C–O Coupling of Hydrazones with Diacetyliminoxyl Radical Leading to Azo Oxime Ethers—Novel Antifungal Agents

Selective oxidative C–O coupling of hydrazones with diacetyliminoxyl is demonstrated, in which diacetyliminoxyl plays a dual role. It is an oxidant (hydrogen atom acceptor) and an O-partner for the oxidative coupling. The reaction is completed within 15–30 min at room temperature, is compatible with a broad scope of hydrazones, provides high yields in most cases, and requires no additives, which makes it robust and practical. The proposed reaction leads to the novel structural family of azo compounds, azo oxime ethers, which were discovered to be highly potent fungicides against a broad spectrum of phytopathogenic fungi (Venturia inaequalis, Rhizoctonia solani, Fusarium oxysporum, Fusarium moniliforme, Bipolaris sorokiniana, Sclerotinia sclerotiorum).


Introduction
The functionalization of organic compounds employing free radicals has emerged as a powerful tool in modern organic chemistry [1,2].In particular, N-oxyl radicals [3] have gained much attention as key agents in oxidative functionalization due to their mild conditions of generation, relatively high stability combined with high reactivity towards organic substrates, and outstanding structural diversity, allowing for control of their properties.However, N-oxyl radicals are usually generated in situ from corresponding N-hydroxy compounds and thus their usage demands oxidants or catalysts and other additives.Frequently, these additional reagents contain transition-metal salts, pose limitations on the substrate scope, and do not correspond to the principles of green chemistry.The peculiar feature of the present work is the use of diacetyliminoxyl [4] as a single ready-to-use free-radical reagent which plays the role of both oxidant and coupling partner for the oxidative functionalization reaction of hydrazones (Scheme 1C).Previously, free-radical chemistry of hydrazones was associated mainly with addition and hydrogen substitution reactions of aldehyde hydrazones [5-7] (Scheme 1A) and cyclizations of hydrazone-derived N-radicals [7][8][9] (Scheme 1B).However, ionic mechanisms were proposed for oxidative cyclizations of α,β-unsaturated N-tosylhydrazones in some cases [8,10].It should also be noted that in some functionalizations of type A (Scheme 1A), an additional synthetic step of chelate complex formation was necessary for effective radical functionalization of hydrazones [11][12][13].Hydrazones are reported to undergo peroxidation by t-BuOOH in the presence of cobalt-salen complexes with the formation of geminal azoperoxides and geminal azoxyperoxides [14].Unstable geminal azohydroperoxides [15] are formed as a result of hydrazone autoxidation by molecular oxygen [16][17][18][19].In general, free-radical functionalization of hydrazones with the formation of azocompounds is less developed compared to methods based on electrophilic attack of hydrazone carbon atoms, such as Michael-type reactions [20][21][22][23], chlorination [24], alkoxylation, or cyanation [25].Geminal azoacetates are synthesized by the oxidation of hydrazones with Pb(OAc) 4 [26][27][28].In the present work (Scheme 1C), diacetyliminoxyl was used as the only necessary reagent for high-yielding oxidative C-O coupling with the broad scope of both ketohydrazones and aldehyde-derived hydrazones at room temperature.It should be noted that none of the products of oxidative functionalization of hydrazones mentioned above were considered as fungicidal compounds.Unexpectedly, synthesized C-O coupling products were discovered as a new structural family of fungicides with activity against phytopathogenic fungi at the level of commercially used crop-protection compounds.This finding is very important in the light of the continuous development of strains of phytopathogenic fungi which are resistant against known synthetic fungicide types [29][30][31].
Molecules 2023, 28, x FOR PEER REVIEW 2 of 20 azoperoxides and geminal azoxyperoxides [14].Unstable geminal azohydroperoxides [15] are formed as a result of hydrazone autoxidation by molecular oxygen [16][17][18][19].In general, free-radical functionalization of hydrazones with the formation of azocompounds is less developed compared to methods based on electrophilic attack of hydrazone carbon atoms, such as Michael-type reactions [20][21][22][23], chlorination [24], alkoxylation, or cyanation [25].Geminal azoacetates are synthesized by the oxidation of hydrazones with Pb(OAc)4 [26][27][28].In the present work (Scheme 1C), diacetyliminoxyl was used as the only necessary reagent for high-yielding oxidative C-O coupling with the broad scope of both ketohydrazones and aldehyde-derived hydrazones at room temperature.It should be noted that none of the products of oxidative functionalization of hydrazones mentioned above were considered as fungicidal compounds.Unexpectedly, synthesized C-O coupling products were discovered as a new structural family of fungicides with activity against phytopathogenic fungi at the level of commercially used crop-protection compounds.This finding is very important in the light of the continuous development of strains of phytopathogenic fungi which are resistant against known synthetic fungicide types [29][30][31].

