and α-disubstituted α-halo-hydroxamates to give α-azidoamide derivatives, demonstrating wide substrate scope and synthetic versatility.
Beyond the use of aza-oxyallyl cations as alkylating agents toward amines by Singh and Kim, other nucleophiles have also been employed, including alcohols (79) (Scheme 27A), water (80) (Scheme 27B), 3-amido oxetane (81) (Scheme 27C), alkyl hydroperoxide (82) (Scheme 27D), and thiol or selenol compounds (83) (Scheme 27E).

Scheme 27

Scheme 27. Other Alkylation of Nucleophiles
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In 2024, Mi et al. reported a transition-metal-free coupling between aryl (or vinyl) boronic acids and aza-oxyallyl cations to afford α-aryl (or vinyl) amides (84) (Scheme 28). The reaction proceeds via a 1,4-metalate shift of a boron-ate complex. Its mild conditions render this method promising for pharmaceutical synthesis. However, for α-alkyl-substituted α-halo-hydroxamates, the coupling with arylboronic acids is limited by modest regioselectivity on the arene ring.

Scheme 28

Scheme 28. Coupling of Boronic Acid and Hydroxamates via 1,4-Metallate Shift
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7. Reduction Reaction

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Hydroxamates contain reducible N–O bonds that can be cleaved to give amides. Under strong bases such as LDA or with TBSOTf/Et3N, the Weinreb amides undergo demethoxylation, releasing formaldehyde via either an E2-elimination (85) or an enolization-retro-ene mechanism. (86) Furthermore, approaches to the reduction of hydroxamates through the utilization of reducing agents have been documented. In 1980, Miller et al. described the TiCl3-mediated reduction of hydroxamates to amides in a buffer system, applicable to both linear amides and β-lactams (87) (Scheme 29A). Later, in 1994, the same group reported reductive N–O cleavage using catalytic hydrogenation, sodium amalgam, or Mo(CO)6 complexes (88) (Scheme 29A).

Scheme 29

Scheme 29. Reduction Reactions of Hydroxamates via Metal Reagents
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In 1999, Keck et al. reported the reduction of hydroxamates to amides using samarium diiodide under mild, base-free conditions, which are also applicable to hydroxylamine-to-amine conversion (89) (Scheme 29B). In 2001, Yus et al. described a DTBB (4,4′-di-tert-butylbiphenyl)/Li-mediated reductive cleavage of hydroxamates, where temperature control allows selective formation of either amide or alkane products (27) (Scheme 29C). In 2011, Ura et al. developed a catalytic RuCl3/Zn–Cu/alcohol system for hydroxamate reduction (90) (Scheme 29D). Control experiments with 2-butanol as the solvent, which generates 2-butanone, confirm that the alcohol acts as a hydrogen donor in this reductive process.
Besides the traditional harsh reducing conditions, in 2019, Du et al. reported a metal-free reduction of N-methoxyamides to primary amides using elemental sulfur as the reductant (91) (Scheme 30A). The mild conditions, with DMSO and DABCO as additives, tolerate diverse functional groups (e.g., nitro, halogens), offering a practical route to primary amides. In 2008, Murphy et al. described an organic superelectron-donor (SED)-mediated reductive cleavage of the N–O bond in hydroxamates (Scheme 30B). The simple protocol accommodates both aryl/heteroaryl and alkyl hydroxamates. Reactivity studies show that substrates with lower LUMO energies undergo N–O cleavage more readily. For aryl-containing substrates, initial electron transfer occurs on the arene, followed by intramolecular electron transfer to the amide. In the absence of an arene, electron transfer becomes more difficult, requiring harsher conditions and resulting in lower yield. (92)

Scheme 30

Scheme 30. Metal-Free Reduction Reactions of Hydroxamates
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8. Umpolung Reaction

