Unusual Formation of 1,2,4-Oxadiazine Core in Reaction of Amidoximes with Maleic or Fumaric Esters

We have developed a simple and convenient method for the synthesis of 3-aryl- and 3-hetaryl-1,2,4-oxadiazin-5-ones bearing an easily functionalizable (methoxycarbonyl)methyl group at position 6 via the reaction of aryl or hetaryl amidoximes with maleates or fumarates. The conditions for this reaction were optimized. Different products can be synthesized selectively in good yields depending on the base used and the ratio of reactants: substituted (1,2,4-oxadiazin-6-yl)acetic acids, corresponding methyl esters, or hybrid 3-(aryl)-6-((3-(aryl)-1,2,4-oxadiazol-5-yl)methyl)-4H-1,2,4-oxadiazin-5(6H)-ones. The reaction is tolerant to substituents’ electronic and steric effects in amidoximes. As a result, a series of 2-(5-oxo-3-(p-tolyl)-5,6-dihydro-4H-1,2,4-oxadiazin-6-yl)acetic acids, their methyl esters, and 1,2,4-oxadiazoles based on them were prepared and characterized by HRMS, 1H, and 13C NMR spectroscopy. The structures of three of them were elucidated with X-ray diffraction.

On the other hand, amidoximes react with α,β-unsaturated carbonyl compounds via the Michael addition to the electron-deficient carbon atom of the double bond followed by N-O bond cleavage and the formation of imidazole or pyrimidine core depending on reaction conditions (Scheme 1B), which are harsh in both cases [25].
At the same time, it seems interesting to develop a convenient route for the preparation of 1,2,4-oxadiazines based on such a reaction. These relatively little-studied heterocyclic compounds have significant potential in medicinal chemistry [26][27][28][29][30][31][32][33]. The research in the field of biological activity of this scaffold is hampered, first of all, by the lack of a suitable simple and universal method for its synthesis [34].
At first glance, the latter method is quite convenient due to the availability of acetylenedicarboxylates. Unfortunately, this reaction for most substrates resulted in low product yields (not exceeding 50%). The reason for this, most likely, is the excessive reactivity of the intermediate, which contains the fumaric acid fragment. In the basic media, this intermediate is able not only to cyclize to the target oxadiazinone but also to react with nucleophiles present in the system, forming the double bond addition by-products [40]. At first glance, the latter method is quite convenient due to the availability of acetylenedicarboxylates. Unfortunately, this reaction for most substrates resulted in low product yields (not exceeding 50%). The reason for this, most likely, is the excessive reactivity of the intermediate, which contains the fumaric acid fragment. In the basic media, this intermediate is able not only to cyclize to the target oxadiazinone but also to react with nucleophiles present in the system, forming the double bond addition by-products [40].
The use of maleates and fumarates as polyelectrophiles, rather than acetylenedicarboxylates, looks much more attractive. However, the literature provides no examples of the oxadiazine systems synthesis in this way. Although the addition of amidoximes to diethyl chlorofumarate led to the formation of an oxadiazinon, the reaction yield was lower than the one with diethyl acetylenedicarboxylate [40]. We have recently shown [41][42][43] that, in the MOH-DMSO medium, both key stages of the coupling between amidoximes and esters, namely O-acylation and 1,2,4-oxadiazole ring closing, occur even at room temperature. We assumed that 1,2,4-oxadiazinones could also be formed in this medium. We have studied this possibility with the example of dimethyl maleate (DMM) and dimethyl fumarate and have shown that this reaction can indeed be a convenient method for the synthesis of 1,2,4-oxadiazinones. Below we present the results of this study.