Results and Discussion
Hydrazone 2aa was used for the initial experiments with diacetyliminoxyl 1 (Table 1).CH2Cl2 was used as a solvent because it is a convenient medium for the synthesis and storage of diacetyliminoxyl 1.The reaction of 2aa with two equivalents of diacetyliminoxyl Scheme 1. Radical functionalization of hydrazones.

Results and Discussion
Hydrazone 2aa was used for the initial experiments with diacetyliminoxyl 1 (Table 1).CH 2 Cl 2 was used as a solvent because it is a convenient medium for the synthesis and storage of diacetyliminoxyl 1.The reaction of 2aa with two equivalents of diacetyliminoxyl under air afforded C-O coupling product 3aa with an 85% yield (Table 1, entry 1), along with diacetyl oxime 1-H.The reaction completed in 15 min, as evidenced by the disappearance of the dark red color characteristic of diacetyliminoxyl (for UV-Vis spectrum of 1, see [32]) and TLC.To check the possible involvement of oxygen as an oxidant [18] in the discovered process, or its possible negative impact on the yield, an experiment under argon was conducted (Table 1, entry 2).However, carrying out the reaction under inert conditions did not lead to a significant change in the yield of 3aa.The increase in the amount of 1 above the stoichiometric ratio increased the yield of 3aa by 10% (entry 3 compared to entry 1).Finally, the reaction with excess of hydrazone 2aa resulted in almost the same yield as in the case of the stoichiometric amount of 2aa (entry 4).under air afforded C-O coupling product 3aa with an 85% yield (Table 1, entry 1), along with diacetyl oxime 1-H.The reaction completed in 15 min, as evidenced by the disappearance of the dark red color characteristic of diacetyliminoxyl (for UV-Vis spectrum of 1, see [32]) and TLC.To check the possible involvement of oxygen as an oxidant [18] in the discovered process, or its possible negative impact on the yield, an experiment under argon was conducted (Table 1, entry 2).However, carrying out the reaction under inert conditions did not lead to a significant change in the yield of 3aa.The increase in the amount of 1 above the stoichiometric ratio increased the yield of 3aa by 10% (entry 3 compared to entry 1).Finally, the reaction with excess of hydrazone 2aa resulted in almost the same yield as in the case of the stoichiometric amount of 2aa (entry 4).The conditions of entry 1 of Table 1 were used to test the scope of the discovered C-O coupling (Scheme 2).The discovered C-O coupling is compatible with a wide range of hydrazones derived from aromatic ketones (Scheme 2A), aromatic aldehydes (Scheme 2B), aliphatic ketones (Scheme 2C), and aliphatic aldehydes (Scheme 2D).
Good yields of C-O coupling products 3aa-3ah (74-96%) were observed for N-arylhydrazones of methylarylketones containing electron-donating or electron-withdrawing substituents at benzene rings.The structure of 3ag was unambiguously confirmed by XRD analysis (see the ESI).The replacement of a methyl group by ethyl did not affect the reaction yield significantly (product 3ai compared to 3aa, yields 84-87%).Hydrazone of benzophenone gave an almost quantitative yield of 3aj despite steric hindrance and the expected low energy of the formed C-O bond [33] due to the steric and electronic effects of phenyl rings.2-Pyridyl moiety at the nitrogen atom of ketohydrazone was also tolerated (product 3ak).The reaction took place even in the case of bulky biphenylalkyl hydrazones with long-chain alkyl groups and a 2,4-dinitrophenyl group at the nitrogen atom, albeit with moderate yields of 42-46% (products 3am, 3an).The reaction of diacetyliminoxyl with β,γ-unsaturated phenylhydrazone 2al delivered the C-O coupling product 3al at 87% with the intact double C=C bond, despite the possible radical cyclization reactions typical of β,γ-unsaturated phenylhydrazones [9].Moreover, diacetyliminoxyl 1 is known to undergo addition to C=C double bonds at room temperature [34].Hydrazones derived from aromatic aldehydes also furnish C-O coupling products (3ba-3bd) in moderate to high yields.Of note, 3bd was obtained at a 68% yield employing N-methyl-substituted hydrazone 2bd.The reaction proceeded with high yields with acetone phenylhydrazone (product 3ca), and somewhat lower yields were obtained with higher homologues of acetone (products 3cd, 3cd).As in the case of unsaturated hydrazone 2al, allylacetone phenylhydrazone 2cd underwent oxidative C-O coupling with the formation of product 3cd containing intact C=C bond.Cyclic phenylhydrazones with ring sizes of 4-6 furnished the corresponding products 3ce-3cg at a 82-89% yield.The hydrazones of  The conditions of entry 1 of Table 1 were used to test the scope of the discovered C-O coupling (Scheme 2).The discovered C-O coupling is compatible with a wide range of hydrazones derived from aromatic ketones (Scheme 2A), aromatic aldehydes (Scheme 2B), aliphatic ketones (Scheme 2C), and aliphatic aldehydes (Scheme 2D).