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“Polarity” refers to the localized charge polarization within a molecule, arising from differences in electronegativity between adjacent functional groups and their associated inductive and conjugative effects. In contrast, “umpolung” encompasses synthetic strategies that reverse the innate polarity of a functional group, thereby enabling reactivity opposite to its inherent tendency. This approach imparts flexibility and chemoselectivity to organic transformations, often enhancing reactivity, and has become an essential tool for expanding accessible chemical space in modern synthesis. Umpolung strategies can be broadly classified into three categories: (1) metal-mediated approaches, such as the generation of organometallic reagents; (93) (2) organic small molecule-based methods, such as the Corey-Seebach reaction, (93) transformations involving enamine/imine intermediates (94) or hydrazone intermediates, (95) triphenylphosphine-mediated processes, (96) and N-heterocyclic carbene catalysis; (97) (3) redox-driven strategies, employing hypervalent iodine reagents (98) and samarium diiodide. (99)
Hydroxamates are amide-type compounds featuring an N–O bond, whose relatively low bond energy distinguishes them from conventional amides. This feature enables an umpolung of amide reactivity through N–O cleavage, paralleling the strategy used in classical electrophilic amide activation. (100) The potential intermediacy of α-lactams was suggested by Leuchs et al. as early as 1908, but direct experimental evidence was not obtained until 1961, when Baumgarten et al. observed the α-lactam by infrared spectroscopy. Following that, they isolated an α-lactam product from N-tert-butyl-N-chloro-phenylacetamide. Subsequently, diverse synthetic routes to α-lactams have been developed. (71) The first synthesis of α-lactam from hydroxamic acid was reported by Kirby et al. in 1982. (101)
However, it was not until 1992 that Hoffmann et al. successfully synthesized functionalized amides from hydroxamates via an α-lactam intermediate using an umpolung strategy. While various nucleophiles could be employed to furnish the corresponding functionalized amides (102) (Scheme 31), this approach is limited to α-aryl, α-heteroaryl, α-alkenyl, or other electron-withdrawing group substituted hydroxamates. The α-lactam intermediate is generated through a 1,3-elimination under basic conditions; (103) thus, the acidity of the α-C–H bond dictates its formation─the more acidic the proton, the more readily the α-lactam forms.

Scheme 31

Scheme 31. α-Functionalized Amide Synthesis from Hydroxamates
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In 2023, Zeng et al. reported an α-lactam-mediated umpolung strategy to synthesize α-heteroatom-functionalized amides (104) (Scheme 32A). Using N-aryloxyamides as starting materials and Et3N as a base, they converted various nucleophiles─amines, alcohols, phenols, thiols, and carboxylic acids into the corresponding α-substituted amides. Although this method exhibits broad substrate generality, it remains limited to α-aryl-substituted hydroxamates, similar to Hoffmann’s earlier work, thereby restricting its wider applicability. In 2024, the same group extended the approach to electron-rich arenes, enabling α-arylation of amides (105) (Scheme 32A).

Scheme 32

Scheme 32. α-Functionalization of Amides via α-Lactam Umpolung
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In 2024, our group reported a general synthesis of α-heteroatom-functionalized amides via an α-lactam intermediate, facilitated by Mg2+ under redox-neutral umpolung conditions (106) (Scheme 32B). This system efficiently incorporates halogen (F, Cl, Br), nitrogen, oxygen, and sulfur nucleophiles. By employing a soft-enolization strategy, the reaction is extended to α-alkyl-substituted hydroxamates, which possess relatively low α-C–H acidity, thereby overcoming a key limitation of prior methods and significantly broadening substrate scope and functional-group tolerance. The protocol also proves effective for late-stage functionalization of complex molecules. DFT studies indicate that the cooperative action of Mg2+ and DIPEA promotes soft enolization, which is crucial for α-lactam formation.
In the same year, by leveraging the same α-lactam-based umpolung strategy, we converted hydroxamates to α-bromoamides, which then underwent a Pd-catalyzed elimination cascade to furnish α, β-unsaturated amides (107) (Scheme 33A). This method delivers the unsaturated products with high efficiency and excellent E-selectivity, offering an attractive route to α, β-unsaturated secondary amides.