Results and Discussion
To start, we evaluated the procedure [43] previously used for the assembly of 1,2,4oxadiazole core via the condensation of amidoximes with esters (Table 1, entry 1). Although acid 3a was isolated as the main product instead of the expected ester 4a, this first experiment encouraged us to conduct a more detailed study. The yield of acid 3a was only 37%, but the following optimization of DMM and NaOH amounts, as well as the reaction time, provided a 65% yield of the product (Table 1, entries 2 and 4). Increasing the reagent amounts and prolonging the reaction time did not improve the product yield; however, it significantly promoted side processes, mainly the formation of 4-methylbenzonitrile (Table 1, entries 3 and 5). Variation of the base used revealed that NaOH is the most suitable alkali metal hydroxide for the preparation of acids 3, while the use of t-BuONa allows one to obtain corresponding esters 4 (Table 1, entries 6-8). In the latter case, hybrid 5a bearing two heterocyclic cores (1,2,4-oxadiazinone and 1,2,4-oxadiazole) with the same aromatic substituent (4-methylphenyl) was detected as a minor byproduct (aside from 4-methylbenzonitrile). Probably, its formation can be explained by the reaction of ester 4a with the second molecule of amidoxime 1a. After reducing the DMM/amidoxime ratio from 2:1 to 0.4:1, we obtain hybrid 5a in yield of 82% (Table 1, entry 11). Finally, we carried out the reactions with isomeric dimethyl fumarate (Table 1, entries 12 and 13) and found no significant difference in the reactivity of the two esters (fumarate and maleate).
Next, we investigated the scope of amidoximes for the synthesis of acids 3 (Scheme 2), esters 4, and hybrids 5 (Scheme 3). Electron-withdrawing substituents at position 4 of amidoximes have almost no effect on the yield of the corresponding products either in NaOH-DMSO medium or in t-BuONa-DMSO medium. The presence of electron donor groups OMe and OPh in amidoximes leads to some decrease in the yields of acids (3c, 3i) or esters (4c), but this effect is small. In the synthesis of hybrids 5, neither the methoxy group nor even the amino group reduced the yields of the desired products (5c, 5h). Substituents in the ortho-position also do not interfere with the reaction, although they slightly reduce the yield of the corresponding products (3j, 3k, 4i, 4k, 5i) compared to their para-substituted isomers.
The use of heterocyclic amidoximes made it possible to obtain 3-hetaryl-substituted derivatives of 1,2,4-oxadiazin-5-ones in acceptable yields. At the same time, for 5-methylthiazol-2-yl and pyridin-4-yl substituents, the corresponding hybrids 5 are formed as well as acids 3 or esters 4. For pyridin-2-yl amidoxime, the yield of hybrid 5k is noticeably lower than that of ester 4p. Nevertheless, we managed to isolate the corresponding hybrid, which confirms the fundamental possibility of using the reaction we discovered for the synthesis of 1,2,4-oxadiazinones bearing additional heterocyclic cores in this case as well.
To additionally confirm the structures of the obtained products, we used X-ray diffraction. Monocrystalline samples were grown for compounds 4d, 5b, and 5f, and their structures were established by XRD (the structures solution and refinement details, as well as numbering plots presented in Supplementary Materials, Table S1 and Figures S1-S3). In cases of hybrids 5b and 5f, molecules in the crystals form dimers via N-H···O hydrogen bonds between two oxadiazinone moieties ( Figure 1B,C), whereas in the structure of ester 4d, the acidic NH proton of the oxadiazinone rings is involved in hydrogen bonds with oxygen atoms of methoxy carbonyl groups ( Figure 1A). Moreover, π···π stacking between aromatic rings was observed in each case. 1,2,4-oxadiazole) with the same aromatic substituent (4-methylphenyl) was detected as a minor byproduct (aside from 4-methylbenzonitrile). Probably, its formation can be explained by the reaction of ester 4a with the second molecule of amidoxime 1a. After reducing the DMM/amidoxime ratio from 2:1 to 0.4:1, we obtain hybrid 5a in yield of 82% (Table 1, entry 11). Finally, we carried out the reactions with isomeric dimethyl fumarate (Table 1, entries 12 and 13) and found no significant difference in the reactivity of the two esters (fumarate and maleate).
a Dimethyl fumarate 2b was used instead of 2a.
Next, we investigated the scope of amidoximes for the synthesis of acids 3 (Scheme 2), esters 4, and hybrids 5 (Scheme 3). Electron-withdrawing substituents at position 4 of amidoximes have almost no effect on the yield of the corresponding products either in NaOH-DMSO medium or in t-BuONa-DMSO medium. The presence of electron donor groups OMe and OPh in amidoximes leads to some decrease in the yields of acids (3c, 3i) or esters (4c), but this effect is small. In the synthesis of hybrids 5, neither the methoxy group nor even the amino group reduced the yields of the desired products (5c, 5h). Substituents in the ortho-position also do not interfere with the reaction, although they slightly reduce the yield of the corresponding products (3j, 3k, 4i, 4k, 5i) compared to their para-substituted isomers. Based on the literature data, we can suggest the following possible mechanism for the formation of oxadiazines (Scheme 4). It is well known that amidoximes deprotonate in basic media and attack (by the oxygen atoms) double bonds activated by electron-withdrawing substituents [1, 40,44]. In our case, intermediate I3 is obtained in this way. Then, in the basic media, the amino group reacts with alkoxycarbonyl group [45], and the cyclization occurs to intermediate I4, which, after protonation and elimination of methanol, forms the target product, similar to the reaction described in [37]. discovered for the synthesis of 1,2,4-oxadiazinones bearing additional heterocyclic cores in this case as well.
To additionally confirm the structures of the obtained products, we used X-ray diffraction. Monocrystalline samples were grown for compounds 4d, 5b, and 5f, and their structures were established by XRD (the structures solution and refinement details, as well as numbering plots presented in Supplementary Materials, Table S1 and Figures S1-3). In cases of hybrids 5b and 5f, molecules in the crystals form dimers via N-H· · · O hydrogen bonds between two oxadiazinone moieties ( Figure 1B,C), whereas in the structure of ester 4d, the acidic NH proton of the oxadiazinone rings is involved in hydrogen bonds with oxygen atoms of methoxy carbonyl groups ( Figure 1A). Moreover, π· · · π stacking between aromatic rings was observed in each case. Based on the literature data, we can suggest the following possible mechanism for the formation of oxadiazines (Scheme 4). It is well known that amidoximes deprotonate in basic media and attack (by the oxygen atoms) double bonds activated by electron-withdrawing substituents [1, 40,44]. In our case, intermediate I3 is obtained in this way. Then, in the basic media, the amino group reacts with alkoxycarbonyl group [45], and the cyclization occurs to intermediate I4, which, after protonation and elimination of methanol, forms the target product, similar to the reaction described in [37].