Good yields of C-O coupling products 3aa-3ah (74-96%) were observed for Narylhydrazones of methylarylketones containing electron-donating or electron-withdrawing substituents at benzene rings.The structure of 3ag was unambiguously confirmed by XRD analysis (see the ESI).The replacement of a methyl group by ethyl did not affect the reaction yield significantly (product 3ai compared to 3aa, yields 84-87%).Hydrazone of benzophenone gave an almost quantitative yield of 3aj despite steric hindrance and the expected low energy of the formed C-O bond [33] due to the steric and electronic effects of phenyl rings.2-Pyridyl moiety at the nitrogen atom of ketohydrazone was also tolerated (product 3ak).The reaction took place even in the case of bulky biphenylalkyl hydrazones with long-chain alkyl groups and a 2,4-dinitrophenyl group at the nitrogen atom, albeit with moderate yields of 42-46% (products 3am, 3an).The reaction of diacetyliminoxyl with β,γ-unsaturated phenylhydrazone 2al delivered the C-O coupling product 3al at 87% with the intact double C=C bond, despite the possible radical cyclization reactions typical of β,γ-unsaturated phenylhydrazones [9].Moreover, diacetyliminoxyl 1 is known to undergo addition to C=C double bonds at room temperature [34].Hydrazones derived from aromatic aldehydes also furnish C-O coupling products (3ba-3bd) in moderate to high yields.Of note, 3bd was obtained at a 68% yield employing N-methyl-substituted hydrazone 2bd.The reaction proceeded with high yields with acetone phenylhydrazone (product 3ca), and somewhat lower yields were obtained with higher homologues of acetone (products 3cd, 3cd).As in the case of unsaturated hydrazone 2al, allylacetone phenylhydrazone 2cd underwent oxidative C-O coupling with the formation of product 3cd containing intact C=C bond.Cyclic phenylhydrazones with ring sizes of 4-6 furnished the corresponding products 3ce-3cg at a 82-89% yield.The hydrazones of aldehydes reacted smoothly with diacetyliminoxyl, providing azocompounds 3da-3de with a 58-74% yield.aldehydes reacted smoothly with diacetyliminoxyl, providing azocompounds 3da-3de with a 58-74% yield.
Scheme 3 demonstrates the practical applicability of the developed protocol for the synthesis at a 4 mmol scale without chromatographic purification or recrystallization (Scheme 3, (1)).Due to the instability of some phenylhydrazones in their pure form, we developed a one-pot procedure delivering the in situ generation of hydrazone that was sequentially added to the solution of diacetyliminoxyl (Scheme 3, (2)).Employing this protocol, the corresponding C-O coupling product 3ca was obtained at a yield of 71%.Scheme 3 demonstrates the practical applicability of the developed protocol for the synthesis at a 4 mmol scale without chromatographic purification or recrystallization (Scheme 3, (1)).Due to the instability of some phenylhydrazones in their pure form, we developed a one-pot procedure delivering the in situ generation of hydrazone that was sequentially added to the solution of diacetyliminoxyl (Scheme 3, (2)).Employing this protocol, the corresponding C-O coupling product 3ca was obtained at a yield of 71%.