Scheme 33

Scheme 33. α, β-Unsaturated Secondary Amides and Aziridination-Containing Amides’ Synthesis via α-Lactam Umpolung
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In 2025, we developed an aziridination of conjugated hydroxamates via α- and, β-addition of primary amines (108) (Scheme 33B). Unlike conventional aziridine syntheses that rely on external oxidants, our approach uses the N–O bond of the hydroxamate as an internal oxidant, thereby avoiding oxidation issues typically encountered with primary amines and electron-rich anilines. The α-addition pathway employed here contrasts with the common β-addition (Michael addition) and represents a less-common mode of reactivity, thus expanding the synthetic toolbox. Mechanistic studies indicate that regioselectivity is controlled by the antiperiplanar requirement of an intramolecular SN2 step. When secondary amines are used as nucleophiles, the same strategy affords 1,2-diamination products, further broadening the utility of this method.
The umpolung strategy based on α-lactam intermediates to achieve the cycloaddition reaction of hydroxamate esters also represents a promising reaction. Zeng et al. developed a novel triethylamine-mediated cascade arylation/cyclization reaction between indoleacetamides and 3-substituted indoles, which proceeds via the umpolung of α-lactams (109) (Scheme 34A). This metal-free and photocatalyst-free protocol enables the synthesis of various indolyl pyrroloindoles in good yields. Gram-scale experiments and downstream derivatization of the products demonstrate the potential utility of this method. Later in 2025, they accomplished a novel [3 + 2] cycloaddition between indolyl hydroxamate esters and phosphonium ylides. This reaction also relies on the umpolung strategy of α-lactams to selectively construct three distinct N-heterocycles: indolyl maleimides, 5-methylene pyrrolones, and 5-hydroxypyrrolones (110) (Scheme 34B). Notably, this study reveals the unprecedented role of phosphonium ylides as two-carbon synthons in the construction of N-heterocycles.

Scheme 34

Scheme 34. Cycloaddition of Hydroxamates via α-Lactam Umpolung
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The umpolung strategy enabled by cleaving the weak N–O bond in hydroxamates proceed not only via the α-lactam intermediate mentioned earlier, but also through a conventional enol intermediate. In 2008, Somfai et al. reported the α-arylation of Weinreb amides, providing access to α-arylglycine amides (111) (Scheme 35A). Later, by replacing aryl Grignard reagents with less nucleophilic arylzinc species, they achieved highly diastereoselective synthesis of α-arylglycines (d.r. >6:1) from chiral substrates. (112) This transformation hinges on the polarity reversal of the enolate upon N–O cleavage in the Weinreb amide.

Scheme 35

Scheme 35. α-Functionalization of Hydroxamates via Enolate Umpolung
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In 2017, Miyata et al. described a synthesis of α-arylamides via electrophilic addition to silyl enol ether intermediates generated from hydroxamates (113) (Scheme 35B). The organoaluminum reagent serves a dual role: as a Lewis acid coordinating oxygen to promote N–O cleavage, and as a nucleophile adding to the polarity-reversed silyl enol ether. The reaction proceeds under simple, noncryogenic conditions. When hydroxamates bearing chiral auxiliaries are used, highly diastereoselective arylation provides optically pure α-aryl carboxylic acids.
Since the publication of Gassman’s pioneering review, (114) nitrenium ions have garnered considerable attention. This interest stems both from their potential role as active metabolites of mutagenic nitro- and amino-aromatic compounds (115) and from their broad synthetic utility. (116) These reactive intermediates can be generated by treating N-chloroamines with silver salts. (117) More recently, hypervalent iodine reagents, serving as milder oxidants, (118) have also been utilized for the in situ generation of nitrenium ions. (119)
The low stability of nitrenium ions has hampered their broad application as reactive intermediates. Nevertheless, when nitrenium ions are bonded to heteroatoms, their stability is significantly enhanced via the interaction between the empty π orbital at the nitrogen center and the filled nonbonding orbitals of adjacent heteroatoms (e.g., oxygen or nitrogen). This phenomenon has been corroborated by MNDO molecular orbital calculations and reactions involving these species. (120) Such stabilized nitrenium ions prove to be valuable in organic synthesis, particularly in a variety of electrophilic reactions.
In 1990, Kikugawa et al. reported the nucleophilic aromatic amination of N-methoxyamides (121) (Scheme 36). In this transformation, N-methoxyamides react with the hypervalent iodine(III) oxidant bis(trifluoroacetoxy)iodobenzene (PIFA), affording N-acyl nitrenium ions as key reactive intermediates. These species subsequently undergo electrophilic aromatic substitution with electron-rich arenes to deliver the corresponding aminated aromatic products. (122)