General
Amidoximes 1 were prepared from commercial nitriles according to the literature procedures [10,17,46]. Maleic and fumaric esters, as well as all other reagents and solvents, were purchased from Merck (Merck KGaA, Darmstadt, Germany) and used as is. Reactions were monitored by analytical thin layer chromatography (TLC) Ma-Scheme 4. Plausible mechanism of 1,2,4-oxadiazine ring formation.

General
Amidoximes 1 were prepared from commercial nitriles according to the literature procedures [10,17,46]. Maleic and fumaric esters, as well as all other reagents and solvents, were purchased from Merck (Merck KGaA, Darmstadt, Germany) and used as is. Reactions were monitored by analytical thin layer chromatography (TLC) Macherey-Nagel, TLC plates Silufol UV-254 using UV light for detection. Column chromatography was carried out with silica gel grade 60 (0.040-0.063 mm) 230-400. NMR spectra were recorded on Bruker Avance DPX 400 (400 MHz, 101 MHz, and 376 MHz for 1 H, 13 C, and 19 F, respectively) or on Bruker Avance III 500 MHz (500 MHz for 1 H, 126 MHz for 13 C) in DMSO-d 6 , CDCl 3 , or acetone-d 6 . Chemical shifts are reported as parts per million (δ, ppm). The 1 H and 13 C spectra were calibrated using the residual signals of nondeuterated solvents as internal reference (2.50 ppm for residual 1 H, 39.50 ppm for 13  Singe crystals for X-ray studying were obtained by slow evaporation of DMSO solutions of corresponding oxadiazines 4d, 5b, and 5f at RT in air. X-ray diffraction data were collected at an Xcalibur Eos (4d) (Agilent Technologies, Santa Clara, CA, USA), at a Rigaku SuperNova (5f), and at a Rigaku XtaLAB Synergy-S (5b) (Rigaku Corporation, Tokyo, Japan) diffractometers using MoKα (λ = 0.71073 nm) or CuKα (λ = 0.154184 nm) radiation. The structures have been solved with the ShelXT [47] structure solution program using Intrinsic Phasing and refined with the ShelXL [48] refinement package incorporated in the OLEX2 program package [49] using Least Squares minimization. Empirical absorption correction was applied in CrysAlisPro [50] program complex using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 30 October 2022). CCDC numbers 2210873 (4d), 2210875 (5b), and 2210876 (5f).

Oxadiazinones Preparation and Characterization
General procedure for preparation acids 3. To a solution of amidoxime 1 (2 mmol) and ester 2 (4 mmol) in DMSO (3 mL), powdered NaOH (240 mg, 4 mmol) was rapidly added. The reaction mixture was stirred at room temperature for 18 h, diluted with cold brine (30 mL), and twice washed with toluene (5 mL). The water solution was acidified to pH of about 1 by hydrochloric acid and cooled to 5 • C. The resulting precipitate was filtered off, washed with cold water (5 mL), and dried in air at 50 • C.  13 4H-1,2,4-oxadiazin-6-yl) General procedure for preparation esters 4. To a solution of amidoxime 1 (2 mmol) in DMSO (3 mL), t-BuONa (192 mg, 2 mmol) was rapidly added. The reaction mixture was stirred at room temperature for 10 min, and ester 2 (4 mmol) was added. The reaction mixture was stirred at room temperature for another 4 h and was diluted with cold brine (30 mL). Compounds 4a, 4d, 4e, 4g, and 4l-p were filtered off, washed with cold water (5 mL) or 2), and dried in air at 50 • C. Compounds 4b, 4c, 4f, 4h-k were extracted with toluene (5 × 3 mL) and dried under reducing pressure. If necessary, the crude product was purified by column chromatography on SiO 2 using EA:Hexane:MeOH mixture as an eluent.