Control experiments were conducted to support the plausible reaction mechanism (Scheme 4).N,N-diphenyl phenylhydrazone 2ao was introduced in the reaction with diacetyliminoxyl at standard reaction conditions (Scheme 4, (1)).There was no C-O coupling product observed by 1 H-NMR monitoring of the crude reaction mixture after 24 h, indicating that hydrogen atom abstraction from the nitrogen atom is a possible crucial step rather than the addition of an oxime radical at the C=N double bond.The experiment with TEMPO (Scheme 4, (2)) is a typical control reaction which is usually employed to intercept possible C-centered radical intermediates.The introduction of two equivalents of TEMPO into the reaction of diacetyliminoxyl 1 with hydrazone 2aa did not lead to significant changes; product 3aa was obtained without yield loss (Scheme 4, (2)).Moreover, no formation of a TEMPO adduct with C-centered radical was observed (TEMPO recovery 91%), highlighting the exceptionally high efficiency of diacetyliminoxyl in scavenging stabilized C-centered radicals [33].Control experiments were conducted to support the plausible reaction mechanism (Scheme 4).N,N-diphenyl phenylhydrazone 2ao was introduced in the reaction with diacetyliminoxyl at standard reaction conditions (Scheme 4, (1)).There was no C-O coupling product observed by 1 H-NMR monitoring of the crude reaction mixture after 24 h, indicating that hydrogen atom abstraction from the nitrogen atom is a possible crucial step rather than the addition of an oxime radical at the C=N double bond.The experiment with TEMPO (Scheme 4, (2)) is a typical control reaction which is usually employed to intercept possible C-centered radical intermediates.The introduction of two equivalents of TEMPO into the reaction of diacetyliminoxyl 1 with hydrazone 2aa did not lead to significant changes; product 3aa was obtained without yield loss (Scheme 4, (2)).Moreover, no formation of a TEMPO adduct with C-centered radical was observed (TEMPO recovery 91%), highlighting the exceptionally high efficiency of diacetyliminoxyl in scavenging stabilized C-centered radicals [33].Two possible reaction pathways can be proposed for the discovered C-O coupling of diacetyliminoxyl with hydrazones (Scheme 5).In path I, the hydrogen atom abstraction from hydrazone 2ca by diacetyliminoxyl 1 is followed by the coupling of the resultant hydrazyl radical A with 1.In path II, diacetyliminoxyl is added to hydrazone 2ca first, Scheme 3. Gram-scale synthesis of 3aa (1) and developed one-pot procedure for the synthesis of 3ca (2).Scheme 3. Gram-scale synthesis of 3aa (1) and developed one-pot procedure for the synthesis of 3ca (2).
Control experiments were conducted to support the plausible reaction mechanism (Scheme 4).N,N-diphenyl phenylhydrazone 2ao was introduced in the reaction with diacetyliminoxyl at standard reaction conditions (Scheme 4, (1)).There was no C-O coupling product observed by 1 H-NMR monitoring of the crude reaction mixture after 24 h, indicating that hydrogen atom abstraction from the nitrogen atom is a possible crucial step rather than the addition of an oxime radical at the C=N double bond.The experiment with TEMPO (Scheme 4, ( 2)) is a typical control reaction which is usually employed to intercept possible C-centered radical intermediates.The introduction of two equivalents of TEMPO into the reaction of diacetyliminoxyl 1 with hydrazone 2aa did not lead to significant changes; product 3aa was obtained without yield loss (Scheme 4, (2)).Moreover, no formation of a TEMPO adduct with C-centered radical was observed (TEMPO recovery 91%), highlighting the exceptionally high efficiency of diacetyliminoxyl in scavenging stabilized C-centered radicals [33].Two possible reaction pathways can be proposed for the discovered C-O coupling of diacetyliminoxyl with hydrazones (Scheme 5).In path I, the hydrogen atom abstraction from hydrazone 2ca by diacetyliminoxyl 1 is followed by the coupling of the resultant hydrazyl radical A with 1.In path II, diacetyliminoxyl is added to hydrazone 2ca first, then hydrogen atom from adduct B is abstracted.In both cases, the first stage is expected to be rate determining, whereas the second is expected to be very fast or even barrierless.In order to evaluate which path is more plausible, DFT calculations were performed by Two possible reaction pathways can be proposed for the discovered C-O coupling of diacetyliminoxyl with hydrazones (Scheme 5).In path I, the hydrogen atom abstraction from hydrazone 2ca by diacetyliminoxyl 1 is followed by the coupling of the resultant hydrazyl radical A with 1.In path II, diacetyliminoxyl is added to hydrazone 2ca first, then hydrogen atom from adduct B is abstracted.In both cases, the first stage is expected to be rate determining, whereas the second is expected to be very fast or even barrierless.In order to evaluate which path is more plausible, DFT calculations were performed by employing the low-cost but robust B97-3c composite method [35].The calculations revealed that path I is favored, both kinetically and thermodynamically, compared to path II; however, both pathways demonstrate activation barriers less than 20 kcal•mol −1 , which are acceptable for room temperature reactions.The fact that path I is energetically more favored than path II is in agreement with the published data on C-O coupling of diacetyliminoxyl with pyrazolones, isoxazolones, and phenols [33].However, it should be noted that diacetyliminoxyl addition reactions to π-systems were reported recently [34].