Scheme 36

Scheme 36. Electrophilic Aromatic Amination Reaction of N-methoxyamides
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In 2015, Shi and Houk et al. reported a novel strategy for intramolecular C(sp3)–H bond amination under mild reaction conditions, utilizing aryl iodides as the catalyst and meta-chloroperoxybenzoic acid (m-CPBA) as the oxidant (Scheme 37). With N-methoxyamides as the starting materials, this protocol enables the synthesis of γ-lactams through the integrated transformation of iodonium ions and nitrenium ions. (123) Density functional theory (DFT) calculations reveal that the construction of the highly stereoselective chiral quaternary carbon center stems from the iodonium cation intermediate dominating the reaction pathway, and the transformation proceeds via a concerted transition state involving the synergistic C–H bond activation and C–N bond formation.

Scheme 37

Scheme 37. γ-Lactam Synthesis via Intramolecular C(sp3)-H Amination of N-Methoxyamides
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In 2021, Li et al. reported the construction of oxindole skeletons bearing all-carbon quaternary carbon centers via an intramolecular C(sp2)–N cross-coupling catalyzed by aryl iodide (124) (Scheme 38A). Using hydroxamic acid esters, this strategy relies on an organocatalytic-oxidative system (4-tert-butyliodobenzene/m-chloroperoxybenzoic acid) to efficiently synthesize rigid oxindole derivatives with potential biological activity in excellent yields (up to 99%). This transition-metal-free protocol features mild reaction conditions and scalability to gram-scale preparation, thereby suggesting its high efficiency.

Scheme 38

Scheme 38. Intramolecular C(sp2)–H Amination of Hydroxamates Catalyzed by Aryl Iodide
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In 2025, Kürti and Das et al. achieved the efficient construction of 2-quinolones using the organocatalytic-oxidative system. Moreover, when N-phenoxyamides are employed as substrates, the resulting products could undergo subsequent [3,3]-σ rearrangement to afford 8-phenyl-2-quinolones (125) (Scheme 38B).
In 2022, Dohi and Kita et al. accomplished the synthesis of six-membered benzolactams using 2,2′-diiodobiphenyl as the catalyst. By varying the carbon chain of the hydroxamates, the synthesis of five-, seven-, and even eight-membered benzolactams are all feasible (126) (Scheme 38C).
Recently, Singh et al. developed a transition-metal-free, one-pot protocol for the synthesis of 1,4-benzoxazinones through aryl C(sp2)–H amination (127) (Scheme 39). Notably, this study represents the first utilization of the 1,3-bielectrophilic character of α-bromohydroxamic acid esters, which enables the direct O-alkylation of phenols and benzyl alcohols via in situ-generated aza-oxyallyl cations. Subsequent aryl C–H amination is then accomplished under hypervalent iodine catalytic conditions, proceeding through the key intermediate of nitrenium ions. When p-methoxy-substituted phenols are used, this catalytic system selectively furnishes spirooxazolidinone derivatives.

Scheme 39

Scheme 39. Synthesis of 1,4-Benzoxazinones and 4,1-Benzoxazepinones via Aryl C(sp2)–H Amination
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Meanwhile, Lovely et al. reported that N-methoxyureas─the stable precursors of nitrenium ions─undergo dearomatizative spirocyclization with p-methoxy-substituted phenols (128) (Scheme 40). This approach further facilitates the synthesis of structural fragments associated with the marine alkaloid KB343, which underscores the practical value of this strategy.