employing the low-cost but robust B97-3c composite method [35].The calculations revealed that path I is favored, both kinetically and thermodynamically, compared to path II; however, both pathways demonstrate activation barriers less than 20 kcal•mol −1 , which are acceptable for room temperature reactions.The fact that path I is energetically more favored than path II is in agreement with the published data on C-O coupling of diacetyliminoxyl with pyrazolones, isoxazolones, and phenols [33].However, it should be noted that diacetyliminoxyl addition reactions to π-systems were reported recently [34].
Scheme 5. Possible reaction pathway of the oxidative C-O coupling of diacetyliminoxyl with hydrazones (ΔG and ΔG ≠ values are calculated by B97-3c composite DFT method [35] and given in kcal•mol −1 ).

In Vitro Fungicidal Activity of the Synthesized Azo Compounds
In the second part of our research, the synthesized azo oxime ethers 3 were discovered as a new class of fungicides.Fungal diseases of agricultural crops represents one of the major threats to crop production [36][37][38][39].Phytopathogenic fungi contribute significantly to reductions in crop yield [37][38][39][40] and produce mycotoxins, which can be extremely dangerous food contaminants [41][42][43][44][45][46] (for example, aflatoxins produced by Aspergillus genus, trichothecenes by Fusarium species, and ergot alkaloids produced by fungi of Claviceps genus).Fungicides remain the most effective tool for crop protection against fungal diseases [47]; however, fungicidal resistance development against known active compound classes [29][30][31]48] is a serious threat to crop production, forcing scientists to search for new types of fungicides.Currently, despite the large number of fungicidal compounds used in agriculture, most of them belong to a limited number of classes and share a common mode of action.Namely, succinate dehydrogenase inhibitors (SDHIs), demethylation inhibitors (DMIs, imidazoles and triazoles), quinone outside inhibitors (QoI, or strobilurins), and quinone inside inhibitors (QiI) dominate the fungicide global market and development [49][50][51].Thus, the discovery of novel antifungal agents with unforeseen modes of action is a primary scientific goal [52][53][54][55][56][57][58].
Synthesized products 3 were tested for fungicidal activity at concentrations of 10-30 µg/mL against six phytopathogenic fungi from different taxonomic classes: 2).Triadimefon and kresoxim-methyl-commercially available fungicides-were used as reference compounds.
Scheme 5. Possible reaction pathway of the oxidative C-O coupling of diacetyliminoxyl with hydrazones (∆G and ∆G = values are calculated by B97-3c composite DFT method [35] and given in kcal•mol −1 ).

In Vitro Fungicidal Activity of the Synthesized Azo Compounds
In the second part of our research, the synthesized azo oxime ethers 3 were discovered as a new class of fungicides.Fungal diseases of agricultural crops represents one of the major threats to crop production [36][37][38][39].Phytopathogenic fungi contribute significantly to reductions in crop yield [37][38][39][40] and produce mycotoxins, which can be extremely dangerous food contaminants [41][42][43][44][45][46] (for example, aflatoxins produced by Aspergillus genus, trichothecenes by Fusarium species, and ergot alkaloids produced by fungi of Claviceps genus).Fungicides remain the most effective tool for crop protection against fungal diseases [47]; however, fungicidal resistance development against known active compound classes [29][30][31]48] is a serious threat to crop production, forcing scientists to search for new types of fungicides.Currently, despite the large number of fungicidal compounds used in agriculture, most of them belong to a limited number of classes and share a common mode of action.Namely, succinate dehydrogenase inhibitors (SDHIs), demethylation inhibitors (DMIs, imidazoles and triazoles), quinone outside inhibitors (QoI, or strobilurins), and quinone inside inhibitors (QiI) dominate the fungicide global market and development [49][50][51].Thus, the discovery of novel antifungal agents with unforeseen modes of action is a primary scientific goal [52][53][54][55][56][57][58].
Synthesized products 3 were tested for fungicidal activity at concentrations of 10-30 µg/mL against six phytopathogenic fungi from different taxonomic classes: V.  2).Triadimefon and kresoximmethyl-commercially available fungicides-were used as reference compounds.As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl chain (3ca, 3cb, 3cc) or ring size (3ce, 3cf, 3cg).AIBN, an alkyl azo derivative frequently As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quaternary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl The data on fungicidal activity exceeding the standard (triadimefon) are highlighted in green.
As can be seen from Table 2, compounds 3da and 3dc exhibit the greatest activity against phytopathogenic fungi.In general, azo oxime ethers with small aliphatic substituents at the quaternary carbon atom (3ca-cg and 3da-de) possess a higher activity compared to azo oxime ethers bearing aromatic substituents at the quaternary center (3aa, 3ae, 3ag, and 3ba).Compounds 3aj, 3al, 3am, and 3an with bulky substituents at quater-nary carbon atom do not show significant fungicidal activity, as well as azo oxime ether 3bd with a Me substituent at the nitrogen atom.Aldehyde-derived azo oxime ethers (3ba and 3da-de), in general, are superior to ketone derivatives (3aa, 3ae, 3ag, and 3ca-cg).In the series of long-chain alkyl or cyclic azo compounds, activity decreases with increasing alkyl chain (3ca, 3cb, 3cc) or ring size (3ce, 3cf, 3cg).AIBN, an alkyl azo derivative frequently used as a radical initiator taken for comparison, does not show significant activity.Compounds with a diacetyl oxime moiety, obtained by oxidative C-O coupling of diacetyliminoxyl with alkenes [34], pyrazolones [33], phenols [33], and dicarbonyl compounds [59], were also tested for fungicidal activity.None of them show essential mycelium growth inhibition, indicating that diacetyl oxime moiety itself is not sufficient for the manifestation of the observed fungicidal activity.It is noteworthy that the activity of the synthesized azo compounds in the present study was not predictable due to their structural novelty.The closest related fungicidal compounds are generally diaryl azo derivatives [60][61][62] and substituted oxime derivatives with a RO-N=C-N=N-Ar fragment at the oxime moiety [63].In contrast to these fungicides, the azo oxime ethers reported in the present work contain a tertiary C(sp 3 ) atom at the azo group.The activity of the azo oxime ethers 3ca, 3cd-cf, and 3da-de is comparable to that of triadimefon and kresoxim-methyl, which are commercially available fungicides widely used in crop protection.
EC 50 values were measured for the most promising azo compounds, 3ca and 3da, and reference compound kresoxim-methyl (Table 3).Synthesized azo compounds 3ca and 3da have a similar activity spectrum that greatly differs from that of kresoxim-methyl.Overall, the EC 50 values of 3ca and 3da are comparable to those of kresoxim-methyl; however, at higher concentrations, these azo compounds demonstrate stronger mycelium growth inhibition (Table 2).
General reaction conditions for Table 1 Hydrazone 2aa (1-2 mmol) was added to a stirred solution of diacetyliminoxyl radical 1 (2-3 mmol) in CH 2 Cl 2 (50 mL), prepared as described earlier in [4], at room temperature.The reaction mixture was stirred for 15 min under air (entries 1, 3, 4) or under argon (entry 2) atmosphere, until the dark red color of diacetyliminoxyl disappeared.After that, the reaction mixture was rotary evaporated under a water-jet vacuum.Yields were determined by 1 H NMR using 1,1,2,2-tetrachloroethane as an internal standard.
Experimental details for Scheme 2 Hydrazone 2 (1 mmol, 134-460 mg) was added to a stirred solution of diacetyliminoxyl 1 (2 mmol) in CH 2 Cl 2 (50 mL) at room temperature.The reaction mixture was stirred at RT for 15-30 min until the red color of diacetyliminoxyl disappeared.After that, the reaction mixture was rotary evaporated under a water-jet vacuum.C-O coupling products 3 were isolated by column chromatography on silica gel.
Experimental details for one-pot procedure for the synthesis of 3ca (Scheme 3) Phenylhydrazine (1 mmol, 108 mg) was dissolved in acetone (5 mL) and stirred at room temperature for 30 min.Then, the resulting solution was added dropwise to a stirred solution of diacetyliminoxyl 1 (2 mmol) in CH 2 Cl 2 (50 mL).The obtained reaction mixture was stirred at RT for 15 min, and was then rotary evaporated under a water-jet vacuum.

Table 1 .
Screening of the reaction parameters.

Table 1 .
Screening of the reaction parameters.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 2 .
In vitro fungicidal activity of the synthesized azo oxime ethers 3.

Table 3 .
5050Values for mycelium growth inhibition by the most active azo oxime ethers 3ca and 3da in comparison with kresoxim-methyl.