Scheme 40

Scheme 40. Dearomatizative Spirocyclization of p-Methoxy-Substituted Phenols
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9. Other Reaction Modes

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In addition to the well-established transformations discussed above, novel reaction patterns involving hydroxamic acid derivatives have also emerged in recent years. This section primarily focuses on two categories: N-centered functionalization of hydroxamates and α-C–H functionalization adjacent to the nitrogen in hydroxamates.

9.1. N-Centered Functionalization of Hydroxamates

In 2017, Wang et al. reported a method for achieving the N-centered arylation (indolation and pyrrolation) of amides using hydroxamates as the electrophilic nitrogen source in DMF promoted by zinc chloride (129) (Scheme 41). In contrast, when the reaction was carried out without zinc chloride and with DMSO as the solvent, the analogous transformation afforded α–C-H functionalized products at the nitrogen-adjacent carbon. It is worth noting that these transformations do not require expensive transition metal catalysts or ligands. These novel reactions provide a straightforward synthetic route toward 3-aminoindoles, which possess significant biological relevance.

Scheme 41

Scheme 41. N-Centered Functionalization Reactions of Hydroxamates
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In 2023, Stoltz and Reisman et al. reported a nickel-catalyzed intermolecular N–N coupling reaction of hydroxamates with various aromatic and aliphatic amines to afford hydrazides (130) (Scheme 42). Experimental studies indicate that an electrophilic nickel-stabilized acyl nitrene intermediate is involved, and the Ni(I) catalyst is generated via a silane-mediated reduction process. Notably, this protocol represents the first intermolecular N–N coupling reaction compatible with secondary aliphatic amines.

Scheme 42

Scheme 42. Ni-Catalyzed N–N Coupling Reaction of Hydroxamates and Amines to Synthesize Hydrazides
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In 2024, Dai et al. reported a method for constructing N–N bonds via an SN2 substitution at the amide nitrogen atom using amine nucleophiles (131) (Scheme 43A). The sulfonate leaving group plays a crucial role in this reaction. This novel N–N coupling reaction employs O-tosyl hydroxamates as the electrophiles and readily accessible amines (including acyclic aliphatic amines and saturated nitrogen heterocycles) as the nucleophiles, obviating the need for transition metals. Featuring mild reaction conditions, a broad substrate scope, and excellent functional group tolerance, this protocol provides a new strategy for the synthesis of hydrazide derivatives with significant biological and medicinal importance. More recently, the same group extended the nucleophile scope to cyanide ion, enabling the synthesis of N-cyanated amides under this reaction paradigm (132) (Scheme 43B).

Scheme 43

Scheme 43. Synthesis of Hydrazides and N-Cyanoamides via N-Amination and Cyanation of Hydroxamates
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9.2. α-C–H Functionalization of Nitrogen in Hydroxamates

In 2023, Pace et al. reported a base-mediated homologation rearrangement reaction of N-methyl-N-alkoxyamides (133) (Scheme 44). Under strongly basic conditions, the N-methyl group of the amide undergoes dehydrogenation to facilitate the departure of the alkoxy anion, which then adds to the generated electrophilic iminium ion to trigger the homologation rearrangement. The essential PMDTA (pentamethyldiethylenetriamine) promotes the complete deaggregation of s-BuLi. Furthermore, employing 2-methyltetrahydrofuran as the solvent prevents α-lithiation more effectively than tetrahydrofuran.

Scheme 44

Scheme 44. Base-Mediated Homologation Rearrangement of N-Methyl-N-Alkoxyamides
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In 2024, Plietker et al. reported a ruthenium-catalyzed α-C–H arylation reaction of the nitrogen atom in hydroxamates (134) (Scheme 45). This reaction tolerates both water and oxygen, featuring robustness of the reaction system. Notably, Rh or Pd-based catalysts fail to yield the target products, highlighting the uniqueness of Ru catalysts. This system provides a novel route for the synthesis of benzylamines, which is complementary to traditional methods.

Scheme 45

Scheme 45. Ru-Catalyzed α-C–H Arylation Reaction of the Nitrogen Atom in Hydroxamates
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Most recently, Dai et al. developed a novel strategy for the synthesis of hemiaminal esters via the direct N-methyl C–H esterification of O-tosyl hydroxamates (135) (Scheme 46). This protocol exhibits excellent tolerance of a wide range of functional groups, including esters, ethers, halides, alkynes, alkenes and heterocycles. Conducted under mild and transition-metal-free conditions, it provides a straightforward and efficient route to access hemiaminal esters. Furthermore, the resulting hemiaminal esters can serve as versatile platforms for further modular derivatization, enabling the construction of diverse valuable molecular scaffolds.

Scheme 46

Scheme 46. α-C–H Esterification of the Nitrogen Atom in Hydroxamates
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10. Summary and Outlook

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This review has summarized key applications of hydroxamate derivatives in organic synthesis, categorized by their structural characteristics. We have discussed essential transformations including Weinreb amide chemistry, rearrangement reactions, C–H activation directed by hydroxamates, their role as precursors to N-centered radicals and aza-oxyallyl cations, reduction reactions, and umpolung transformations initiated by N–O bond cleavage. These examples illustrate the versatility and synthetic utility of the hydroxamate scaffold. We anticipate that this area of chemistry will continue to develop and expand in the coming years.

Data Availability

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The data underlying this study are available in the published article.

Author Information

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  • Corresponding Author
  • Author
    • Rui Wang - School of Chemistry, Sun Yat-Sen University, Guangzhou 510006, P. R. China
  • Author Contributions

    R.W.: Writing─original draft─review and editing. W.L Writing─review and editing, supervision, funding acquisition, conceptualization.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the National Natural Science Foundation of China (22471297), and the Guangzhou Municipal Science and Technology Bureau (2025A04J2067).

References

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  • Abstract

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    Scheme 1

    Scheme 1. Hydroxamic Acid and Related Pharmaceuticals
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    Scheme 2

    Scheme 2. Addition Reaction of Weinreb Amides with Organometallic Reagents
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    Scheme 3

    Scheme 3. Classical Lossen Rearrangement and the “Direct Lossen Rearrangement”
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    Scheme 4

    Scheme 4. HERON Rearrangement
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    Scheme 5

    Scheme 5. Oxidative HERON Rearrangement
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    Scheme 6

    Scheme 6. Pd-Catalyzed Cross-Coupling Reaction of Hydroxamates with Boronic Acid
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    Scheme 7

    Scheme 7. Pd-Catalyzed Alkoxylation of N-Methoxybenzamides
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    Scheme 8

    Scheme 8. PdII-Catalyzed Synthesis of Alkylidene Isoindolinones and Phthalimide
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    Scheme 9

    Scheme 9. Pd0-Catalyzed Directed C–H Functionalization of Heterocycles
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    Scheme 10

    Scheme 10. Pd0-Catalyzed Directed C–H Functionalization of Alkenes
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    Scheme 11

    Scheme 11. Ligand-Enabled β-C(sp3)-H Arylation of α-Amino Acids
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    Scheme 12

    Scheme 12. Rh-Catalyzed Isoquinolone Synthesis from N-methoxybenzamide and Alkyne
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    Scheme 13

    Scheme 13. Rh-Catalyzed Azepinone Synthesis from N-methoxybenzamide and α, β-Unsaturated Aldehydes or Ketones
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    Scheme 14

    Scheme 14. Rh-Catalyzed C(sp2)-H Activation Reactions Involving 1,1-Disubstituted Allenes and Trisubstituted Allenes
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    Scheme 15

    Scheme 15. Rh-Catalyzed C(sp2)-H Activation Reactions
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    Scheme 16

    Scheme 16. Rh-Catalyzed C(sp2)-H Activation Reactions Involving N-methoxyamides and Organometallic Reagents
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    Scheme 17

    Scheme 17. Ru-Catalyzed C(sp2)-H Activation Reactions Involving N-Methoxyamide and Alkynes
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    Scheme 18

    Scheme 18. Methods for the Generation of N-Centered Radicals
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    Scheme 19

    Scheme 19. Hydroamination of Amidyl Radicals Derived from Hydroxamates
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    Scheme 20

    Scheme 20. Intramolecular 5-exo-dig Radical Cyclization of Amidyl Radicals and Alkynes
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    Scheme 21

    Scheme 21. Synthesis of Pyrroloindoline via an Amidyl Radical Cyclization/Carbon Radical Addition Cascade
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    Scheme 22

    Scheme 22. γ-Selective Pyridylation of Hydroxamate
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    Scheme 23

    Scheme 23. Amidyl Radicals α-C(sp3)–H Coupling via Net-1,2-HAT
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    Scheme 24

    Scheme 24. Photo-Enzyme Co-Catalyzed Hydroamination of Alkenes and Hydroxamates
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    Scheme 25

    Scheme 25. [3 + 4] Cycloaddition of Aza-Oxyallyl Cationic Intermediates
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    Scheme 26

    Scheme 26. Aza-Oxyallyl Cations as Alkylating Reagents for Amines
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    Scheme 27

    Scheme 27. Other Alkylation of Nucleophiles
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    Scheme 28

    Scheme 28. Coupling of Boronic Acid and Hydroxamates via 1,4-Metallate Shift
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    Scheme 29

    Scheme 29. Reduction Reactions of Hydroxamates via Metal Reagents
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    Scheme 30

    Scheme 30. Metal-Free Reduction Reactions of Hydroxamates
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    Scheme 31

    Scheme 31. α-Functionalized Amide Synthesis from Hydroxamates
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    Scheme 32

    Scheme 32. α-Functionalization of Amides via α-Lactam Umpolung
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    Scheme 33

    Scheme 33. α, β-Unsaturated Secondary Amides and Aziridination-Containing Amides’ Synthesis via α-Lactam Umpolung
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    Scheme 34

    Scheme 34. Cycloaddition of Hydroxamates via α-Lactam Umpolung
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    Scheme 35

    Scheme 35. α-Functionalization of Hydroxamates via Enolate Umpolung
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    Scheme 36

    Scheme 36. Electrophilic Aromatic Amination Reaction of N-methoxyamides
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    Scheme 37

    Scheme 37. γ-Lactam Synthesis via Intramolecular C(sp3)-H Amination of N-Methoxyamides
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    Scheme 38

    Scheme 38. Intramolecular C(sp2)–H Amination of Hydroxamates Catalyzed by Aryl Iodide
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    Scheme 39

    Scheme 39. Synthesis of 1,4-Benzoxazinones and 4,1-Benzoxazepinones via Aryl C(sp2)–H Amination
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    Scheme 40

    Scheme 40. Dearomatizative Spirocyclization of p-Methoxy-Substituted Phenols
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    Scheme 41

    Scheme 41. N-Centered Functionalization Reactions of Hydroxamates
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    Scheme 42

    Scheme 42. Ni-Catalyzed N–N Coupling Reaction of Hydroxamates and Amines to Synthesize Hydrazides
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    Scheme 43

    Scheme 43. Synthesis of Hydrazides and N-Cyanoamides via N-Amination and Cyanation of Hydroxamates
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    Scheme 44

    Scheme 44. Base-Mediated Homologation Rearrangement of N-Methyl-N-Alkoxyamides
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    Scheme 45

    Scheme 45. Ru-Catalyzed α-C–H Arylation Reaction of the Nitrogen Atom in Hydroxamates
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    Scheme 46

    Scheme 46. α-C–H Esterification of the Nitrogen Atom in Hydroxamates
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